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<title>Week 34: Introduction to the course, Logistics and Practicalities &#8212; Applied Data Analysis and Machine Learning</title>
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Applied Data Analysis and Machine Learning
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About the course
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Teaching schedule with links to material
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Teachers and Grading
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Textbooks
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Review of Statistics with Resampling Techniques and Linear Algebra
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1. Elements of Probability Theory and Statistical Data Analysis
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2. Linear Algebra, Handling of Arrays and more Python Features
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From Regression to Support Vector Machines
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3. Linear Regression
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4. Ridge and Lasso Regression
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5. Resampling Methods
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6. Logistic Regression
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7. Optimization, the central part of any Machine Learning algortithm
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8. Support Vector Machines, overarching aims
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Decision Trees, Ensemble Methods and Boosting
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9. Decision trees, overarching aims
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10. Ensemble Methods: From a Single Tree to Many Trees and Extreme Boosting, Meet the Jungle of Methods
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Dimensionality Reduction
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11. Basic ideas of the Principal Component Analysis (PCA)
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12. Clustering and Unsupervised Learning
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Deep Learning Methods
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13. Neural networks
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14. Building a Feed Forward Neural Network
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15. Solving Differential Equations with Deep Learning
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16. Convolutional Neural Networks
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17. Recurrent neural networks: Overarching view
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Weekly material, notes and exercises
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Exercises week 34
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Week 34: Introduction to the course, Logistics and Practicalities
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Exercises week 35
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Week 35: From Ordinary Linear Regression to Ridge and Lasso Regression
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Exercises week 36
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Week 36: Linear Regression and Statistical interpretations
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Exercises week 37
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Week 37: Statistical interpretations and Resampling Methods
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Exercises week 38
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Week 38: Logistic Regression and Optimization
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Exercises week 39
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Week 39: Optimization and Gradient Methods
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Week 40: Gradient descent methods (continued) and start Neural networks
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Exercises week 41
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Week 41 Neural networks and constructing a neural network code
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Projects
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Project 1 on Machine Learning, deadline October 7 (midnight), 2024
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Project 2 on Machine Learning, deadline November 4 (Midnight)
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<i class="fas fa-list"></i> Contents
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<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#overview-of-first-week">
Overview of first week
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#schedule-first-week">
Schedule first week
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#lectures-and-computerlab">
Lectures and ComputerLab
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#communication-channels">
Communication channels
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#course-format">
Course Format
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#teachers">
Teachers
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#deadlines-for-projects-tentative">
Deadlines for projects (tentative)
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#grading">
Grading
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#reading-material">
Reading material
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#main-textbooks">
Main textbooks
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-popular-texts">
Other popular texts
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#reading-suggestions-week-34">
Reading suggestions week 34
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#prerequisites">
Prerequisites
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#topics-covered-in-this-course-statistical-analysis-and-optimization-of-data">
Topics covered in this course: Statistical analysis and optimization of data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#statistical-analysis-and-optimization-of-data">
Statistical analysis and optimization of data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#machine-learning">
Machine Learning
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#deep-learning-methods">
Deep learning methods
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#extremely-useful-tools-strongly-recommended">
Extremely useful tools, strongly recommended
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-courses-on-data-science-and-machine-learning-at-uio">
Other courses on Data science and Machine Learning at UiO
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-courses-on-data-science-and-machine-learning-at-uio-contn">
Other courses on Data science and Machine Learning at UiO, contn
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#learning-outcomes">
Learning outcomes
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#types-of-machine-learning">
Types of Machine Learning
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#essential-elements-of-ml">
Essential elements of ML
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#an-optimization-minimization-problem">
An optimization/minimization problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#a-frequentist-approach-to-data-analysis">
A Frequentist approach to data analysis
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#what-is-a-good-model">
What is a good model?
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#what-is-a-good-model-can-we-define-it">
What is a good model? Can we define it?
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#software-and-needed-installations">
Software and needed installations
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#python-installers">
Python installers
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#useful-python-libraries">
Useful Python libraries
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#installing-r-c-cython-or-julia">
Installing R, C++, cython or Julia
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#installing-r-c-cython-numba-etc">
Installing R, C++, cython, Numba etc
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#numpy-examples-and-important-matrix-and-vector-handling-packages">
Numpy examples and Important Matrix and vector handling packages
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#numpy-and-arrays">
Numpy and arrays
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#matrices-in-python">
Matrices in Python
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#meet-the-pandas">
Meet the Pandas
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#pandas-ai">
Pandas AI
</a>
<ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#simple-linear-regression-model-using-scikit-learn">
Simple linear regression model using
<strong>
scikit-learn
</strong>
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#to-our-real-data-nuclear-binding-energies-brief-reminder-on-masses-and-binding-energies">
To our real data: nuclear binding energies. Brief reminder on masses and binding energies
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#organizing-our-data">
Organizing our data
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#and-what-about-using-neural-networks">
And what about using neural networks?
</a>
</li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#a-first-summary">
A first summary
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#why-linear-regression-aka-ordinary-least-squares-and-family">
Why Linear Regression (aka Ordinary Least Squares and family)
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#regression-analysis-overarching-aims">
Regression analysis, overarching aims
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#regression-analysis-overarching-aims-ii">
Regression analysis, overarching aims II
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#examples">
Examples
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#general-linear-models-and-linear-algebra">
General linear models and linear algebra
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem">
Rewriting the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem-more-details">
Rewriting the fitting procedure as a linear algebra problem, more details
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#generalizing-the-fitting-procedure-as-a-linear-algebra-problem">
Generalizing the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id1">
Generalizing the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#optimizing-our-parameters">
Optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#our-model-for-the-nuclear-binding-energies">
Our model for the nuclear binding energies
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#optimizing-our-parameters-more-details">
Optimizing our parameters, more details
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#interpretations-and-optimizing-our-parameters">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id2">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id3">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#own-code-for-ordinary-least-squares">
Own code for Ordinary Least Squares
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#adding-error-analysis-and-training-set-up">
Adding error analysis and training set up
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#the-chi-2-function">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id4">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id5">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id6">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id7">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id8">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#fitting-an-equation-of-state-for-dense-nuclear-matter">
Fitting an Equation of State for Dense Nuclear Matter
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#the-code">
The code
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#splitting-our-data-in-training-and-test-data">
Splitting our Data in Training and Test data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercises">
Exercises
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-1-setting-up-various-python-environments">
Exercise 1: Setting up various Python environments
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-2-making-your-own-data-and-exploring-scikit-learn">
Exercise 2: making your own data and exploring scikit-learn
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-3-split-data-in-test-and-training-data">
Exercise 3: Split data in test and training data
</a>
</li>
</ul>
</nav>
</div>
</div>
</div>
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<h1>Week 34: Introduction to the course, Logistics and Practicalities</h1>
<!-- Table of contents -->
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<div id="jb-print-toc">
<div>
<h2> Contents </h2>
</div>
<nav aria-label="Page">
<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#overview-of-first-week">
Overview of first week
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#schedule-first-week">
Schedule first week
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#lectures-and-computerlab">
Lectures and ComputerLab
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#communication-channels">
Communication channels
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#course-format">
Course Format
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#teachers">
Teachers
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#deadlines-for-projects-tentative">
Deadlines for projects (tentative)
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#grading">
Grading
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#reading-material">
Reading material
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#main-textbooks">
Main textbooks
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-popular-texts">
Other popular texts
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#reading-suggestions-week-34">
Reading suggestions week 34
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#prerequisites">
Prerequisites
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#topics-covered-in-this-course-statistical-analysis-and-optimization-of-data">
Topics covered in this course: Statistical analysis and optimization of data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#statistical-analysis-and-optimization-of-data">
Statistical analysis and optimization of data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#machine-learning">
Machine Learning
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#deep-learning-methods">
Deep learning methods
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#extremely-useful-tools-strongly-recommended">
Extremely useful tools, strongly recommended
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-courses-on-data-science-and-machine-learning-at-uio">
Other courses on Data science and Machine Learning at UiO
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#other-courses-on-data-science-and-machine-learning-at-uio-contn">
Other courses on Data science and Machine Learning at UiO, contn
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#learning-outcomes">
Learning outcomes
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#types-of-machine-learning">
Types of Machine Learning
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#essential-elements-of-ml">
Essential elements of ML
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#an-optimization-minimization-problem">
An optimization/minimization problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#a-frequentist-approach-to-data-analysis">
A Frequentist approach to data analysis
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#what-is-a-good-model">
What is a good model?
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#what-is-a-good-model-can-we-define-it">
What is a good model? Can we define it?
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#software-and-needed-installations">
Software and needed installations
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#python-installers">
Python installers
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#useful-python-libraries">
Useful Python libraries
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#installing-r-c-cython-or-julia">
Installing R, C++, cython or Julia
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#installing-r-c-cython-numba-etc">
Installing R, C++, cython, Numba etc
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#numpy-examples-and-important-matrix-and-vector-handling-packages">
Numpy examples and Important Matrix and vector handling packages
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#numpy-and-arrays">
Numpy and arrays
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#matrices-in-python">
Matrices in Python
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#meet-the-pandas">
Meet the Pandas
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#pandas-ai">
Pandas AI
</a>
<ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#simple-linear-regression-model-using-scikit-learn">
Simple linear regression model using
<strong>
scikit-learn
</strong>
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#to-our-real-data-nuclear-binding-energies-brief-reminder-on-masses-and-binding-energies">
To our real data: nuclear binding energies. Brief reminder on masses and binding energies
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#organizing-our-data">
Organizing our data
</a>
</li>
<li class="toc-h3 nav-item toc-entry">
<a class="reference internal nav-link" href="#and-what-about-using-neural-networks">
And what about using neural networks?
</a>
</li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#a-first-summary">
A first summary
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#why-linear-regression-aka-ordinary-least-squares-and-family">
Why Linear Regression (aka Ordinary Least Squares and family)
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#regression-analysis-overarching-aims">
Regression analysis, overarching aims
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#regression-analysis-overarching-aims-ii">
Regression analysis, overarching aims II
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#examples">
Examples
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#general-linear-models-and-linear-algebra">
General linear models and linear algebra
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem">
Rewriting the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem-more-details">
Rewriting the fitting procedure as a linear algebra problem, more details
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#generalizing-the-fitting-procedure-as-a-linear-algebra-problem">
Generalizing the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id1">
Generalizing the fitting procedure as a linear algebra problem
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#optimizing-our-parameters">
Optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#our-model-for-the-nuclear-binding-energies">
Our model for the nuclear binding energies
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#optimizing-our-parameters-more-details">
Optimizing our parameters, more details
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#interpretations-and-optimizing-our-parameters">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id2">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id3">
Interpretations and optimizing our parameters
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#own-code-for-ordinary-least-squares">
Own code for Ordinary Least Squares
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#adding-error-analysis-and-training-set-up">
Adding error analysis and training set up
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#the-chi-2-function">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id4">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id5">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id6">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id7">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#id8">
The
<span class="math notranslate nohighlight">
\(\chi^2\)
</span>
function
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#fitting-an-equation-of-state-for-dense-nuclear-matter">
Fitting an Equation of State for Dense Nuclear Matter
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#the-code">
The code
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#splitting-our-data-in-training-and-test-data">
Splitting our Data in Training and Test data
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercises">
Exercises
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-1-setting-up-various-python-environments">
Exercise 1: Setting up various Python environments
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-2-making-your-own-data-and-exploring-scikit-learn">
Exercise 2: making your own data and exploring scikit-learn
</a>
</li>
<li class="toc-h2 nav-item toc-entry">
<a class="reference internal nav-link" href="#exercise-3-split-data-in-test-and-training-data">
Exercise 3: Split data in test and training data
</a>
</li>
</ul>
</nav>
</div>
</div>
</div>
<div>
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<h1>Week 34: Introduction to the course, Logistics and Practicalities<a class="headerlink" href="#week-34-introduction-to-the-course-logistics-and-practicalities" title="Permalink to this headline"></a></h1>
<p><strong>Morten Hjorth-Jensen</strong>, Department of Physics and Center for Computing in Science Education, University of Oslo, Norway and Department of Physics and Astronomy and Facility for Rare Isotope Beams, Michigan State University, USA</p>
<p>Date: <strong>Week 34, August 19-23, 2024</strong></p>
<div class="section" id="overview-of-first-week">
<h2>Overview of first week<a class="headerlink" href="#overview-of-first-week" title="Permalink to this headline"></a></h2>
<ol class="simple">
<li><p>The sessions on Tuesdays and Wednesdays last four hours for each group (four groups in total) and will include lectures in a flipped mode (promoting active learning) and work on exercices and projects.</p></li>
<li><p>The sessions will begin with lectures, discussions, questions and answers about the material to be covered every week. Videos and teaching material will be announced in due time.</p></li>
<li><p>There are four groups:</p></li>
</ol>
<ul class="simple">
<li><p>Tuesdays 815am-12pm and 1215pm-4pm</p></li>
<li><p>Wednesdays 815am-12pm and 1215pm-4pm.</p></li>
</ul>
<ol class="simple">
<li><p>On Mondays we have a regular lecture which will be organized as a mix of active learning sessions and regular lectures. These lectures/active learning sessions start at 1015am and end at 12pm and serve the aims of giving an overview over various topics as well as solving specific problems. These lectures will also be recorded.</p></li>
</ol>
<ul class="simple">
<li><p><a class="reference external" href="https://youtu.be/82IPtCrzbhs">Link to recording of lecture</a></p></li>
</ul>
<p>The labs are also available till 6pm Tuesdays and Wednesdays. Videos and learning material with reading suggestions will be made available before each week starts.</p>
</div>
<div class="section" id="schedule-first-week">
<h2>Schedule first week<a class="headerlink" href="#schedule-first-week" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>August 19: Lecture: Presentation of course, Linear regression, examples and theory</p></li>
<li><p>August 20: Introduction to software and repetition of Python Programming, linear algebra and basic elements of statistics. Please select group.</p></li>
<li><p>August 23: Introduction to software and repetition of Python Programming, linear algebra and basic elements of statistics. Please select group.</p></li>
</ul>
</div>
<div class="section" id="lectures-and-computerlab">
<h2>Lectures and ComputerLab<a class="headerlink" href="#lectures-and-computerlab" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Mondays: regular lectures/active learning sessions (10.15am-12pm)</p></li>
<li><p>The sessions on Tuesdays and Wednesdays last four hours and will include partly lectures and discussions in the beginning.</p></li>
<li><p>Weekly reading assignments and videos needed to solve projects and exercises.</p></li>
<li><p>Weekly exercises. You can hand in exercises if you want and get an extra score, see below.</p></li>
<li><p>Detailed lecture notes, exercises, all programs presented, projects etc can be found at the homepage of the course.</p></li>
<li><p>Weekly plans and all other information are on the official website. This info will also be conveyed via weekly emails.</p></li>
<li><p>No final exam, three projects that are graded and have to be approved.</p></li>
</ul>
</div>
<div class="section" id="communication-channels">
<h2>Communication channels<a class="headerlink" href="#communication-channels" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Communications (email and more) via &lt;<a class="reference external" href="http://canvas.uio.no">canvas.uio.no</a>&gt;</p></li>
<li><p><strong>Discord</strong> channel at <a class="reference external" href="https://discord.gg/XBKjd4ccGq">https://discord.gg/XBKjd4ccGq</a></p></li>
</ul>
</div>
<div class="section" id="course-format">
<h2>Course Format<a class="headerlink" href="#course-format" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Three compulsory projects. Electronic reports only using <a class="reference external" href="https://www.uio.no/english/services/it/education/canvas/">Canvas</a> to hand in projects and <a class="reference external" href="https://git-scm.com/">git</a> as version control software and <a class="reference external" href="https://github.com/">GitHub</a> for repository (or <a class="reference external" href="https://about.gitlab.com/">GitLab</a>) of all your material.</p></li>
<li><p>Evaluation and grading: The three projects are graded and each counts 1/3 of the final mark. No final written or oral exam.</p></li>
</ul>
<p>a. For the last project each group/participant submits a proposal or works with suggested (by us) proposals for the project.</p>
<p>b. If possible, we would like to organize the last project as a workshop where each group presents this to all other participants of the course</p>
<p>c. Based on feedback etc, each group finalizes the report and submits for grading.</p>
<ul class="simple">
<li><p>Python is the default programming language, but feel free to use C/C++, Julia and/or Fortran or other programming languages. All source codes discussed during the lectures can be found at the webpage and <a class="reference external" href="https://github.com/CompPhysics/MachineLearning/tree/master/doc/Programs">github address</a> of the course.</p></li>
</ul>
</div>
<div class="section" id="teachers">
<h2>Teachers<a class="headerlink" href="#teachers" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Morten Hjorth-Jensen, <a class="reference external" href="mailto:morten&#46;hjorth-jensen&#37;&#52;&#48;fys&#46;uio&#46;no">morten<span>&#46;</span>hjorth-jensen<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p>
<ul>
<li><p><strong>Phone</strong>: +47-48257387</p></li>
<li><p><strong>Office</strong>: Department of Physics, University of Oslo, Eastern wing, room FØ470</p></li>
<li><p><strong>Office hours</strong>: <em>Anytime</em>! Individual or group office hours can be arranged either in person or via zoom. Feel free to send an email for planning.</p></li>
</ul>
</li>
<li><p>Ida Torkjellsdatter Storehaug, <a class="reference external" href="mailto:i&#46;t&#46;storehaug&#37;&#52;&#48;fys&#46;uio&#46;no">i<span>&#46;</span>t<span>&#46;</span>storehaug<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
<li><p>Fahimeh Najafi, <a class="reference external" href="mailto:fahimeh&#46;najafi&#37;&#52;&#48;fys&#46;uio&#46;no">fahimeh<span>&#46;</span>najafi<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
<li><p>Mia-Katrin Ose Kvalsund, <a class="reference external" href="mailto:m&#46;k&#46;o&#46;kvalsund&#37;&#52;&#48;fys&#46;uio&#46;no">m<span>&#46;</span>k<span>&#46;</span>o<span>&#46;</span>kvalsund<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
<li><p>Karl Henrik Fredly, <a class="reference external" href="mailto:k&#46;h&#46;fredly&#37;&#52;&#48;fys&#46;uio&#46;no">k<span>&#46;</span>h<span>&#46;</span>fredly<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
<li><p>Sigurd k. Huse, <a class="reference external" href="mailto:s&#46;k&#46;huse&#37;&#52;&#48;fys&#46;uio&#46;no">s<span>&#46;</span>k<span>&#46;</span>huse<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
<li><p>Odin Johansen, <a class="reference external" href="mailto:odin&#46;johansen&#37;&#52;&#48;fys&#46;uio&#46;no">odin<span>&#46;</span>johansen<span>&#64;</span>fys<span>&#46;</span>uio<span>&#46;</span>no</a></p></li>
</ul>
</div>
<div class="section" id="deadlines-for-projects-tentative">
<h2>Deadlines for projects (tentative)<a class="headerlink" href="#deadlines-for-projects-tentative" title="Permalink to this headline"></a></h2>
<ol class="simple">
<li><p>Project 1: October 7 (available September 2) graded with feedback)</p></li>
<li><p>Project 2: November 4 (available October 8, graded with feedback)</p></li>
<li><p>Project 3: December 9 (available November 5, graded with feedback)</p></li>
</ol>
<p>Extra Credit (not mandatory), weekly exercise assignments, 10 in total (due Friday same week), 10% additional score. The extra credit assignments are due each Friday and can be uploaed to <strong>Canvas</strong> in your preferred format (although we prefer jupyter-notebooks). First assignment is for week 35. Each weekly exercise set gives one additional point to the final score, see below on grading.</p>
</div>
<div class="section" id="grading">
<h2>Grading<a class="headerlink" href="#grading" title="Permalink to this headline"></a></h2>
<p>Grades are awarded on a scale from A to F, where A is the best grade and F is a fail. There are three projects which are graded and each project counts 1/3 of the final grade. The total score is thus the average from all three projects.</p>
<p>The final number of points is based on the average of all projects and the grade follows the following table:</p>
<ul class="simple">
<li><p>92-100 points: A</p></li>
<li><p>77-91 points: B</p></li>
<li><p>58-76 points: C</p></li>
<li><p>46-57 points: D</p></li>
<li><p>40-45 points: E</p></li>
<li><p>0-39 points: F-failed</p></li>
</ul>
<p>In addition you can get an extra score for weekly assignments (10 in total and due each Friday). Each weekly assignment counts 1 point. As an example, this means that if your average after three projects is 88 points and you have handed in and gotten approved four weekly exercises, the total score is 88+4=92, which translates into an A.</p>
</div>
<div class="section" id="reading-material">
<h2>Reading material<a class="headerlink" href="#reading-material" title="Permalink to this headline"></a></h2>
<p>The lecture notes are collected as a jupyter-book at <a class="reference external" href="https://compphysics.github.io/MachineLearning/doc/LectureNotes/_build/html/intro.html">https://compphysics.github.io/MachineLearning/doc/LectureNotes/_build/html/intro.html</a>.
The lecture notes can also be retrieved as a standard PDF file at <a class="reference external" href="https://compphysics.github.io/MachineLearning/doc/LectureNotes/MLbook.pdf">https://compphysics.github.io/MachineLearning/doc/LectureNotes/MLbook.pdf</a>.</p>
<p>In addition to the lecture notes, we recommend the books of Rasckha et
al and Goodfellow et al. We will follow these texts closely and the
weekly reading assignments refer to these texts. The text by Hastie et
al is also widely used in the Machine Learning community. See next slide for link to textbooks.</p>
</div>
<div class="section" id="main-textbooks">
<h2>Main textbooks<a class="headerlink" href="#main-textbooks" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Goodfellow, Bengio, and Courville (GBC), Deep Learning <a class="reference external" href="https://www.deeplearningbook.org/">https://www.deeplearningbook.org/</a></p></li>
<li><p>Sebastian Raschka, Yuxi Lie, and Vahid Mirjalili (RLM), Machine Learning with PyTorch and Scikit-Learn at <a class="reference external" href="https://www.packtpub.com/product/machine-learning-with-pytorch-and-scikit-learn/9781801819312">https://www.packtpub.com/product/machine-learning-with-pytorch-and-scikit-learn/9781801819312</a>, see also <a class="reference external" href="https://sebastianraschka.com/blog/2022/ml-pytorch-book.html">https://sebastianraschka.com/blog/2022/ml-pytorch-book.html</a></p></li>
</ul>
<p>The weekly reading suggestions are all from these two texts. The text by GBC can be accessed chapter by chapter from the abovementioned URL.
Each chapter of RLM gives access to the pertinent notebooks. These notebooks are highly recommended.</p>
</div>
<div class="section" id="other-popular-texts">
<h2>Other popular texts<a class="headerlink" href="#other-popular-texts" title="Permalink to this headline"></a></h2>
<p><strong>Other texts.</strong></p>
<ul class="simple">
<li><p>Christopher M. Bishop (CB), Pattern Recognition and Machine Learning</p></li>
<li><p><a class="reference external" href="https://www.springer.com/gp/book/9780387848570">Hastie, Tibshirani, and Friedman (HTF), The Elements of Statistical Learning, Springer</a>.</p></li>
<li><p><a class="reference external" href="https://www.oreilly.com/library/view/hands-on-machine-learning/9781492032632/">Aurelien Geron (AG), Hands‑On Machine Learning with Scikit‑Learn and TensorFlow, O’Reilly</a>. This text is very useful since it contains many code examples and hands-on applications of all algorithms discussed in this course.</p></li>
<li><p><a class="reference external" href="https://probml.github.io/pml-book/book1.html">Kevin Murphy (KM), Probabilistic Machine Learning, an Introduction</a></p></li>
<li><p>David Foster (DF), Generative Deep Learning, <a class="reference external" href="https://www.oreilly.com/library/view/generative-deep-learning/9781098134174/">https://www.oreilly.com/library/view/generative-deep-learning/9781098134174/</a></p></li>
<li><p>Babcock and Gavras (BG), Generative AI with Python and TensorFlow, <a class="reference external" href="https://github.com/PacktPublishing/Hands-On-Generative-AI-with-Python-and-TensorFlow-2">https://github.com/PacktPublishing/Hands-On-Generative-AI-with-Python-and-TensorFlow-2</a></p></li>
</ul>
</div>
<div class="section" id="reading-suggestions-week-34">
<h2>Reading suggestions week 34<a class="headerlink" href="#reading-suggestions-week-34" title="Permalink to this headline"></a></h2>
<p>This week: Refresh linear algebra, GBC chapter 2. Install scikit-learn. See lecture notes for week 34 at <a class="reference external" href="https://compphysics.github.io/MachineLearning/doc/web/course.html">https://compphysics.github.io/MachineLearning/doc/web/course.html</a> (these notes).</p>
</div>
<div class="section" id="prerequisites">
<h2>Prerequisites<a class="headerlink" href="#prerequisites" title="Permalink to this headline"></a></h2>
<p>Basic knowledge in programming and mathematics, with an emphasis on
linear algebra. Knowledge of Python or/and C++ as programming
languages is strongly recommended and experience with Jupiter notebook
is recommended. Required courses are the equivalents to the University
of Oslo mathematics courses MAT1100, MAT1110, MAT1120 and at least one
of the corresponding computing and programming courses INF1000/INF1110
or MAT-INF1100/MAT-INF1100L/BIOS1100/KJM-INF1100. Most universities
offer nowadays a basic programming course (often compulsory) where
Python is the recurring programming language.</p>
</div>
<div class="section" id="topics-covered-in-this-course-statistical-analysis-and-optimization-of-data">
<h2>Topics covered in this course: Statistical analysis and optimization of data<a class="headerlink" href="#topics-covered-in-this-course-statistical-analysis-and-optimization-of-data" title="Permalink to this headline"></a></h2>
<p>The course has two central parts</p>
<ol class="simple">
<li><p>Statistical analysis and optimization of data</p></li>
<li><p>Machine learning</p></li>
</ol>
<p>These topics will be scattered thorughout the course and may not necessarily be taught separately. Rather, we will often take an approach (during the lectures and project/exercise sessions) where say elements from statistical data analysis are mixed with specific Machine Learning algorithms.</p>
</div>
<div class="section" id="statistical-analysis-and-optimization-of-data">
<h2>Statistical analysis and optimization of data<a class="headerlink" href="#statistical-analysis-and-optimization-of-data" title="Permalink to this headline"></a></h2>
<p>We plan to cover the following topics:</p>
<ul class="simple">
<li><p>Basic concepts, expectation values, variance, covariance, correlation functions and errors;</p></li>
<li><p>Simpler models, binomial distribution, the Poisson distribution, simple and multivariate normal distributions;</p></li>
<li><p>Central elements of Bayesian statistics and modeling;</p></li>
<li><p>Gradient methods for data optimization;</p></li>
<li><p>Monte Carlo methods, Markov chains, Gibbs sampling and Metropolis-Hastings sampling (tentative);</p></li>
<li><p>Estimation of errors and resampling techniques such as the cross-validation, blocking, bootstrapping and jackknife methods;</p></li>
<li><p>Principal Component Analysis (PCA) and its mathematical foundation;</p></li>
</ul>
</div>
<div class="section" id="machine-learning">
<h2>Machine Learning<a class="headerlink" href="#machine-learning" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Pre deep-learning revolution (2008 approx)</p>
<ul>
<li><p>Linear Regression and Logistic Regression, classification and regression problems;</p></li>
<li><p>Bayesian linear and logistic regression, kernel regression;</p></li>
<li><p>Decisions trees, Random Forests, Bagging and Boosting methods;</p></li>
<li><p>Support vector machines (only survey);</p></li>
<li><p>Unsupervised learning and dimensionality reduction, from PCA to clustering;</p></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="deep-learning-methods">
<h2>Deep learning methods<a class="headerlink" href="#deep-learning-methods" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Deep learning</p>
<ul>
<li><p>Neural networks and deep learning;</p></li>
<li><p>Convolutional neural networks;</p></li>
<li><p>Recurrent neural networks;</p></li>
<li><p>Autoencoders</p></li>
<li><p>Generative methods with an emphasis on Boltzmann Machines, Variational Autoencoders and Generalized Adversarial Networks(covered by FYS5429);</p></li>
</ul>
</li>
</ul>
<p>Hands-on demonstrations, exercises and projects aim at deepening your understanding of these topics.</p>
</div>
<div class="section" id="extremely-useful-tools-strongly-recommended">
<h2>Extremely useful tools, strongly recommended<a class="headerlink" href="#extremely-useful-tools-strongly-recommended" title="Permalink to this headline"></a></h2>
<p><strong>and discussed at the lab sessions.</strong></p>
<ul class="simple">
<li><p>GIT for version control, and GitHub or GitLab as repositories, highly recommended. This will be discussed during the first exercise session</p></li>
<li><p>Anaconda and other Python environments, see intro slides and links to programming resources at <a class="reference external" href="https://computationalscienceuio.github.io/RefreshProgrammingSkills/intro.html">https://computationalscienceuio.github.io/RefreshProgrammingSkills/intro.html</a></p></li>
</ul>
</div>
<div class="section" id="other-courses-on-data-science-and-machine-learning-at-uio">
<h2>Other courses on Data science and Machine Learning at UiO<a class="headerlink" href="#other-courses-on-data-science-and-machine-learning-at-uio" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/fys/FYS5419/index-eng.html">FYS5419 Quantum Computing and Quantum Machine Learning</a></p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/fys/FYS5429/index-eng.html">FYS5429 Advanced Machine Learning for the Physical Sciences</a></p></li>
<li><p><a class="reference external" href="http://www.uio.no/studier/emner/matnat/math/STK2100/index-eng.html">STK2100 Machine learning and statistical methods for prediction and classification</a>.</p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/ifi/IN3050/index-eng.html">IN3050/4050 Introduction to Artificial Intelligence and Machine Learning</a>. Introductory course in machine learning and AI with an algorithmic approach.</p></li>
<li><p><a class="reference external" href="http://www.uio.no/studier/emner/matnat/math/STK-INF3000/index-eng.html">STK-INF3000/4000 Selected Topics in Data Science</a>. The course provides insight into selected contemporary relevant topics within Data Science.</p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/ifi/IN4080/index.html">IN4080 Natural Language Processing</a>. Probabilistic and machine learning techniques applied to natural language processing.</p></li>
</ul>
</div>
<div class="section" id="other-courses-on-data-science-and-machine-learning-at-uio-contn">
<h2>Other courses on Data science and Machine Learning at UiO, contn<a class="headerlink" href="#other-courses-on-data-science-and-machine-learning-at-uio-contn" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/math/STK-IN4300/index-eng.html">STK-IN4300 Statistical learning methods in Data Science</a>. An advanced introduction to statistical and machine learning. For students with a good mathematics and statistics background.</p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/ifi/IN4310/index.html">IN3310/4310 Deep Learnig for Image Analysis</a></p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/math/STK4051/index-eng.html">STK4051 Computational Statistics</a></p></li>
<li><p><a class="reference external" href="https://www.uio.no/studier/emner/matnat/math/STK4021/index-eng.html">STK4021 Applied Bayesian Analysis and Numerical Methods</a></p></li>
</ul>
</div>
<div class="section" id="learning-outcomes">
<h2>Learning outcomes<a class="headerlink" href="#learning-outcomes" title="Permalink to this headline"></a></h2>
<ul class="simple">
<li><p>Learn about basic data analysis, statistical analysis, Bayesian statistics, Monte Carlo sampling, data optimization and machine learning;</p></li>
<li><p>Be capable of extending the acquired knowledge to other systems and cases;</p></li>
<li><p>Have an understanding of central algorithms used in data analysis and machine learning;</p></li>
<li><p>Understand linear methods for regression and classification, from ordinary least squares, via Lasso and Ridge to Logistic regression;</p></li>
<li><p>Learn about neural networks and deep learning methods for supervised and unsupervised learning. Emphasis on feed forward neural networks, convolutional and recurrent neural networks;</p></li>
<li><p>Learn about about decision trees, random forests, bagging and boosting methods;</p></li>
<li><p>Learn about support vector machines and kernel transformations;</p></li>
<li><p>Reduction of data sets, from PCA to clustering;</p></li>
<li><p>Generative models</p></li>
<li><p>Work on numerical projects to illustrate the theory. The projects play a central role and you are expected to know modern programming languages like Python or C++ and/or Fortran (Fortran2003 or later) or Julia or other.</p></li>
</ul>
</div>
<div class="section" id="types-of-machine-learning">
<h2>Types of Machine Learning<a class="headerlink" href="#types-of-machine-learning" title="Permalink to this headline"></a></h2>
<p>The approaches to machine learning are many, but are often split into
two main categories. In <em>supervised learning</em> we know the answer to a
problem, and let the computer deduce the logic behind it. On the other
hand, <em>unsupervised learning</em> is a method for finding patterns and
relationship in data sets without any prior knowledge of the system.
Some authours also operate with a third category, namely
<em>reinforcement learning</em>. This is a paradigm of learning inspired by
behavioral psychology, where learning is achieved by trial-and-error,
solely from rewards and punishment.</p>
<p>Another way to categorize machine learning tasks is to consider the
desired output of a system. Some of the most common tasks are:</p>
<ul class="simple">
<li><p>Classification: Outputs are divided into two or more classes. The goal is to produce a model that assigns inputs into one of these classes. An example is to identify digits based on pictures of hand-written ones. Classification is typically supervised learning.</p></li>
<li><p>Regression: Finding a functional relationship between an input data set and a reference data set. The goal is to construct a function that maps input data to continuous output values.</p></li>
<li><p>Clustering: Data are divided into groups with certain common traits, without knowing the different groups beforehand. It is thus a form of unsupervised learning.</p></li>
</ul>
</div>
<div class="section" id="essential-elements-of-ml">
<h2>Essential elements of ML<a class="headerlink" href="#essential-elements-of-ml" title="Permalink to this headline"></a></h2>
<p>The methods we cover have three main topics in common, irrespective of
whether we deal with supervised or unsupervised learning.</p>
<ul class="simple">
<li><p>The first ingredient is normally our data set (which can be subdivided into training, validation and test data). Many find the most difficult part of using Machine Learning to be the set up of your data in a meaningful way.</p></li>
<li><p>The second item is a model which is normally a function of some parameters. The model reflects our knowledge of the system (or lack thereof). As an example, if we know that our data show a behavior similar to what would be predicted by a polynomial, fitting our data to a polynomial of some degree would then determin our model.</p></li>
<li><p>The last ingredient is a so-called <strong>cost/loss</strong> function (or error or risk function) which allows us to present an estimate on how good our model is in reproducing the data it is supposed to train.</p></li>
</ul>
</div>
<div class="section" id="an-optimization-minimization-problem">
<h2>An optimization/minimization problem<a class="headerlink" href="#an-optimization-minimization-problem" title="Permalink to this headline"></a></h2>
<p>At the heart of basically all Machine Learning algorithms we will encounter so-called minimization or optimization algorithms. A large family of such methods are so-called <strong>gradient methods</strong>.</p>
</div>
<div class="section" id="a-frequentist-approach-to-data-analysis">
<h2>A Frequentist approach to data analysis<a class="headerlink" href="#a-frequentist-approach-to-data-analysis" title="Permalink to this headline"></a></h2>
<p>When you hear phrases like <strong>predictions and estimations</strong> and
<strong>correlations and causations</strong>, what do you think of? May be you think
of the difference between classifying new data points and generating
new data points.
Or perhaps you consider that correlations represent some kind of symmetric statements like
if <span class="math notranslate nohighlight">\(A\)</span> is correlated with <span class="math notranslate nohighlight">\(B\)</span>, then <span class="math notranslate nohighlight">\(B\)</span> is correlated with
<span class="math notranslate nohighlight">\(A\)</span>. Causation on the other hand is directional, that is if <span class="math notranslate nohighlight">\(A\)</span> causes <span class="math notranslate nohighlight">\(B\)</span>, <span class="math notranslate nohighlight">\(B\)</span> does not
necessarily cause <span class="math notranslate nohighlight">\(A\)</span>.</p>
<p>These concepts are in some sense the difference between machine
learning and statistics. In machine learning and prediction based
tasks, we are often interested in developing algorithms that are
capable of learning patterns from given data in an automated fashion,
and then using these learned patterns to make predictions or
assessments of newly given data. In many cases, our primary concern
is the quality of the predictions or assessments, and we are less
concerned about the underlying patterns that were learned in order
to make these predictions.</p>
<p>In machine learning we normally use <a class="reference external" href="https://en.wikipedia.org/wiki/Frequentist_inference">a so-called frequentist approach</a>,
where the aim is to make predictions and find correlations. We focus
less on for example extracting a probability distribution function (PDF). The PDF can be
used in turn to make estimations and find causations such as given <span class="math notranslate nohighlight">\(A\)</span>
what is the likelihood of finding <span class="math notranslate nohighlight">\(B\)</span>.</p>
</div>
<div class="section" id="what-is-a-good-model">
<h2>What is a good model?<a class="headerlink" href="#what-is-a-good-model" title="Permalink to this headline"></a></h2>
<p>In science and engineering we often end up in situations where we want to infer (or learn) a
quantitative model <span class="math notranslate nohighlight">\(M\)</span> for a given set of sample points <span class="math notranslate nohighlight">\(\boldsymbol{X} \in [x_1, x_2,\dots x_N]\)</span>.</p>
<p>As we will see repeatedely in these lectures, we could try to fit these data points to a model given by a
straight line, or if we wish to be more sophisticated to a more complex
function.</p>
<p>The reason for inferring such a model is that it
serves many useful purposes. On the one hand, the model can reveal information
encoded in the data or underlying mechanisms from which the data were generated. For instance, we could discover important
corelations that relate interesting physics interpretations.</p>
<p>In addition, it can simplify the representation of the given data set and help
us in making predictions about future data samples.</p>
<p>A first important consideration to keep in mind is that inferring the <em>correct</em> model
for a given data set is an elusive, if not impossible, task. The fundamental difficulty
is that if we are not specific about what we mean by a <em>correct</em> model, there
could easily be many different models that fit the given data set <em>equally well</em>.</p>
</div>
<div class="section" id="what-is-a-good-model-can-we-define-it">
<h2>What is a good model? Can we define it?<a class="headerlink" href="#what-is-a-good-model-can-we-define-it" title="Permalink to this headline"></a></h2>
<p>The central question is this: what leads us to say that a model is correct or
optimal for a given data set? To make the model inference problem well posed, i.e.,
to guarantee that there is a unique optimal model for the given data, we need to
impose additional assumptions or restrictions on the class of models considered. To
this end, we should not be looking for just any model that can describe the data.
Instead, we should look for a <strong>model</strong> <span class="math notranslate nohighlight">\(M\)</span> that is the best among a restricted class
of models. In addition, to make the model inference problem computationally
tractable, we need to specify how restricted the class of models needs to be. A
common strategy is to start
with the simplest possible class of models that is just necessary to describe the data
or solve the problem at hand. More precisely, the model class should be rich enough
to contain at least one model that can fit the data to a desired accuracy and yet be
restricted enough that it is relatively simple to find the best model for the given data.</p>
<p>Thus, the most popular strategy is to start from the
simplest class of models and increase the complexity of the models only when the
simpler models become inadequate. For instance, if we work with a regression problem to fit a set of sample points, one
may first try the simplest class of models, namely linear models, followed obviously by more complex models.</p>
<p>How to evaluate which model fits best the data is something we will come back to over and over again in these sets of lectures.</p>
</div>
<div class="section" id="software-and-needed-installations">
<h2>Software and needed installations<a class="headerlink" href="#software-and-needed-installations" title="Permalink to this headline"></a></h2>
<p>We will make extensive use of Python as programming language and its
myriad of available libraries. You will find
Jupyter notebooks invaluable in your work. You can run <strong>R</strong>
codes in the Jupyter/IPython notebooks, with the immediate benefit of
visualizing your data. You can also use compiled languages like C++,
Rust, Julia, Fortran etc if you prefer. The focus in these lectures will be
on Python.</p>
<p>If you have Python installed (we strongly recommend Python3) and you feel
pretty familiar with installing different packages, we recommend that
you install the following Python packages via <strong>pip</strong> as</p>
<ol class="simple">
<li><p>pip install numpy scipy matplotlib ipython scikit-learn mglearn sympy pandas pillow</p></li>
</ol>
<p>For Python3, replace <strong>pip</strong> with <strong>pip3</strong>.</p>
<p>For OSX users we recommend, after having installed Xcode, to
install <strong>brew</strong>. Brew allows for a seamless installation of additional
software via for example</p>
<ol class="simple">
<li><p>brew install python3</p></li>
</ol>
<p>For Linux users, with its variety of distributions like for example the widely popular Ubuntu distribution,
you can use <strong>pip</strong> as well and simply install Python as</p>
<ol class="simple">
<li><p>sudo apt-get install python3 (or python for pyhton2.7)</p></li>
</ol>
<p>etc etc.</p>
</div>
<div class="section" id="python-installers">
<h2>Python installers<a class="headerlink" href="#python-installers" title="Permalink to this headline"></a></h2>
<p>If you don’t want to perform these operations separately and venture
into the hassle of exploring how to set up dependencies and paths, we
recommend two widely used distrubutions which set up all relevant
dependencies for Python, namely</p>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.anaconda.com/">Anaconda</a>,</p></li>
</ul>
<p>which is an open source
distribution of the Python and R programming languages for large-scale
data processing, predictive analytics, and scientific computing, that
aims to simplify package management and deployment. Package versions
are managed by the package management system <strong>conda</strong>.</p>
<ul class="simple">
<li><p><a class="reference external" href="https://www.enthought.com/product/canopy/">Enthought canopy</a></p></li>
</ul>
<p>is a Python
distribution for scientific and analytic computing distribution and
analysis environment, available for free and under a commercial
license.</p>
<p>Furthermore, <a class="reference external" href="https://colab.research.google.com/notebooks/welcome.ipynb">Google’s Colab</a> is a free Jupyter notebook environment that requires
no setup and runs entirely in the cloud. Try it out!</p>
</div>
<div class="section" id="useful-python-libraries">
<h2>Useful Python libraries<a class="headerlink" href="#useful-python-libraries" title="Permalink to this headline"></a></h2>
<p>Here we list several useful Python libraries we strongly recommend (if you use anaconda many of these are already there)</p>
<ul class="simple">
<li><p><a class="reference external" href="https://www.numpy.org/">NumPy</a> is a highly popular library for large, multi-dimensional arrays and matrices, along with a large collection of high-level mathematical functions to operate on these arrays</p></li>
<li><p><a class="reference external" href="https://pandas.pydata.org/">The pandas</a> library provides high-performance, easy-to-use data structures and data analysis tools</p></li>
<li><p><a class="reference external" href="http://xarray.pydata.org/en/stable/">Xarray</a> is a Python package that makes working with labelled multi-dimensional arrays simple, efficient, and fun!</p></li>
<li><p><a class="reference external" href="https://www.scipy.org/">Scipy</a> (pronounced “Sigh Pie”) is a Python-based ecosystem of open-source software for mathematics, science, and engineering.</p></li>
<li><p><a class="reference external" href="https://matplotlib.org/">Matplotlib</a> is a Python 2D plotting library which produces publication quality figures in a variety of hardcopy formats and interactive environments across platforms.</p></li>
<li><p><a class="reference external" href="https://github.com/HIPS/autograd">Autograd</a> can automatically differentiate native Python and Numpy code. It can handle a large subset of Python’s features, including loops, ifs, recursion and closures, and it can even take derivatives of derivatives of derivatives</p></li>
<li><p><a class="reference external" href="https://jax.readthedocs.io/en/latest/index.html">JAX</a> has now more or less replaced <strong>Autograd</strong>. JAX is Autograd and XLA, brought together for high-performance numerical computing and machine learning research. It provides composable transformations of Python+NumPy programs: differentiate, vectorize, parallelize, Just-In-Time compile to GPU/TPU, and more.</p></li>
<li><p><a class="reference external" href="https://www.sympy.org/en/index.html">SymPy</a> is a Python library for symbolic mathematics.</p></li>
<li><p><a class="reference external" href="https://scikit-learn.org/stable/">scikit-learn</a> has simple and efficient tools for machine learning, data mining and data analysis</p></li>
<li><p><a class="reference external" href="https://www.tensorflow.org/">TensorFlow</a> is a Python library for fast numerical computing created and released by Google</p></li>
<li><p><a class="reference external" href="https://keras.io/">Keras</a> is a high-level neural networks API, written in Python and capable of running on top of TensorFlow, CNTK, or Theano</p></li>
<li><p><a class="reference external" href="https://pytorch.org/">Pytorch</a>, highly recommened</p></li>
<li><p><a class="reference external" href="https://pypi.org/project/Theano/">Theano</a> and many other</p></li>
</ul>
</div>
<div class="section" id="installing-r-c-cython-or-julia">
<h2>Installing R, C++, cython or Julia<a class="headerlink" href="#installing-r-c-cython-or-julia" title="Permalink to this headline"></a></h2>
<p>You will also find it convenient to utilize <strong>R</strong>. We will mainly
use Python during our lectures and in various projects and exercises.
Those of you
already familiar with <strong>R</strong> should feel free to continue using <strong>R</strong>, keeping
however an eye on the parallel Python set ups. Similarly, if you are a
Python afecionado, feel free to explore <strong>R</strong> as well. Jupyter(Julia, Python and R) /Ipython
notebook allows you to run <strong>R</strong> codes and <strong>Julia</strong> codes interactively in your
browser. The software library <strong>R</strong> is really tailored for statistical data analysis
and allows for an easy usage of the tools and algorithms we will discuss in these
lectures.</p>
<p>To install <strong>R</strong> with Jupyter notebook
<a class="reference external" href="https://mpacer.org/maths/r-kernel-for-ipython-notebook">follow the link here</a></p>
</div>
<div class="section" id="installing-r-c-cython-numba-etc">
<h2>Installing R, C++, cython, Numba etc<a class="headerlink" href="#installing-r-c-cython-numba-etc" title="Permalink to this headline"></a></h2>
<p>For the C++ aficionados, Jupyter/IPython notebook allows you also to
install C++ and run codes written in this language interactively in
the browser. Since we will emphasize writing many of the algorithms
yourself, you can thus opt for either Python or C++ (or Fortran or other compiled languages) as programming
languages.</p>
<p>To add more entropy, <strong>cython</strong> can also be used when running your
notebooks. It means that Python with the jupyter notebook
setup allows you to integrate widely popular softwares and tools for
scientific computing. Similarly, the
<a class="reference external" href="https://numba.pydata.org/">Numba Python package</a> delivers increased performance
capabilities with minimal rewrites of your codes. With its
versatility, including symbolic operations, Python offers a unique
computational environment. Your jupyter notebook can easily be
converted into a nicely rendered <strong>PDF</strong> file or a Latex file for
further processing. For example, convert to latex as</p>
<div class="highlight-none notranslate"><div class="highlight"><pre><span></span> pycod jupyter nbconvert filename.ipynb --to latex
</pre></div>
</div>
<p>And to add more versatility, the Python package <a class="reference external" href="http://www.sympy.org/en/index.html">SymPy</a> is a Python library for symbolic mathematics. It aims to become a full-featured computer algebra system (CAS) and is entirely written in Python.</p>
<p>Finally, we recommend strongly using Autograd or JAX for automatic differentiation.</p>
</div>
<div class="section" id="numpy-examples-and-important-matrix-and-vector-handling-packages">
<h2>Numpy examples and Important Matrix and vector handling packages<a class="headerlink" href="#numpy-examples-and-important-matrix-and-vector-handling-packages" title="Permalink to this headline"></a></h2>
<p>There are several central software libraries for linear algebra and eigenvalue problems. Several of the more
popular ones have been wrapped into ofter software packages like those from the widely used text <strong>Numerical Recipes</strong>. The original source codes in many of the available packages are often taken from the widely used
software package LAPACK, which follows two other popular packages
developed in the 1970s, namely EISPACK and LINPACK. We describe them shortly here.</p>
<ul class="simple">
<li><p>LINPACK: package for linear equations and least square problems.</p></li>
<li><p>LAPACK:package for solving symmetric, unsymmetric and generalized eigenvalue problems. From LAPACK’s website <a class="reference external" href="http://www.netlib.org">http://www.netlib.org</a> it is possible to download for free all source codes from this library. Both C/C++ and Fortran versions are available.</p></li>
<li><p>BLAS (I, II and III): (Basic Linear Algebra Subprograms) are routines that provide standard building blocks for performing basic vector and matrix operations. Blas I is vector operations, II vector-matrix operations and III matrix-matrix operations. Highly parallelized and efficient codes, all available for download from <a class="reference external" href="http://www.netlib.org">http://www.netlib.org</a>.</p></li>
</ul>
</div>
<div class="section" id="numpy-and-arrays">
<h2>Numpy and arrays<a class="headerlink" href="#numpy-and-arrays" title="Permalink to this headline"></a></h2>
<p><a class="reference external" href="http://www.numpy.org/">Numpy</a> provides an easy way to handle arrays in Python. The standard way to import this library is as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
</pre></div>
</div>
</div>
</div>
<p>Here follows a simple example where we set up an array of ten elements, all determined by random numbers drawn according to the normal distribution,</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">n</span> <span class="o">=</span> <span class="mi">10</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">normal</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">n</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[ 1.51262599 0.63980912 -1.25680702 0.97680846 -1.33095972 -0.41396339
-0.81478187 -0.6087346 2.11164003 -1.21061589]
</pre></div>
</div>
</div>
</div>
<p>We defined a vector <span class="math notranslate nohighlight">\(x\)</span> with <span class="math notranslate nohighlight">\(n=10\)</span> elements with its values given by the Normal distribution <span class="math notranslate nohighlight">\(N(0,1)\)</span>.
Another alternative is to declare a vector as follows</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">])</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1 2 3]
</pre></div>
</div>
</div>
</div>
<p>Here we have defined a vector with three elements, with <span class="math notranslate nohighlight">\(x_0=1\)</span>, <span class="math notranslate nohighlight">\(x_1=2\)</span> and <span class="math notranslate nohighlight">\(x_2=3\)</span>. Note that both Python and C++
start numbering array elements from <span class="math notranslate nohighlight">\(0\)</span> and on. This means that a vector with <span class="math notranslate nohighlight">\(n\)</span> elements has a sequence of entities <span class="math notranslate nohighlight">\(x_0, x_1, x_2, \dots, x_{n-1}\)</span>. We could also let (recommended) Numpy to compute the logarithms of a specific array as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">4</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">]))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1.38629436 1.94591015 2.07944154]
</pre></div>
</div>
</div>
</div>
<p>In the last example we used Numpy’s unary function <span class="math notranslate nohighlight">\(np.log\)</span>. This function is
highly tuned to compute array elements since the code is vectorized
and does not require looping. We normaly recommend that you use the
Numpy intrinsic functions instead of the corresponding <strong>log</strong> function
from Python’s <strong>math</strong> module. The looping is done explicitely by the
<strong>np.log</strong> function. The alternative, and slower way to compute the
logarithms of a vector would be to write</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">log</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">4</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">])</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)):</span>
<span class="n">x</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">log</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="n">i</span><span class="p">])</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1 1 2]
</pre></div>
</div>
</div>
</div>
<p>We note that our code is much longer already and we need to import the <strong>log</strong> function from the <strong>math</strong> module.
The attentive reader will also notice that the output is <span class="math notranslate nohighlight">\([1, 1, 2]\)</span>. Python interprets automagically our numbers as integers (like the <strong>automatic</strong> keyword in C++). To change this we could define our array elements to be double precision numbers as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mi">4</span><span class="p">,</span> <span class="mi">7</span><span class="p">,</span> <span class="mi">8</span><span class="p">],</span> <span class="n">dtype</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">float64</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1.38629436 1.94591015 2.07944154]
</pre></div>
</div>
</div>
</div>
<p>or simply write them as double precision numbers (Python uses 64 bits as default for floating point type variables), that is</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">,</span> <span class="mf">8.0</span><span class="p">]))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1.38629436 1.94591015 2.07944154]
</pre></div>
</div>
</div>
</div>
<p>To check the number of bytes (remember that one byte contains eight bits for double precision variables), you can use simple use the <strong>itemsize</strong> functionality (the array <span class="math notranslate nohighlight">\(x\)</span> is actually an object which inherits the functionalities defined in Numpy) as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">,</span> <span class="mf">8.0</span><span class="p">]))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">x</span><span class="o">.</span><span class="n">itemsize</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>8
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="matrices-in-python">
<h2>Matrices in Python<a class="headerlink" href="#matrices-in-python" title="Permalink to this headline"></a></h2>
<p>Having defined vectors, we are now ready to try out matrices. We can
define a <span class="math notranslate nohighlight">\(3 \times 3 \)</span> real matrix <span class="math notranslate nohighlight">\(\boldsymbol{A}\)</span> as (recall that we user
lowercase letters for vectors and uppercase letters for matrices)</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">,</span> <span class="mf">8.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">3.0</span><span class="p">,</span> <span class="mf">10.0</span><span class="p">,</span> <span class="mf">11.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">5.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">]</span> <span class="p">]))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[[1.38629436 1.94591015 2.07944154]
[1.09861229 2.30258509 2.39789527]
[1.38629436 1.60943791 1.94591015]]
</pre></div>
</div>
</div>
</div>
<p>If we use the <strong>shape</strong> function we would get <span class="math notranslate nohighlight">\((3, 3)\)</span> as output, that is verifying that our matrix is a <span class="math notranslate nohighlight">\(3\times 3\)</span> matrix. We can slice the matrix and print for example the first column (Python organized matrix elements in a row-major order, see below) as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">,</span> <span class="mf">8.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">3.0</span><span class="p">,</span> <span class="mf">10.0</span><span class="p">,</span> <span class="mf">11.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">5.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">]</span> <span class="p">]))</span>
<span class="c1"># print the first column, row-major order and elements start with 0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">[:,</span><span class="mi">0</span><span class="p">])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1.38629436 1.09861229 1.38629436]
</pre></div>
</div>
</div>
</div>
<p>We can continue this was by printing out other columns or rows. The example here prints out the second column</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">,</span> <span class="mf">8.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">3.0</span><span class="p">,</span> <span class="mf">10.0</span><span class="p">,</span> <span class="mf">11.0</span><span class="p">],</span> <span class="p">[</span><span class="mf">4.0</span><span class="p">,</span> <span class="mf">5.0</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">]</span> <span class="p">]))</span>
<span class="c1"># print the first column, row-major order and elements start with 0</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">[</span><span class="mi">1</span><span class="p">,:])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[1.09861229 2.30258509 2.39789527]
</pre></div>
</div>
</div>
</div>
<p>Numpy contains many other functionalities that allow us to slice, subdivide etc etc arrays. We strongly recommend that you look up the <a class="reference external" href="http://www.numpy.org/">Numpy website for more details</a>. Useful functions when defining a matrix are the <strong>np.zeros</strong> function which declares a matrix of a given dimension and sets all elements to zero</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">n</span> <span class="o">=</span> <span class="mi">10</span>
<span class="c1"># define a matrix of dimension 10 x 10 and set all elements to zero</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span> <span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span> <span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]
[0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]]
</pre></div>
</div>
</div>
</div>
<p>or initializing all elements to</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">n</span> <span class="o">=</span> <span class="mi">10</span>
<span class="c1"># define a matrix of dimension 10 x 10 and set all elements to one</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">ones</span><span class="p">(</span> <span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span> <span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]
[1. 1. 1. 1. 1. 1. 1. 1. 1. 1.]]
</pre></div>
</div>
</div>
</div>
<p>or as unitarily distributed random numbers (see the material on random number generators in the statistics part)</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">n</span> <span class="o">=</span> <span class="mi">10</span>
<span class="c1"># define a matrix of dimension 10 x 10 and set all elements to random numbers with x \in [0, 1]</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">rand</span><span class="p">(</span><span class="n">n</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">A</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[[0.1169204 0.51615779 0.40961688 0.169299 0.08009874 0.67925887
0.8475889 0.92080432 0.07712724 0.2863391 ]
[0.36161658 0.84155431 0.70135856 0.1576057 0.04686491 0.67511113
0.29593305 0.22946401 0.78385675 0.20527785]
[0.66575697 0.37717637 0.52407775 0.55094784 0.68989446 0.30013135
0.39991048 0.20300793 0.25294371 0.91433102]
[0.05080769 0.92665802 0.77039278 0.13455019 0.89692576 0.09621323
0.48511333 0.8529175 0.32738537 0.12206812]
[0.98108401 0.73397147 0.62288579 0.66003032 0.18712313 0.63307537
0.2032806 0.17418673 0.06061276 0.92991181]
[0.53480404 0.69484973 0.09821823 0.93019783 0.34478594 0.18646225
0.11861803 0.25646067 0.55225408 0.84907109]
[0.50352245 0.92678221 0.27037635 0.9833205 0.84985833 0.82844656
0.34112554 0.9306628 0.89155606 0.24149532]
[0.37137157 0.65751456 0.63693246 0.25068519 0.75674251 0.43724406
0.34131583 0.74180248 0.63801791 0.76426396]
[0.2311959 0.77594586 0.52606333 0.54222783 0.86434639 0.72364915
0.4008393 0.68827947 0.56408898 0.68640031]
[0.90137794 0.02599188 0.40848657 0.94114646 0.67199457 0.02124568
0.32717946 0.59030403 0.59188296 0.81707832]]
</pre></div>
</div>
</div>
</div>
<p>As we will see throughout these lectures, there are several extremely useful functionalities in Numpy.
As an example, consider the discussion of the covariance matrix. Suppose we have defined three vectors
<span class="math notranslate nohighlight">\(\boldsymbol{x}, \boldsymbol{y}, \boldsymbol{z}\)</span> with <span class="math notranslate nohighlight">\(n\)</span> elements each. The covariance matrix is defined as</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\boldsymbol{\Sigma} = \begin{bmatrix} \sigma_{xx} &amp; \sigma_{xy} &amp; \sigma_{xz} \\
\sigma_{yx} &amp; \sigma_{yy} &amp; \sigma_{yz} \\
\sigma_{zx} &amp; \sigma_{zy} &amp; \sigma_{zz}
\end{bmatrix},
\end{split}\]</div>
<p>where for example</p>
<div class="math notranslate nohighlight">
\[
\sigma_{xy} =\frac{1}{n} \sum_{i=0}^{n-1}(x_i- \overline{x})(y_i- \overline{y}).
\]</div>
<p>The Numpy function <strong>np.cov</strong> calculates the covariance elements using the factor <span class="math notranslate nohighlight">\(1/(n-1)\)</span> instead of <span class="math notranslate nohighlight">\(1/n\)</span> since it assumes we do not have the exact mean values.
The following simple function uses the <strong>np.vstack</strong> function which takes each vector of dimension <span class="math notranslate nohighlight">\(1\times n\)</span> and produces a <span class="math notranslate nohighlight">\(3\times n\)</span> matrix <span class="math notranslate nohighlight">\(\boldsymbol{W}\)</span></p>
<div class="math notranslate nohighlight">
\[\begin{split}
\boldsymbol{W} = \begin{bmatrix} x_0 &amp; x_1 &amp; x_2 &amp; \dots &amp; x_{n-2} &amp; x_{n-1} \\
y_0 &amp; y_1 &amp; y_2 &amp; \dots &amp; y_{n-2} &amp; y_{n-1} \\
z_0 &amp; z_1 &amp; z_2 &amp; \dots &amp; z_{n-2} &amp; z_{n-1} \\
\end{bmatrix},
\end{split}\]</div>
<p>which in turn is converted into into the <span class="math notranslate nohighlight">\(3\times 3\)</span> covariance matrix
<span class="math notranslate nohighlight">\(\boldsymbol{\Sigma}\)</span> via the Numpy function <strong>np.cov()</strong>. We note that we can also calculate
the mean value of each set of samples <span class="math notranslate nohighlight">\(\boldsymbol{x}\)</span> etc using the Numpy
function <strong>np.mean(x)</strong>. We can also extract the eigenvalues of the
covariance matrix through the <strong>np.linalg.eig()</strong> function.</p>
<div class="cell docutils container">
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Importing various packages</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">n</span> <span class="o">=</span> <span class="mi">100</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">normal</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">n</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">x</span><span class="p">))</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">4</span><span class="o">+</span><span class="mi">3</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">normal</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">n</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">y</span><span class="p">))</span>
<span class="n">z</span> <span class="o">=</span> <span class="n">x</span><span class="o">**</span><span class="mi">3</span><span class="o">+</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">normal</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">n</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">np</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">z</span><span class="p">))</span>
<span class="n">W</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">vstack</span><span class="p">((</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">))</span>
<span class="n">Sigma</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">cov</span><span class="p">(</span><span class="n">W</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">Sigma</span><span class="p">)</span>
<span class="n">Eigvals</span><span class="p">,</span> <span class="n">Eigvecs</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linalg</span><span class="o">.</span><span class="n">eig</span><span class="p">(</span><span class="n">Sigma</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">Eigvals</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>-0.04372067685794603
3.777112373675558
-0.028188673105960467
[[ 1.0680225 3.36841864 2.42592679]
[ 3.36841864 11.49312535 7.50233673]
[ 2.42592679 7.50233673 7.94272259]]
[18.418656 0.05992237 2.02529207]
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="k">matplotlib</span> inline
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">scipy</span> <span class="kn">import</span> <span class="n">sparse</span>
<span class="n">eye</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">eye</span><span class="p">(</span><span class="mi">4</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">eye</span><span class="p">)</span>
<span class="n">sparse_mtx</span> <span class="o">=</span> <span class="n">sparse</span><span class="o">.</span><span class="n">csr_matrix</span><span class="p">(</span><span class="n">eye</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">sparse_mtx</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linspace</span><span class="p">(</span><span class="o">-</span><span class="mi">10</span><span class="p">,</span><span class="mi">10</span><span class="p">,</span><span class="mi">100</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">sin</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span><span class="n">marker</span><span class="o">=</span><span class="s1">&#39;x&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>[[1. 0. 0. 0.]
[0. 1. 0. 0.]
[0. 0. 1. 0.]
[0. 0. 0. 1.]]
(0, 0) 1.0
(1, 1) 1.0
(2, 2) 1.0
(3, 3) 1.0
</pre></div>
</div>
<img alt="_images/week34_73_1.png" src="_images/week34_73_1.png" />
</div>
</div>
</div>
<div class="section" id="meet-the-pandas">
<h2>Meet the Pandas<a class="headerlink" href="#meet-the-pandas" title="Permalink to this headline"></a></h2>
<!-- dom:FIGURE: [fig/pandas.jpg, width=600 frac=0.8] -->
<!-- begin figure -->
<p><img src="fig/pandas.jpg" width="600"><p style="font-size: 0.9em"><i>Figure 1: </i></p></p>
<!-- end figure -->
<p>Another useful Python package is
<a class="reference external" href="https://pandas.pydata.org/">pandas</a>, which is an open source library
providing high-performance, easy-to-use data structures and data
analysis tools for Python. <strong>pandas</strong> stands for panel data, a term borrowed from econometrics and is an efficient library for data analysis with an emphasis on tabular data.
<strong>pandas</strong> has two major classes, the <strong>DataFrame</strong> class with two-dimensional data objects and tabular data organized in columns and the class <strong>Series</strong> with a focus on one-dimensional data objects. Both classes allow you to index data easily as we will see in the examples below.
<strong>pandas</strong> allows you also to perform mathematical operations on the data, spanning from simple reshapings of vectors and matrices to statistical operations.</p>
<p>The following simple example shows how we can, in an easy way make tables of our data. Here we define a data set which includes names, place of birth and date of birth, and displays the data in an easy to read way. We will see repeated use of <strong>pandas</strong>, in particular in connection with classification of data.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">from</span> <span class="nn">IPython.display</span> <span class="kn">import</span> <span class="n">display</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;First Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Frodo&quot;</span><span class="p">,</span> <span class="s2">&quot;Bilbo&quot;</span><span class="p">,</span> <span class="s2">&quot;Aragorn II&quot;</span><span class="p">,</span> <span class="s2">&quot;Samwise&quot;</span><span class="p">],</span>
<span class="s1">&#39;Last Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Baggins&quot;</span><span class="p">,</span> <span class="s2">&quot;Baggins&quot;</span><span class="p">,</span><span class="s2">&quot;Elessar&quot;</span><span class="p">,</span><span class="s2">&quot;Gamgee&quot;</span><span class="p">],</span>
<span class="s1">&#39;Place of birth&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Shire&quot;</span><span class="p">,</span> <span class="s2">&quot;Shire&quot;</span><span class="p">,</span> <span class="s2">&quot;Eriador&quot;</span><span class="p">,</span> <span class="s2">&quot;Shire&quot;</span><span class="p">],</span>
<span class="s1">&#39;Date of Birth T.A.&#39;</span><span class="p">:</span> <span class="p">[</span><span class="mi">2968</span><span class="p">,</span> <span class="mi">2890</span><span class="p">,</span> <span class="mi">2931</span><span class="p">,</span> <span class="mi">2980</span><span class="p">]</span>
<span class="p">}</span>
<span class="n">data_pandas</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">data</span><span class="p">)</span>
<span class="n">display</span><span class="p">(</span><span class="n">data_pandas</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_html"><div>
<style scoped>
.dataframe tbody tr th:only-of-type {
vertical-align: middle;
}
.dataframe tbody tr th {
vertical-align: top;
}
.dataframe thead th {
text-align: right;
}
</style>
<table border="1" class="dataframe">
<thead>
<tr style="text-align: right;">
<th></th>
<th>First Name</th>
<th>Last Name</th>
<th>Place of birth</th>
<th>Date of Birth T.A.</th>
</tr>
</thead>
<tbody>
<tr>
<th>0</th>
<td>Frodo</td>
<td>Baggins</td>
<td>Shire</td>
<td>2968</td>
</tr>
<tr>
<th>1</th>
<td>Bilbo</td>
<td>Baggins</td>
<td>Shire</td>
<td>2890</td>
</tr>
<tr>
<th>2</th>
<td>Aragorn II</td>
<td>Elessar</td>
<td>Eriador</td>
<td>2931</td>
</tr>
<tr>
<th>3</th>
<td>Samwise</td>
<td>Gamgee</td>
<td>Shire</td>
<td>2980</td>
</tr>
</tbody>
</table>
</div></div></div>
</div>
<p>In the above we have imported <strong>pandas</strong> with the shorthand <strong>pd</strong>, the latter has become the standard way we import <strong>pandas</strong>. We make then a list of various variables
and reorganize the aboves lists into a <strong>DataFrame</strong> and then print out a neat table with specific column labels as <em>Name</em>, <em>place of birth</em> and <em>date of birth</em>.
Displaying these results, we see that the indices are given by the default numbers from zero to three.
<strong>pandas</strong> is extremely flexible and we can easily change the above indices by defining a new type of indexing as</p>
<div class="cell docutils container">
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">data_pandas</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">data</span><span class="p">,</span><span class="n">index</span><span class="o">=</span><span class="p">[</span><span class="s1">&#39;Frodo&#39;</span><span class="p">,</span><span class="s1">&#39;Bilbo&#39;</span><span class="p">,</span><span class="s1">&#39;Aragorn&#39;</span><span class="p">,</span><span class="s1">&#39;Sam&#39;</span><span class="p">])</span>
<span class="n">display</span><span class="p">(</span><span class="n">data_pandas</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_html"><div>
<style scoped>
.dataframe tbody tr th:only-of-type {
vertical-align: middle;
}
.dataframe tbody tr th {
vertical-align: top;
}
.dataframe thead th {
text-align: right;
}
</style>
<table border="1" class="dataframe">
<thead>
<tr style="text-align: right;">
<th></th>
<th>First Name</th>
<th>Last Name</th>
<th>Place of birth</th>
<th>Date of Birth T.A.</th>
</tr>
</thead>
<tbody>
<tr>
<th>Frodo</th>
<td>Frodo</td>
<td>Baggins</td>
<td>Shire</td>
<td>2968</td>
</tr>
<tr>
<th>Bilbo</th>
<td>Bilbo</td>
<td>Baggins</td>
<td>Shire</td>
<td>2890</td>
</tr>
<tr>
<th>Aragorn</th>
<td>Aragorn II</td>
<td>Elessar</td>
<td>Eriador</td>
<td>2931</td>
</tr>
<tr>
<th>Sam</th>
<td>Samwise</td>
<td>Gamgee</td>
<td>Shire</td>
<td>2980</td>
</tr>
</tbody>
</table>
</div></div></div>
</div>
<p>Thereafter we display the content of the row which begins with the index <strong>Aragorn</strong></p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">display</span><span class="p">(</span><span class="n">data_pandas</span><span class="o">.</span><span class="n">loc</span><span class="p">[</span><span class="s1">&#39;Aragorn&#39;</span><span class="p">])</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>First Name Aragorn II
Last Name Elessar
Place of birth Eriador
Date of Birth T.A. 2931
Name: Aragorn, dtype: object
</pre></div>
</div>
</div>
</div>
<p>We can easily append data to this, for example</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">new_hobbit</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;First Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Peregrin&quot;</span><span class="p">],</span>
<span class="s1">&#39;Last Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Took&quot;</span><span class="p">],</span>
<span class="s1">&#39;Place of birth&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Shire&quot;</span><span class="p">],</span>
<span class="s1">&#39;Date of Birth T.A.&#39;</span><span class="p">:</span> <span class="p">[</span><span class="mi">2990</span><span class="p">]</span>
<span class="p">}</span>
<span class="n">data_pandas</span><span class="o">=</span><span class="n">data_pandas</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">new_hobbit</span><span class="p">,</span> <span class="n">index</span><span class="o">=</span><span class="p">[</span><span class="s1">&#39;Pippin&#39;</span><span class="p">]))</span>
<span class="n">display</span><span class="p">(</span><span class="n">data_pandas</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output traceback highlight-ipythontb notranslate"><div class="highlight"><pre><span></span><span class="gt">---------------------------------------------------------------------------</span>
<span class="ne">AttributeError</span><span class="g g-Whitespace"> </span>Traceback (most recent call last)
<span class="nn">/var/folders/td/3yk470mj5p931p9dtkk0y6jw0000gn/T/ipykernel_22556/1326197715.py</span> in <span class="ni">?</span><span class="nt">()</span>
<span class="ne">----&gt; </span><span class="mi">6</span> <span class="n">new_hobbit</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;First Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Peregrin&quot;</span><span class="p">],</span>
<span class="g g-Whitespace"> </span><span class="mi">7</span> <span class="s1">&#39;Last Name&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Took&quot;</span><span class="p">],</span>
<span class="g g-Whitespace"> </span><span class="mi">8</span> <span class="s1">&#39;Place of birth&#39;</span><span class="p">:</span> <span class="p">[</span><span class="s2">&quot;Shire&quot;</span><span class="p">],</span>
<span class="g g-Whitespace"> </span><span class="mi">9</span> <span class="s1">&#39;Date of Birth T.A.&#39;</span><span class="p">:</span> <span class="p">[</span><span class="mi">2990</span><span class="p">]</span>
<span class="nn">~/miniforge3/envs/myenv/lib/python3.9/site-packages/pandas/core/generic.py</span> in <span class="ni">?</span><span class="nt">(self, name)</span>
<span class="g g-Whitespace"> </span><span class="mi">6200</span> <span class="ow">and</span> <span class="n">name</span> <span class="ow">not</span> <span class="ow">in</span> <span class="bp">self</span><span class="o">.</span><span class="n">_accessors</span>
<span class="g g-Whitespace"> </span><span class="mi">6201</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">_info_axis</span><span class="o">.</span><span class="n">_can_hold_identifiers_and_holds_name</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="g g-Whitespace"> </span><span class="mi">6202</span> <span class="p">):</span>
<span class="g g-Whitespace"> </span><span class="mi">6203</span> <span class="k">return</span> <span class="bp">self</span><span class="p">[</span><span class="n">name</span><span class="p">]</span>
<span class="ne">-&gt; </span><span class="mi">6204</span> <span class="k">return</span> <span class="nb">object</span><span class="o">.</span><span class="fm">__getattribute__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
<span class="ne">AttributeError</span>: &#39;DataFrame&#39; object has no attribute &#39;append&#39;
</pre></div>
</div>
</div>
</div>
<p>Here are other examples where we use the <strong>DataFrame</strong> functionality to handle arrays, now with more interesting features for us, namely numbers. We set up a matrix
of dimensionality <span class="math notranslate nohighlight">\(10\times 5\)</span> and compute the mean value and standard deviation of each column. Similarly, we can perform mathematial operations like squaring the matrix elements and many other operations.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">from</span> <span class="nn">IPython.display</span> <span class="kn">import</span> <span class="n">display</span>
<span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">seed</span><span class="p">(</span><span class="mi">100</span><span class="p">)</span>
<span class="c1"># setting up a 10 x 5 matrix</span>
<span class="n">rows</span> <span class="o">=</span> <span class="mi">10</span>
<span class="n">cols</span> <span class="o">=</span> <span class="mi">5</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="n">rows</span><span class="p">,</span><span class="n">cols</span><span class="p">)</span>
<span class="n">df</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">a</span><span class="p">)</span>
<span class="n">display</span><span class="p">(</span><span class="n">df</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df</span><span class="o">.</span><span class="n">mean</span><span class="p">())</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df</span><span class="o">.</span><span class="n">std</span><span class="p">())</span>
<span class="n">display</span><span class="p">(</span><span class="n">df</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>Thereafter we can select specific columns only and plot final results</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">df</span><span class="o">.</span><span class="n">columns</span> <span class="o">=</span> <span class="p">[</span><span class="s1">&#39;First&#39;</span><span class="p">,</span> <span class="s1">&#39;Second&#39;</span><span class="p">,</span> <span class="s1">&#39;Third&#39;</span><span class="p">,</span> <span class="s1">&#39;Fourth&#39;</span><span class="p">,</span> <span class="s1">&#39;Fifth&#39;</span><span class="p">]</span>
<span class="n">df</span><span class="o">.</span><span class="n">index</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
<span class="n">display</span><span class="p">(</span><span class="n">df</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df</span><span class="p">[</span><span class="s1">&#39;Second&#39;</span><span class="p">]</span><span class="o">.</span><span class="n">mean</span><span class="p">()</span> <span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df</span><span class="o">.</span><span class="n">info</span><span class="p">())</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df</span><span class="o">.</span><span class="n">describe</span><span class="p">())</span>
<span class="kn">from</span> <span class="nn">pylab</span> <span class="kn">import</span> <span class="n">plt</span><span class="p">,</span> <span class="n">mpl</span>
<span class="n">plt</span><span class="o">.</span><span class="n">style</span><span class="o">.</span><span class="n">use</span><span class="p">(</span><span class="s1">&#39;seaborn&#39;</span><span class="p">)</span>
<span class="n">mpl</span><span class="o">.</span><span class="n">rcParams</span><span class="p">[</span><span class="s1">&#39;font.family&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="s1">&#39;serif&#39;</span>
<span class="n">df</span><span class="o">.</span><span class="n">cumsum</span><span class="p">()</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">lw</span><span class="o">=</span><span class="mf">2.0</span><span class="p">,</span> <span class="n">figsize</span><span class="o">=</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="mi">6</span><span class="p">))</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
<span class="n">df</span><span class="o">.</span><span class="n">plot</span><span class="o">.</span><span class="n">bar</span><span class="p">(</span><span class="n">figsize</span><span class="o">=</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="mi">6</span><span class="p">),</span> <span class="n">rot</span><span class="o">=</span><span class="mi">15</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<p>We can produce a <span class="math notranslate nohighlight">\(4\times 4\)</span> matrix</p>
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<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">b</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">16</span><span class="p">)</span><span class="o">.</span><span class="n">reshape</span><span class="p">((</span><span class="mi">4</span><span class="p">,</span><span class="mi">4</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="n">df1</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">b</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">df1</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>and many other operations.</p>
<p>The <strong>Series</strong> class is another important class included in
<strong>pandas</strong>. You can view it as a specialization of <strong>DataFrame</strong> but where
we have just a single column of data. It shares many of the same features as <strong>DataFrame</strong>. As with <strong>DataFrame</strong>,
most operations are vectorized, achieving thereby a high performance when dealing with computations of arrays, in particular labeled arrays.
As we will see below it leads also to a very concice code close to the mathematical operations we may be interested in.
For multidimensional arrays, we recommend strongly <a class="reference external" href="http://xarray.pydata.org/en/stable/">xarray</a>. <strong>xarray</strong> has much of the same flexibility as <strong>pandas</strong>, but allows for the extension to higher dimensions than two. We will see examples later of the usage of both <strong>pandas</strong> and <strong>xarray</strong>.</p>
</div>
<div class="section" id="pandas-ai">
<h2>Pandas AI<a class="headerlink" href="#pandas-ai" title="Permalink to this headline"></a></h2>
<p>Try out <a class="reference external" href="https://pandas-ai.com/">Pandas AI</a></p>
<div class="section" id="simple-linear-regression-model-using-scikit-learn">
<h3>Simple linear regression model using <strong>scikit-learn</strong><a class="headerlink" href="#simple-linear-regression-model-using-scikit-learn" title="Permalink to this headline"></a></h3>
<p>We start with perhaps our simplest possible example, using <strong>Scikit-Learn</strong> to perform linear regression analysis on a data set produced by us.</p>
<p>What follows is a simple Python code where we have defined a function
<span class="math notranslate nohighlight">\(y\)</span> in terms of the variable <span class="math notranslate nohighlight">\(x\)</span>. Both are defined as vectors with <span class="math notranslate nohighlight">\(100\)</span> entries.
The numbers in the vector <span class="math notranslate nohighlight">\(\boldsymbol{x}\)</span> are given
by random numbers generated with a uniform distribution with entries
<span class="math notranslate nohighlight">\(x_i \in [0,1]\)</span> (more about probability distribution functions
later). These values are then used to define a function <span class="math notranslate nohighlight">\(y(x)\)</span>
(tabulated again as a vector) with a linear dependence on <span class="math notranslate nohighlight">\(x\)</span> plus a
random noise added via the normal distribution.</p>
<p>The Numpy functions are imported used the <strong>import numpy as np</strong>
statement and the random number generator for the uniform distribution
is called using the function <strong>np.random.rand()</strong>, where we specificy
that we want <span class="math notranslate nohighlight">\(100\)</span> random variables. Using Numpy we define
automatically an array with the specified number of elements, <span class="math notranslate nohighlight">\(100\)</span> in
our case. With the Numpy function <strong>randn()</strong> we can compute random
numbers with the normal distribution (mean value <span class="math notranslate nohighlight">\(\mu\)</span> equal to zero and
variance <span class="math notranslate nohighlight">\(\sigma^2\)</span> set to one) and produce the values of <span class="math notranslate nohighlight">\(y\)</span> assuming a linear
dependence as function of <span class="math notranslate nohighlight">\(x\)</span></p>
<div class="math notranslate nohighlight">
\[
y = 2x+N(0,1),
\]</div>
<p>where <span class="math notranslate nohighlight">\(N(0,1)\)</span> represents random numbers generated by the normal
distribution. From <strong>Scikit-Learn</strong> we import then the
<strong>LinearRegression</strong> functionality and make a prediction <span class="math notranslate nohighlight">\(\tilde{y} =
\alpha + \beta x\)</span> using the function <strong>fit(x,y)</strong>. We call the set of
data <span class="math notranslate nohighlight">\((\boldsymbol{x},\boldsymbol{y})\)</span> for our training data. The Python package
<strong>scikit-learn</strong> has also a functionality which extracts the above
fitting parameters <span class="math notranslate nohighlight">\(\alpha\)</span> and <span class="math notranslate nohighlight">\(\beta\)</span> (see below). Later we will
distinguish between training data and test data.</p>
<p>For plotting we use the Python package
<a class="reference external" href="https://matplotlib.org/">matplotlib</a> which produces publication
quality figures. Feel free to explore the extensive
<a class="reference external" href="https://matplotlib.org/gallery/index.html">gallery</a> of examples. In
this example we plot our original values of <span class="math notranslate nohighlight">\(x\)</span> and <span class="math notranslate nohighlight">\(y\)</span> as well as the
prediction <strong>ypredict</strong> (<span class="math notranslate nohighlight">\(\tilde{y}\)</span>), which attempts at fitting our
data with a straight line.</p>
<p>The Python code follows here.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Importing various packages</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">sklearn.linear_model</span> <span class="kn">import</span> <span class="n">LinearRegression</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">rand</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">2</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">linreg</span> <span class="o">=</span> <span class="n">LinearRegression</span><span class="p">()</span>
<span class="n">linreg</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">)</span>
<span class="n">xnew</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">array</span><span class="p">([[</span><span class="mi">0</span><span class="p">],[</span><span class="mi">1</span><span class="p">]])</span>
<span class="n">ypredict</span> <span class="o">=</span> <span class="n">linreg</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">xnew</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">xnew</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">,</span> <span class="s2">&quot;r-&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="p">,</span><span class="s1">&#39;ro&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">axis</span><span class="p">([</span><span class="mi">0</span><span class="p">,</span><span class="mf">1.0</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span> <span class="mf">5.0</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$x$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$y$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;Simple Linear Regression&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<p>This example serves several aims. It allows us to demonstrate several
aspects of data analysis and later machine learning algorithms. The
immediate visualization shows that our linear fit is not
impressive. It goes through the data points, but there are many
outliers which are not reproduced by our linear regression. We could
now play around with this small program and change for example the
factor in front of <span class="math notranslate nohighlight">\(x\)</span> and the normal distribution. Try to change the
function <span class="math notranslate nohighlight">\(y\)</span> to</p>
<div class="math notranslate nohighlight">
\[
y = 10x+0.01 \times N(0,1),
\]</div>
<p>where <span class="math notranslate nohighlight">\(x\)</span> is defined as before. Does the fit look better? Indeed, by
reducing the role of the noise given by the normal distribution we see immediately that
our linear prediction seemingly reproduces better the training
set. However, this testing ‘by the eye’ is obviouly not satisfactory in the
long run. Here we have only defined the training data and our model, and
have not discussed a more rigorous approach to the <strong>cost</strong> function.</p>
<p>We need more rigorous criteria in defining whether we have succeeded or
not in modeling our training data. You will be surprised to see that
many scientists seldomly venture beyond this ‘by the eye’ approach. A
standard approach for the <em>cost</em> function is the so-called <span class="math notranslate nohighlight">\(\chi^2\)</span>
function (a variant of the mean-squared error (MSE))</p>
<div class="math notranslate nohighlight">
\[
\chi^2 = \frac{1}{n}
\sum_{i=0}^{n-1}\frac{(y_i-\tilde{y}_i)^2}{\sigma_i^2},
\]</div>
<p>where <span class="math notranslate nohighlight">\(\sigma_i^2\)</span> is the variance (to be defined later) of the entry
<span class="math notranslate nohighlight">\(y_i\)</span>. We may not know the explicit value of <span class="math notranslate nohighlight">\(\sigma_i^2\)</span>, it serves
however the aim of scaling the equations and make the cost function
dimensionless.</p>
<p>Minimizing the cost function is a central aspect of
our discussions to come. Finding its minima as function of the model
parameters (<span class="math notranslate nohighlight">\(\alpha\)</span> and <span class="math notranslate nohighlight">\(\beta\)</span> in our case) will be a recurring
theme in these series of lectures. Essentially all machine learning
algorithms we will discuss center around the minimization of the
chosen cost function. This depends in turn on our specific
model for describing the data, a typical situation in supervised
learning. Automatizing the search for the minima of the cost function is a
central ingredient in all algorithms. Typical methods which are
employed are various variants of <strong>gradient</strong> methods. These will be
discussed in more detail later. Again, you’ll be surprised to hear that
many practitioners minimize the above function ‘’by the eye’, popularly dubbed as
‘chi by the eye’. That is, change a parameter and see (visually and numerically) that
the <span class="math notranslate nohighlight">\(\chi^2\)</span> function becomes smaller.</p>
<p>There are many ways to define the cost function. A simpler approach is to look at the relative difference between the training data and the predicted data, that is we define
the relative error (why would we prefer the MSE instead of the relative error?) as</p>
<div class="math notranslate nohighlight">
\[
\epsilon_{\mathrm{relative}}= \frac{\vert \boldsymbol{y} -\boldsymbol{\tilde{y}}\vert}{\vert \boldsymbol{y}\vert}.
\]</div>
<p>The squared cost function results in an arithmetic mean-unbiased
estimator, and the absolute-value cost function results in a
median-unbiased estimator (in the one-dimensional case, and a
geometric median-unbiased estimator for the multi-dimensional
case). The squared cost function has the disadvantage that it has the tendency
to be dominated by outliers.</p>
<p>We can modify easily the above Python code and plot the relative error instead</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">sklearn.linear_model</span> <span class="kn">import</span> <span class="n">LinearRegression</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">rand</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="mi">5</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="mf">0.01</span><span class="o">*</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">linreg</span> <span class="o">=</span> <span class="n">LinearRegression</span><span class="p">()</span>
<span class="n">linreg</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">)</span>
<span class="n">ypredict</span> <span class="o">=</span> <span class="n">linreg</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">np</span><span class="o">.</span><span class="n">abs</span><span class="p">(</span><span class="n">ypredict</span><span class="o">-</span><span class="n">y</span><span class="p">)</span><span class="o">/</span><span class="nb">abs</span><span class="p">(</span><span class="n">y</span><span class="p">),</span> <span class="s2">&quot;ro&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">axis</span><span class="p">([</span><span class="mi">0</span><span class="p">,</span><span class="mf">1.0</span><span class="p">,</span><span class="mf">0.0</span><span class="p">,</span> <span class="mf">0.5</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$x$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$\epsilon_{\mathrm</span><span class="si">{relative}</span><span class="s1">}$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;Relative error&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<p>Depending on the parameter in front of the normal distribution, we may
have a small or larger relative error. Try to play around with
different training data sets and study (graphically) the value of the
relative error.</p>
<p>As mentioned above, <strong>Scikit-Learn</strong> has an impressive functionality.
We can for example extract the values of <span class="math notranslate nohighlight">\(\alpha\)</span> and <span class="math notranslate nohighlight">\(\beta\)</span> and
their error estimates, or the variance and standard deviation and many
other properties from the statistical data analysis.</p>
<p>Here we show an
example of the functionality of <strong>Scikit-Learn</strong>.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">sklearn.linear_model</span> <span class="kn">import</span> <span class="n">LinearRegression</span>
<span class="kn">from</span> <span class="nn">sklearn.metrics</span> <span class="kn">import</span> <span class="n">mean_squared_error</span><span class="p">,</span> <span class="n">r2_score</span><span class="p">,</span> <span class="n">mean_squared_log_error</span><span class="p">,</span> <span class="n">mean_absolute_error</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">rand</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="mf">2.0</span><span class="o">+</span> <span class="mi">5</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="mf">0.5</span><span class="o">*</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">linreg</span> <span class="o">=</span> <span class="n">LinearRegression</span><span class="p">()</span>
<span class="n">linreg</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">)</span>
<span class="n">ypredict</span> <span class="o">=</span> <span class="n">linreg</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;The intercept alpha: </span><span class="se">\n</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">linreg</span><span class="o">.</span><span class="n">intercept_</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Coefficient beta : </span><span class="se">\n</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">linreg</span><span class="o">.</span><span class="n">coef_</span><span class="p">)</span>
<span class="c1"># The mean squared error </span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Mean squared error: </span><span class="si">%.2f</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">y</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">))</span>
<span class="c1"># Explained variance score: 1 is perfect prediction </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Variance score: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">r2_score</span><span class="p">(</span><span class="n">y</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">))</span>
<span class="c1"># Mean squared log error </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Mean squared log error: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">mean_squared_log_error</span><span class="p">(</span><span class="n">y</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">)</span> <span class="p">)</span>
<span class="c1"># Mean absolute error </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Mean absolute error: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">mean_absolute_error</span><span class="p">(</span><span class="n">y</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">))</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">ypredict</span><span class="p">,</span> <span class="s2">&quot;r-&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span> <span class="p">,</span><span class="s1">&#39;ro&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">axis</span><span class="p">([</span><span class="mf">0.0</span><span class="p">,</span><span class="mf">1.0</span><span class="p">,</span><span class="mf">1.5</span><span class="p">,</span> <span class="mf">7.0</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$x$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$y$&#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">title</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;Linear Regression fit &#39;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<p>The function <strong>coef</strong> gives us the parameter <span class="math notranslate nohighlight">\(\beta\)</span> of our fit while <strong>intercept</strong> yields
<span class="math notranslate nohighlight">\(\alpha\)</span>. Depending on the constant in front of the normal distribution, we get values near or far from <span class="math notranslate nohighlight">\(\alpha =2\)</span> and <span class="math notranslate nohighlight">\(\beta =5\)</span>. Try to play around with different parameters in front of the normal distribution. The function <strong>meansquarederror</strong> gives us the mean square error, a risk metric corresponding to the expected value of the squared (quadratic) error or loss defined as</p>
<div class="math notranslate nohighlight">
\[
MSE(\boldsymbol{y},\boldsymbol{\tilde{y}}) = \frac{1}{n}
\sum_{i=0}^{n-1}(y_i-\tilde{y}_i)^2,
\]</div>
<p>The smaller the value, the better the fit. Ideally we would like to
have an MSE equal zero. The attentive reader has probably recognized
this function as being similar to the <span class="math notranslate nohighlight">\(\chi^2\)</span> function defined above.</p>
<p>The <strong>r2score</strong> function computes <span class="math notranslate nohighlight">\(R^2\)</span>, the coefficient of
determination. It provides a measure of how well future samples are
likely to be predicted by the model. Best possible score is 1.0 and it
can be negative (because the model can be arbitrarily worse). A
constant model that always predicts the expected value of <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span>,
disregarding the input features, would get a <span class="math notranslate nohighlight">\(R^2\)</span> score of <span class="math notranslate nohighlight">\(0.0\)</span>.</p>
<p>If <span class="math notranslate nohighlight">\(\tilde{\boldsymbol{y}}_i\)</span> is the predicted value of the <span class="math notranslate nohighlight">\(i-th\)</span> sample and <span class="math notranslate nohighlight">\(y_i\)</span> is the corresponding true value, then the score <span class="math notranslate nohighlight">\(R^2\)</span> is defined as</p>
<div class="math notranslate nohighlight">
\[
R^2(\boldsymbol{y}, \tilde{\boldsymbol{y}}) = 1 - \frac{\sum_{i=0}^{n - 1} (y_i - \tilde{y}_i)^2}{\sum_{i=0}^{n - 1} (y_i - \bar{y})^2},
\]</div>
<p>where we have defined the mean value of <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> as</p>
<div class="math notranslate nohighlight">
\[
\bar{y} = \frac{1}{n} \sum_{i=0}^{n - 1} y_i.
\]</div>
<p>Another quantity taht we will meet again in our discussions of regression analysis is
the mean absolute error (MAE), a risk metric corresponding to the expected value of the absolute error loss or what we call the <span class="math notranslate nohighlight">\(l1\)</span>-norm loss. In our discussion above we presented the relative error.
The MAE is defined as follows</p>
<div class="math notranslate nohighlight">
\[
\text{MAE}(\boldsymbol{y}, \boldsymbol{\tilde{y}}) = \frac{1}{n} \sum_{i=0}^{n-1} \left| y_i - \tilde{y}_i \right|.
\]</div>
<p>We present the
squared logarithmic (quadratic) error</p>
<div class="math notranslate nohighlight">
\[
\text{MSLE}(\boldsymbol{y}, \boldsymbol{\tilde{y}}) = \frac{1}{n} \sum_{i=0}^{n - 1} (\log_e (1 + y_i) - \log_e (1 + \tilde{y}_i) )^2,
\]</div>
<p>where <span class="math notranslate nohighlight">\(\log_e (x)\)</span> stands for the natural logarithm of <span class="math notranslate nohighlight">\(x\)</span>. This error
estimate is best to use when targets having exponential growth, such
as population counts, average sales of a commodity over a span of
years etc.</p>
<p>Finally, another cost function is the Huber cost function used in robust regression.</p>
<p>The rationale behind this possible cost function is its reduced
sensitivity to outliers in the data set. In our discussions on
dimensionality reduction and normalization of data we will meet other
ways of dealing with outliers.</p>
<p>The Huber cost function is defined as</p>
<div class="math notranslate nohighlight">
\[\begin{split}
H_{\delta}(\boldsymbol{a})=\left\{\begin{array}{cc}\frac{1}{2} \boldsymbol{a}^{2}&amp; \text{for }|\boldsymbol{a}|\leq \delta\\ \delta (|\boldsymbol{a}|-\frac{1}{2}\delta ),&amp;\text{otherwise}.\end{array}\right.
\end{split}\]</div>
<p>Here <span class="math notranslate nohighlight">\(\boldsymbol{a}=\boldsymbol{y} - \boldsymbol{\tilde{y}}\)</span>.</p>
<p>We will discuss in more detail these and other functions in the
various lectures and lab sessions.</p>
</div>
<div class="section" id="to-our-real-data-nuclear-binding-energies-brief-reminder-on-masses-and-binding-energies">
<h3>To our real data: nuclear binding energies. Brief reminder on masses and binding energies<a class="headerlink" href="#to-our-real-data-nuclear-binding-energies-brief-reminder-on-masses-and-binding-energies" title="Permalink to this headline"></a></h3>
<p>Let us now dive into nuclear physics and remind ourselves briefly about some basic features about binding
energies. A basic quantity which can be measured for the ground
states of nuclei is the atomic mass <span class="math notranslate nohighlight">\(M(N, Z)\)</span> of the neutral atom with
atomic mass number <span class="math notranslate nohighlight">\(A\)</span> and charge <span class="math notranslate nohighlight">\(Z\)</span>. The number of neutrons is <span class="math notranslate nohighlight">\(N\)</span>. There are indeed several sophisticated experiments worldwide which allow us to measure this quantity to high precision (parts per million even).</p>
<p>Atomic masses are usually tabulated in terms of the mass excess defined by</p>
<div class="math notranslate nohighlight">
\[
\Delta M(N, Z) = M(N, Z) - uA,
\]</div>
<p>where <span class="math notranslate nohighlight">\(u\)</span> is the Atomic Mass Unit</p>
<div class="math notranslate nohighlight">
\[
u = M(^{12}\mathrm{C})/12 = 931.4940954(57) \hspace{0.1cm} \mathrm{MeV}/c^2.
\]</div>
<p>The nucleon masses are</p>
<div class="math notranslate nohighlight">
\[
m_p = 1.00727646693(9)u,
\]</div>
<p>and</p>
<div class="math notranslate nohighlight">
\[
m_n = 939.56536(8)\hspace{0.1cm} \mathrm{MeV}/c^2 = 1.0086649156(6)u.
\]</div>
<p>In the <a class="reference external" href="http://nuclearmasses.org/resources_folder/Wang_2017_Chinese_Phys_C_41_030003.pdf">2016 mass evaluation of by W.J.Huang, G.Audi, M.Wang, F.G.Kondev, S.Naimi and X.Xu</a>
there are data on masses and decays of 3437 nuclei.</p>
<p>The nuclear binding energy is defined as the energy required to break
up a given nucleus into its constituent parts of <span class="math notranslate nohighlight">\(N\)</span> neutrons and <span class="math notranslate nohighlight">\(Z\)</span>
protons. In terms of the atomic masses <span class="math notranslate nohighlight">\(M(N, Z)\)</span> the binding energy is
defined by</p>
<div class="math notranslate nohighlight">
\[
BE(N, Z) = ZM_H c^2 + Nm_n c^2 - M(N, Z)c^2 ,
\]</div>
<p>where <span class="math notranslate nohighlight">\(M_H\)</span> is the mass of the hydrogen atom and <span class="math notranslate nohighlight">\(m_n\)</span> is the mass of the neutron.
In terms of the mass excess the binding energy is given by</p>
<div class="math notranslate nohighlight">
\[
BE(N, Z) = Z\Delta_H c^2 + N\Delta_n c^2 -\Delta(N, Z)c^2 ,
\]</div>
<p>where <span class="math notranslate nohighlight">\(\Delta_H c^2 = 7.2890\)</span> MeV and <span class="math notranslate nohighlight">\(\Delta_n c^2 = 8.0713\)</span> MeV.</p>
<p>A popular and physically intuitive model which can be used to parametrize
the experimental binding energies as function of <span class="math notranslate nohighlight">\(A\)</span>, is the so-called
<strong>liquid drop model</strong>. The ansatz is based on the following expression</p>
<div class="math notranslate nohighlight">
\[
BE(N,Z) = a_1A-a_2A^{2/3}-a_3\frac{Z^2}{A^{1/3}}-a_4\frac{(N-Z)^2}{A},
\]</div>
<p>where <span class="math notranslate nohighlight">\(A\)</span> stands for the number of nucleons and the <span class="math notranslate nohighlight">\(a_i\)</span>s are parameters which are determined by a fit
to the experimental data.</p>
<p>To arrive at the above expression we have assumed that we can make the following assumptions:</p>
<ul class="simple">
<li><p>There is a volume term <span class="math notranslate nohighlight">\(a_1A\)</span> proportional with the number of nucleons (the energy is also an extensive quantity). When an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. This contribution is proportional to the volume.</p></li>
<li><p>There is a surface energy term <span class="math notranslate nohighlight">\(a_2A^{2/3}\)</span>. The assumption here is that a nucleon at the surface of a nucleus interacts with fewer other nucleons than one in the interior of the nucleus and hence its binding energy is less. This surface energy term takes that into account and is therefore negative and is proportional to the surface area.</p></li>
<li><p>There is a Coulomb energy term <span class="math notranslate nohighlight">\(a_3\frac{Z^2}{A^{1/3}}\)</span>. The electric repulsion between each pair of protons in a nucleus yields less binding.</p></li>
<li><p>There is an asymmetry term <span class="math notranslate nohighlight">\(a_4\frac{(N-Z)^2}{A}\)</span>. This term is associated with the Pauli exclusion principle and reflects the fact that the proton-neutron interaction is more attractive on the average than the neutron-neutron and proton-proton interactions.</p></li>
</ul>
<p>We could also add a so-called pairing term, which is a correction term that
arises from the tendency of proton pairs and neutron pairs to
occur. An even number of particles is more stable than an odd number.</p>
</div>
<div class="section" id="organizing-our-data">
<h3>Organizing our data<a class="headerlink" href="#organizing-our-data" title="Permalink to this headline"></a></h3>
<p>Let us start with reading and organizing our data.
We start with the compilation of masses and binding energies from 2016.
After having downloaded this file to our own computer, we are now ready to read the file and start structuring our data.</p>
<p>We start with preparing folders for storing our calculations and the data file over masses and binding energies. We import also various modules that we will find useful in order to present various Machine Learning methods. Here we focus mainly on the functionality of <strong>scikit-learn</strong>.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Common imports</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">import</span> <span class="nn">sklearn.linear_model</span> <span class="k">as</span> <span class="nn">skl</span>
<span class="kn">from</span> <span class="nn">sklearn.model_selection</span> <span class="kn">import</span> <span class="n">train_test_split</span>
<span class="kn">from</span> <span class="nn">sklearn.metrics</span> <span class="kn">import</span> <span class="n">mean_squared_error</span><span class="p">,</span> <span class="n">r2_score</span><span class="p">,</span> <span class="n">mean_absolute_error</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="c1"># Where to save the figures and data files</span>
<span class="n">PROJECT_ROOT_DIR</span> <span class="o">=</span> <span class="s2">&quot;Results&quot;</span>
<span class="n">FIGURE_ID</span> <span class="o">=</span> <span class="s2">&quot;Results/FigureFiles&quot;</span>
<span class="n">DATA_ID</span> <span class="o">=</span> <span class="s2">&quot;DataFiles/&quot;</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">mkdir</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">,</span> <span class="n">fig_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">data_path</span><span class="p">(</span><span class="n">dat_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">,</span> <span class="n">dat_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">save_fig</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="n">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;.png&quot;</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">&#39;png&#39;</span><span class="p">)</span>
<span class="n">infile</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">data_path</span><span class="p">(</span><span class="s2">&quot;MassEval2016.dat&quot;</span><span class="p">),</span><span class="s1">&#39;r&#39;</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>Before we proceed, we define also a function for making our plots. You can obviously avoid this and simply set up various <strong>matplotlib</strong> commands every time you need them. You may however find it convenient to collect all such commands in one function and simply call this function.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">pylab</span> <span class="kn">import</span> <span class="n">plt</span><span class="p">,</span> <span class="n">mpl</span>
<span class="n">plt</span><span class="o">.</span><span class="n">style</span><span class="o">.</span><span class="n">use</span><span class="p">(</span><span class="s1">&#39;seaborn&#39;</span><span class="p">)</span>
<span class="n">mpl</span><span class="o">.</span><span class="n">rcParams</span><span class="p">[</span><span class="s1">&#39;font.family&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="s1">&#39;serif&#39;</span>
<span class="k">def</span> <span class="nf">MakePlot</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">,</span> <span class="n">styles</span><span class="p">,</span> <span class="n">labels</span><span class="p">,</span> <span class="n">axlabels</span><span class="p">):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">figure</span><span class="p">(</span><span class="n">figsize</span><span class="o">=</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="mi">6</span><span class="p">))</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="nb">len</span><span class="p">(</span><span class="n">x</span><span class="p">)):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">x</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">y</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">styles</span><span class="p">[</span><span class="n">i</span><span class="p">],</span> <span class="n">label</span> <span class="o">=</span> <span class="n">labels</span><span class="p">[</span><span class="n">i</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">xlabel</span><span class="p">(</span><span class="n">axlabels</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">ylabel</span><span class="p">(</span><span class="n">axlabels</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
<span class="n">plt</span><span class="o">.</span><span class="n">legend</span><span class="p">(</span><span class="n">loc</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>Our next step is to read the data on experimental binding energies and
reorganize them as functions of the mass number <span class="math notranslate nohighlight">\(A\)</span>, the number of
protons <span class="math notranslate nohighlight">\(Z\)</span> and neutrons <span class="math notranslate nohighlight">\(N\)</span> using <strong>pandas</strong>. Before we do this it is
always useful (unless you have a binary file or other types of compressed
data) to actually open the file and simply take a look at it!</p>
<p>In particular, the program that outputs the final nuclear masses is written in Fortran with a specific format. It means that we need to figure out the format and which columns contain the data we are interested in. Pandas comes with a function that reads formatted output. After having admired the file, we are now ready to start massaging it with <strong>pandas</strong>. The file begins with some basic format information.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="sd">&quot;&quot;&quot; </span>
<span class="sd">This is taken from the data file of the mass 2016 evaluation. </span>
<span class="sd">All files are 3436 lines long with 124 character per line. </span>
<span class="sd"> Headers are 39 lines long. </span>
<span class="sd"> col 1 : Fortran character control: 1 = page feed 0 = line feed </span>
<span class="sd"> format : a1,i3,i5,i5,i5,1x,a3,a4,1x,f13.5,f11.5,f11.3,f9.3,1x,a2,f11.3,f9.3,1x,i3,1x,f12.5,f11.5 </span>
<span class="sd"> These formats are reflected in the pandas widths variable below, see the statement </span>
<span class="sd"> widths=(1,3,5,5,5,1,3,4,1,13,11,11,9,1,2,11,9,1,3,1,12,11,1), </span>
<span class="sd"> Pandas has also a variable header, with length 39 in this case. </span>
<span class="sd">&quot;&quot;&quot;</span>
</pre></div>
</div>
</div>
</div>
<p>The data we are interested in are in columns 2, 3, 4 and 11, giving us
the number of neutrons, protons, mass numbers and binding energies,
respectively. We add also for the sake of completeness the element name. The data are in fixed-width formatted lines and we will
covert them into the <strong>pandas</strong> DataFrame structure.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Read the experimental data with Pandas</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">read_fwf</span><span class="p">(</span><span class="n">infile</span><span class="p">,</span> <span class="n">usecols</span><span class="o">=</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">6</span><span class="p">,</span><span class="mi">11</span><span class="p">),</span>
<span class="n">names</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;N&#39;</span><span class="p">,</span> <span class="s1">&#39;Z&#39;</span><span class="p">,</span> <span class="s1">&#39;A&#39;</span><span class="p">,</span> <span class="s1">&#39;Element&#39;</span><span class="p">,</span> <span class="s1">&#39;Ebinding&#39;</span><span class="p">),</span>
<span class="n">widths</span><span class="o">=</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">13</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">9</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">9</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">12</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">1</span><span class="p">),</span>
<span class="n">header</span><span class="o">=</span><span class="mi">39</span><span class="p">,</span>
<span class="n">index_col</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="c1"># Extrapolated values are indicated by &#39;#&#39; in place of the decimal place, so</span>
<span class="c1"># the Ebinding column won&#39;t be numeric. Coerce to float and drop these entries.</span>
<span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">to_numeric</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">],</span> <span class="n">errors</span><span class="o">=</span><span class="s1">&#39;coerce&#39;</span><span class="p">)</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">dropna</span><span class="p">()</span>
<span class="c1"># Convert from keV to MeV.</span>
<span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span> <span class="o">/=</span> <span class="mi">1000</span>
<span class="c1"># Group the DataFrame by nucleon number, A.</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">groupby</span><span class="p">(</span><span class="s1">&#39;A&#39;</span><span class="p">)</span>
<span class="c1"># Find the rows of the grouped DataFrame with the maximum binding energy.</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">apply</span><span class="p">(</span><span class="k">lambda</span> <span class="n">t</span><span class="p">:</span> <span class="n">t</span><span class="p">[</span><span class="n">t</span><span class="o">.</span><span class="n">Ebinding</span><span class="o">==</span><span class="n">t</span><span class="o">.</span><span class="n">Ebinding</span><span class="o">.</span><span class="n">max</span><span class="p">()])</span>
</pre></div>
</div>
</div>
</div>
<p>We have now read in the data, grouped them according to the variables we are interested in.
We see how easy it is to reorganize the data using <strong>pandas</strong>. If we
were to do these operations in C/C++ or Fortran, we would have had to
write various functions/subroutines which perform the above
reorganizations for us. Having reorganized the data, we can now start
to make some simple fits using both the functionalities in <strong>numpy</strong> and
<strong>Scikit-Learn</strong> afterwards.</p>
<p>Now we define five variables which contain
the number of nucleons <span class="math notranslate nohighlight">\(A\)</span>, the number of protons <span class="math notranslate nohighlight">\(Z\)</span> and the number of neutrons <span class="math notranslate nohighlight">\(N\)</span>, the element name and finally the energies themselves.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">A</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">]</span>
<span class="n">Z</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Z&#39;</span><span class="p">]</span>
<span class="n">N</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;N&#39;</span><span class="p">]</span>
<span class="n">Element</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Element&#39;</span><span class="p">]</span>
<span class="n">Energies</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span>
<span class="nb">print</span><span class="p">(</span><span class="n">Masses</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>The next step, and we will define this mathematically later, is to set up the so-called <strong>design matrix</strong>. We will throughout call this matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span>.
It has dimensionality <span class="math notranslate nohighlight">\(p\times n\)</span>, where <span class="math notranslate nohighlight">\(n\)</span> is the number of data points and <span class="math notranslate nohighlight">\(p\)</span> are the so-called predictors. In our case here they are given by the number of polynomials in <span class="math notranslate nohighlight">\(A\)</span> we wish to include in the fit.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Now we set up the design matrix X</span>
<span class="n">X</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">A</span><span class="p">),</span><span class="mi">5</span><span class="p">))</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="mf">2.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="o">-</span><span class="mf">1.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">4</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="o">-</span><span class="mf">1.0</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>With <strong>scikitlearn</strong> we are now ready to use linear regression and fit our data.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">clf</span> <span class="o">=</span> <span class="n">skl</span><span class="o">.</span><span class="n">LinearRegression</span><span class="p">()</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">Energies</span><span class="p">)</span>
<span class="n">fity</span> <span class="o">=</span> <span class="n">clf</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">X</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>Pretty simple!<br />
Now we can print measures of how our fit is doing, the coefficients from the fits and plot the final fit together with our data.</p>
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<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># The mean squared error </span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Mean squared error: </span><span class="si">%.2f</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">fity</span><span class="p">))</span>
<span class="c1"># Explained variance score: 1 is perfect prediction </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Variance score: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">r2_score</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">fity</span><span class="p">))</span>
<span class="c1"># Mean absolute error </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Mean absolute error: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">mean_absolute_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">fity</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">clf</span><span class="o">.</span><span class="n">coef_</span><span class="p">,</span> <span class="n">clf</span><span class="o">.</span><span class="n">intercept_</span><span class="p">)</span>
<span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Eapprox&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">fity</span>
<span class="c1"># Generate a plot comparing the experimental with the fitted values values.</span>
<span class="n">fig</span><span class="p">,</span> <span class="n">ax</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">subplots</span><span class="p">()</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$A = N + Z$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$E_\mathrm</span><span class="si">{bind}</span><span class="s1">\,/\mathrm</span><span class="si">{MeV}</span><span class="s1">$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">],</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Ame2016&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">],</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Eapprox&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">c</span><span class="o">=</span><span class="s1">&#39;m&#39;</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Fit&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<span class="n">save_fig</span><span class="p">(</span><span class="s2">&quot;Masses2016&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="and-what-about-using-neural-networks">
<h3>And what about using neural networks?<a class="headerlink" href="#and-what-about-using-neural-networks" title="Permalink to this headline"></a></h3>
<p>The <strong>seaborn</strong> package allows us to visualize data in an efficient way. Note that we use <strong>scikit-learn</strong>’s multi-layer perceptron (or feed forward neural network)
functionality.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">sklearn.neural_network</span> <span class="kn">import</span> <span class="n">MLPRegressor</span>
<span class="kn">from</span> <span class="nn">sklearn.metrics</span> <span class="kn">import</span> <span class="n">accuracy_score</span>
<span class="kn">import</span> <span class="nn">seaborn</span> <span class="k">as</span> <span class="nn">sns</span>
<span class="n">X_train</span> <span class="o">=</span> <span class="n">X</span>
<span class="n">Y_train</span> <span class="o">=</span> <span class="n">Energies</span>
<span class="n">n_hidden_neurons</span> <span class="o">=</span> <span class="mi">50</span>
<span class="n">epochs</span> <span class="o">=</span> <span class="mi">100</span>
<span class="c1"># store models for later use</span>
<span class="n">eta_vals</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">logspace</span><span class="p">(</span><span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
<span class="n">lmbd_vals</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">logspace</span><span class="p">(</span><span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">4</span><span class="p">)</span>
<span class="c1"># store the models for later use</span>
<span class="n">DNN_scikit</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">eta_vals</span><span class="p">),</span> <span class="nb">len</span><span class="p">(</span><span class="n">lmbd_vals</span><span class="p">)),</span> <span class="n">dtype</span><span class="o">=</span><span class="nb">object</span><span class="p">)</span>
<span class="n">train_accuracy</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">eta_vals</span><span class="p">),</span> <span class="nb">len</span><span class="p">(</span><span class="n">lmbd_vals</span><span class="p">)))</span>
<span class="n">sns</span><span class="o">.</span><span class="n">set</span><span class="p">()</span>
<span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">eta</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">eta_vals</span><span class="p">):</span>
<span class="k">for</span> <span class="n">j</span><span class="p">,</span> <span class="n">lmbd</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">lmbd_vals</span><span class="p">):</span>
<span class="n">dnn</span> <span class="o">=</span> <span class="n">MLPRegressor</span><span class="p">(</span><span class="n">hidden_layer_sizes</span><span class="o">=</span><span class="p">(</span><span class="n">n_hidden_neurons</span><span class="p">),</span> <span class="n">activation</span><span class="o">=</span><span class="s1">&#39;relu&#39;</span><span class="p">,</span> <span class="n">solver</span><span class="o">=</span><span class="s1">&#39;adam&#39;</span><span class="p">,</span>
<span class="n">alpha</span><span class="o">=</span><span class="n">lmbd</span><span class="p">,</span> <span class="n">learning_rate_init</span><span class="o">=</span><span class="n">eta</span><span class="p">,</span> <span class="n">max_iter</span><span class="o">=</span><span class="n">epochs</span><span class="p">)</span>
<span class="n">dnn</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">X_train</span><span class="p">,</span> <span class="n">Y_train</span><span class="p">)</span>
<span class="n">DNN_scikit</span><span class="p">[</span><span class="n">i</span><span class="p">][</span><span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="n">dnn</span>
<span class="n">train_accuracy</span><span class="p">[</span><span class="n">i</span><span class="p">][</span><span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="n">dnn</span><span class="o">.</span><span class="n">score</span><span class="p">(</span><span class="n">X_train</span><span class="p">,</span> <span class="n">Y_train</span><span class="p">)</span>
<span class="n">fity</span> <span class="o">=</span> <span class="n">dnn</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">X_train</span><span class="p">)</span>
<span class="n">MSE</span> <span class="o">=</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">Y_train</span><span class="p">,</span> <span class="n">fity</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Mean squared error: </span><span class="si">%.2f</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">Y_train</span><span class="p">,</span> <span class="n">fity</span><span class="p">))</span>
<span class="n">train_accuracy</span><span class="p">[</span><span class="n">i</span><span class="p">][</span><span class="n">j</span><span class="p">]</span> <span class="o">=</span> <span class="n">MSE</span>
<span class="n">fig</span><span class="p">,</span> <span class="n">ax</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">subplots</span><span class="p">(</span><span class="n">figsize</span> <span class="o">=</span> <span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="mi">10</span><span class="p">))</span>
<span class="n">sns</span><span class="o">.</span><span class="n">heatmap</span><span class="p">(</span><span class="n">train_accuracy</span><span class="p">,</span> <span class="n">annot</span><span class="o">=</span><span class="kc">True</span><span class="p">,</span> <span class="n">ax</span><span class="o">=</span><span class="n">ax</span><span class="p">,</span> <span class="n">cmap</span><span class="o">=</span><span class="s2">&quot;viridis&quot;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_title</span><span class="p">(</span><span class="s2">&quot;Training Accuracy&quot;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="s2">&quot;$\eta$&quot;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="s2">&quot;$\lambda$&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">train_accuracy</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
</div>
</div>
<div class="section" id="a-first-summary">
<h2>A first summary<a class="headerlink" href="#a-first-summary" title="Permalink to this headline"></a></h2>
<p>The aim behind these introductory words was to present to you various
Python libraries and their functionalities, in particular libraries like
<strong>numpy</strong>, <strong>pandas</strong>, <strong>xarray</strong> and <strong>matplotlib</strong> and other that make our life much easier
in handling various data sets and visualizing data.</p>
<p>Furthermore,
<strong>Scikit-Learn</strong> allows us with few lines of code to implement popular
Machine Learning algorithms for supervised learning. Later we will meet <strong>Tensorflow</strong>, a powerful library for deep learning.
Now it is time to dive more into the details of various methods. We will start with linear regression and try to take a deeper look at what it entails.</p>
</div>
<div class="section" id="why-linear-regression-aka-ordinary-least-squares-and-family">
<h2>Why Linear Regression (aka Ordinary Least Squares and family)<a class="headerlink" href="#why-linear-regression-aka-ordinary-least-squares-and-family" title="Permalink to this headline"></a></h2>
<p>Fitting a continuous function with linear parameterization in terms of the parameters <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span>.</p>
<ul class="simple">
<li><p>Method of choice for fitting a continuous function!</p></li>
<li><p>Gives an excellent introduction to central Machine Learning features with <strong>understandable pedagogical</strong> links to other methods like <strong>Neural Networks</strong>, <strong>Support Vector Machines</strong> etc</p></li>
<li><p>Analytical expression for the fitting parameters <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span></p></li>
<li><p>Analytical expressions for statistical propertiers like mean values, variances, confidence intervals and more</p></li>
<li><p>Analytical relation with probabilistic interpretations</p></li>
<li><p>Easy to introduce basic concepts like bias-variance tradeoff, cross-validation, resampling and regularization techniques and many other ML topics</p></li>
<li><p>Easy to code! And links well with classification problems and logistic regression and neural networks</p></li>
<li><p>Allows for <strong>easy</strong> hands-on understanding of gradient descent methods</p></li>
<li><p>and many more features</p></li>
</ul>
<p>For more discussions of Ridge and Lasso regression, <a class="reference external" href="https://arxiv.org/abs/1509.09169">Wessel van Wieringen’s</a> article is highly recommended.
Similarly, <a class="reference external" href="https://arxiv.org/abs/1803.08823">Mehta et al’s article</a> is also recommended.</p>
</div>
<div class="section" id="regression-analysis-overarching-aims">
<h2>Regression analysis, overarching aims<a class="headerlink" href="#regression-analysis-overarching-aims" title="Permalink to this headline"></a></h2>
<p>Regression modeling deals with the description of the sampling distribution of a given random variable <span class="math notranslate nohighlight">\(y\)</span> and how it varies as function of another variable or a set of such variables <span class="math notranslate nohighlight">\(\boldsymbol{x} =[x_0, x_1,\dots, x_{n-1}]^T\)</span>.
The first variable is called the <strong>dependent</strong>, the <strong>outcome</strong> or the <strong>response</strong> variable while the set of variables <span class="math notranslate nohighlight">\(\boldsymbol{x}\)</span> is called the independent variable, or the predictor variable or the explanatory variable, or simply just the <strong>inputs</strong>.</p>
<p>A regression model aims at finding a likelihood function <span class="math notranslate nohighlight">\(p(\boldsymbol{y}\vert \boldsymbol{x})\)</span> or in the more traditional sense a function <span class="math notranslate nohighlight">\(\boldsymbol{y}(\boldsymbol{x})\)</span>, that is the conditional distribution for <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> with a given <span class="math notranslate nohighlight">\(\boldsymbol{x}\)</span>. The estimation of <span class="math notranslate nohighlight">\(p(\boldsymbol{y}\vert \boldsymbol{x})\)</span> is made using a data set with</p>
<ul class="simple">
<li><p><span class="math notranslate nohighlight">\(n\)</span> cases <span class="math notranslate nohighlight">\(i = 0, 1, 2, \dots, n-1\)</span></p></li>
<li><p>Response (target, dependent or outcome) variable <span class="math notranslate nohighlight">\(y_i\)</span> with <span class="math notranslate nohighlight">\(i = 0, 1, 2, \dots, n-1\)</span></p></li>
<li><p><span class="math notranslate nohighlight">\(p\)</span> so-called explanatory (independent or predictor or feature) variables <span class="math notranslate nohighlight">\(\boldsymbol{x}_i=[x_{i0}, x_{i1}, \dots, x_{ip-1}]\)</span> with <span class="math notranslate nohighlight">\(i = 0, 1, 2, \dots, n-1\)</span> and explanatory variables running from <span class="math notranslate nohighlight">\(0\)</span> to <span class="math notranslate nohighlight">\(p-1\)</span>. See below for more explicit examples.</p></li>
</ul>
<p>The goal of the regression analysis is to extract/exploit relationship between <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> and <span class="math notranslate nohighlight">\(\boldsymbol{x}\)</span> in order to infer specific dependencies, approximations to the likelihood functions, functional relationships and to make predictions, making fits and many other things.</p>
</div>
<div class="section" id="regression-analysis-overarching-aims-ii">
<h2>Regression analysis, overarching aims II<a class="headerlink" href="#regression-analysis-overarching-aims-ii" title="Permalink to this headline"></a></h2>
<p>Consider an experiment in which <span class="math notranslate nohighlight">\(p\)</span> characteristics/features of <span class="math notranslate nohighlight">\(n\)</span> samples are
measured. The data from this experiment, for various explanatory variables <span class="math notranslate nohighlight">\(p\)</span> are normally represented by a matrix<br />
<span class="math notranslate nohighlight">\(\mathbf{X}\)</span>.</p>
<p>The matrix <span class="math notranslate nohighlight">\(\mathbf{X}\)</span> is called the <em>design
matrix</em>. Additional information of the samples is available in the
form of <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> (also as above). The variable <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> is
generally referred to as the <em>response variable</em>. The aim of
regression analysis is to explain <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> in terms of
<span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> through a functional relationship like <span class="math notranslate nohighlight">\(y_i =
f(\mathbf{X}_{i,\ast})\)</span>. When no prior knowledge on the form of
<span class="math notranslate nohighlight">\(f(\cdot)\)</span> is available, it is common to assume a linear relationship
between <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> and <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span>. This assumption gives rise to
the <em>linear regression model</em> where <span class="math notranslate nohighlight">\(\boldsymbol{\beta} = [\beta_0, \ldots,
\beta_{p-1}]^{T}\)</span> are the <em>regression parameters</em>.</p>
<p>Linear regression gives us a set of analytical equations for the parameters <span class="math notranslate nohighlight">\(\beta_j\)</span>.</p>
</div>
<div class="section" id="examples">
<h2>Examples<a class="headerlink" href="#examples" title="Permalink to this headline"></a></h2>
<p>In order to understand the relation among the predictors (or features or properties) <span class="math notranslate nohighlight">\(p\)</span>, the set of data <span class="math notranslate nohighlight">\(n\)</span> and the target (outcome, output etc) <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span>,
consider the model we discussed for describing nuclear binding energies.</p>
<p>There we assumed that we could parametrize the data using a polynomial approximation based on the liquid drop model.
Assuming</p>
<div class="math notranslate nohighlight">
\[
BE(A) = a_0+a_1A+a_2A^{2/3}+a_3A^{-1/3}+a_4A^{-1},
\]</div>
<p>we have five predictors, that is the intercept, the <span class="math notranslate nohighlight">\(A\)</span> dependent term, the <span class="math notranslate nohighlight">\(A^{2/3}\)</span> term and the <span class="math notranslate nohighlight">\(A^{-1/3}\)</span> and <span class="math notranslate nohighlight">\(A^{-1}\)</span> terms.
This gives <span class="math notranslate nohighlight">\(p=0,1,2,3,4\)</span>. Furthermore we have <span class="math notranslate nohighlight">\(n\)</span> entries for each predictor. It means that our design matrix is a
<span class="math notranslate nohighlight">\(p\times n\)</span> matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span>.</p>
<p>Here the predictors are based on a model we have made. A popular data set which is widely encountered in ML applications is the
so-called <a class="reference external" href="https://www.sciencedirect.com/science/article/pii/S0957417407006719?via%3Dihub">credit card default data from Taiwan</a>. The data set contains data on <span class="math notranslate nohighlight">\(n=30000\)</span> credit card holders with predictors like gender, marital status, age, profession, education, etc. In total there are <span class="math notranslate nohighlight">\(24\)</span> such predictors or attributes leading to a design matrix of dimensionality <span class="math notranslate nohighlight">\(24 \times 30000\)</span>. This is however a classification problem and we will come back to it when we discuss Logistic Regression.</p>
</div>
<div class="section" id="general-linear-models-and-linear-algebra">
<h2>General linear models and linear algebra<a class="headerlink" href="#general-linear-models-and-linear-algebra" title="Permalink to this headline"></a></h2>
<p>Before we proceed let us study a case where we aim at fitting a set of data <span class="math notranslate nohighlight">\(\boldsymbol{y}=[y_0,y_1,\dots,y_{n-1}]\)</span>. We could think of these data as a result of an experiment or a complicated numerical experiment. These data are functions of a series of variables <span class="math notranslate nohighlight">\(\boldsymbol{x}=[x_0,x_1,\dots,x_{n-1}]\)</span>, that is <span class="math notranslate nohighlight">\(y_i = y(x_i)\)</span> with <span class="math notranslate nohighlight">\(i=0,1,2,\dots,n-1\)</span>. The variables <span class="math notranslate nohighlight">\(x_i\)</span> could represent physical quantities like time, temperature, position etc. We assume that <span class="math notranslate nohighlight">\(y(x)\)</span> is a smooth function.</p>
<p>Since obtaining these data points may not be trivial, we want to use these data to fit a function which can allow us to make predictions for values of <span class="math notranslate nohighlight">\(y\)</span> which are not in the present set. The perhaps simplest approach is to assume we can parametrize our function in terms of a polynomial of degree <span class="math notranslate nohighlight">\(n-1\)</span> with <span class="math notranslate nohighlight">\(n\)</span> points, that is</p>
<div class="math notranslate nohighlight">
\[
y=y(x) \rightarrow y(x_i)=\tilde{y}_i+\epsilon_i=\sum_{j=0}^{n-1} \beta_j x_i^j+\epsilon_i,
\]</div>
<p>where <span class="math notranslate nohighlight">\(\epsilon_i\)</span> is the error in our approximation.</p>
</div>
<div class="section" id="rewriting-the-fitting-procedure-as-a-linear-algebra-problem">
<h2>Rewriting the fitting procedure as a linear algebra problem<a class="headerlink" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem" title="Permalink to this headline"></a></h2>
<p>For every set of values <span class="math notranslate nohighlight">\(y_i,x_i\)</span> we have thus the corresponding set of equations</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\begin{align*}
y_0&amp;=\beta_0+\beta_1x_0^1+\beta_2x_0^2+\dots+\beta_{n-1}x_0^{n-1}+\epsilon_0\\
y_1&amp;=\beta_0+\beta_1x_1^1+\beta_2x_1^2+\dots+\beta_{n-1}x_1^{n-1}+\epsilon_1\\
y_2&amp;=\beta_0+\beta_1x_2^1+\beta_2x_2^2+\dots+\beta_{n-1}x_2^{n-1}+\epsilon_2\\
\dots &amp; \dots \\
y_{n-1}&amp;=\beta_0+\beta_1x_{n-1}^1+\beta_2x_{n-1}^2+\dots+\beta_{n-1}x_{n-1}^{n-1}+\epsilon_{n-1}.\\
\end{align*}
\end{split}\]</div>
</div>
<div class="section" id="rewriting-the-fitting-procedure-as-a-linear-algebra-problem-more-details">
<h2>Rewriting the fitting procedure as a linear algebra problem, more details<a class="headerlink" href="#rewriting-the-fitting-procedure-as-a-linear-algebra-problem-more-details" title="Permalink to this headline"></a></h2>
<p>Defining the vectors</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{y} = [y_0,y_1, y_2,\dots, y_{n-1}]^T,
\]</div>
<p>and</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\beta} = [\beta_0,\beta_1, \beta_2,\dots, \beta_{n-1}]^T,
\]</div>
<p>and</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\epsilon} = [\epsilon_0,\epsilon_1, \epsilon_2,\dots, \epsilon_{n-1}]^T,
\]</div>
<p>and the design matrix</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\boldsymbol{X}=
\begin{bmatrix}
1&amp; x_{0}^1 &amp;x_{0}^2&amp; \dots &amp; \dots &amp;x_{0}^{n-1}\\
1&amp; x_{1}^1 &amp;x_{1}^2&amp; \dots &amp; \dots &amp;x_{1}^{n-1}\\
1&amp; x_{2}^1 &amp;x_{2}^2&amp; \dots &amp; \dots &amp;x_{2}^{n-1}\\
\dots&amp; \dots &amp;\dots&amp; \dots &amp; \dots &amp;\dots\\
1&amp; x_{n-1}^1 &amp;x_{n-1}^2&amp; \dots &amp; \dots &amp;x_{n-1}^{n-1}\\
\end{bmatrix}
\end{split}\]</div>
<p>we can rewrite our equations as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{y} = \boldsymbol{X}\boldsymbol{\beta}+\boldsymbol{\epsilon}.
\]</div>
<p>The above design matrix is called a <a class="reference external" href="https://en.wikipedia.org/wiki/Vandermonde_matrix">Vandermonde matrix</a>.</p>
</div>
<div class="section" id="generalizing-the-fitting-procedure-as-a-linear-algebra-problem">
<h2>Generalizing the fitting procedure as a linear algebra problem<a class="headerlink" href="#generalizing-the-fitting-procedure-as-a-linear-algebra-problem" title="Permalink to this headline"></a></h2>
<p>We are obviously not limited to the above polynomial expansions. We
could replace the various powers of <span class="math notranslate nohighlight">\(x\)</span> with elements of Fourier
series or instead of <span class="math notranslate nohighlight">\(x_i^j\)</span> we could have <span class="math notranslate nohighlight">\(\cos{(j x_i)}\)</span> or <span class="math notranslate nohighlight">\(\sin{(j
x_i)}\)</span>, or time series or other orthogonal functions. For every set
of values <span class="math notranslate nohighlight">\(y_i,x_i\)</span> we can then generalize the equations to</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\begin{align*}
y_0&amp;=\beta_0x_{00}+\beta_1x_{01}+\beta_2x_{02}+\dots+\beta_{n-1}x_{0n-1}+\epsilon_0\\
y_1&amp;=\beta_0x_{10}+\beta_1x_{11}+\beta_2x_{12}+\dots+\beta_{n-1}x_{1n-1}+\epsilon_1\\
y_2&amp;=\beta_0x_{20}+\beta_1x_{21}+\beta_2x_{22}+\dots+\beta_{n-1}x_{2n-1}+\epsilon_2\\
\dots &amp; \dots \\
y_{i}&amp;=\beta_0x_{i0}+\beta_1x_{i1}+\beta_2x_{i2}+\dots+\beta_{n-1}x_{in-1}+\epsilon_i\\
\dots &amp; \dots \\
y_{n-1}&amp;=\beta_0x_{n-1,0}+\beta_1x_{n-1,2}+\beta_2x_{n-1,2}+\dots+\beta_{n-1}x_{n-1,n-1}+\epsilon_{n-1}.\\
\end{align*}
\end{split}\]</div>
<p><strong>Note that we have <span class="math notranslate nohighlight">\(p=n\)</span> here. The matrix is symmetric. This is generally not the case!</strong></p>
</div>
<div class="section" id="id1">
<h2>Generalizing the fitting procedure as a linear algebra problem<a class="headerlink" href="#id1" title="Permalink to this headline"></a></h2>
<p>We redefine in turn the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> as</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\boldsymbol{X}=
\begin{bmatrix}
x_{00}&amp; x_{01} &amp;x_{02}&amp; \dots &amp; \dots &amp;x_{0,n-1}\\
x_{10}&amp; x_{11} &amp;x_{12}&amp; \dots &amp; \dots &amp;x_{1,n-1}\\
x_{20}&amp; x_{21} &amp;x_{22}&amp; \dots &amp; \dots &amp;x_{2,n-1}\\
\dots&amp; \dots &amp;\dots&amp; \dots &amp; \dots &amp;\dots\\
x_{n-1,0}&amp; x_{n-1,1} &amp;x_{n-1,2}&amp; \dots &amp; \dots &amp;x_{n-1,n-1}\\
\end{bmatrix}
\end{split}\]</div>
<p>and without loss of generality we rewrite again our equations as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{y} = \boldsymbol{X}\boldsymbol{\beta}+\boldsymbol{\epsilon}.
\]</div>
<p>The left-hand side of this equation is kwown. Our error vector <span class="math notranslate nohighlight">\(\boldsymbol{\epsilon}\)</span> and the parameter vector <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span> are our unknow quantities. How can we obtain the optimal set of <span class="math notranslate nohighlight">\(\beta_i\)</span> values?</p>
</div>
<div class="section" id="optimizing-our-parameters">
<h2>Optimizing our parameters<a class="headerlink" href="#optimizing-our-parameters" title="Permalink to this headline"></a></h2>
<p>We have defined the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> via the equations</p>
<div class="math notranslate nohighlight">
\[\begin{split}
\begin{align*}
y_0&amp;=\beta_0x_{00}+\beta_1x_{01}+\beta_2x_{02}+\dots+\beta_{n-1}x_{0n-1}+\epsilon_0\\
y_1&amp;=\beta_0x_{10}+\beta_1x_{11}+\beta_2x_{12}+\dots+\beta_{n-1}x_{1n-1}+\epsilon_1\\
y_2&amp;=\beta_0x_{20}+\beta_1x_{21}+\beta_2x_{22}+\dots+\beta_{n-1}x_{2n-1}+\epsilon_1\\
\dots &amp; \dots \\
y_{i}&amp;=\beta_0x_{i0}+\beta_1x_{i1}+\beta_2x_{i2}+\dots+\beta_{n-1}x_{in-1}+\epsilon_1\\
\dots &amp; \dots \\
y_{n-1}&amp;=\beta_0x_{n-1,0}+\beta_1x_{n-1,2}+\beta_2x_{n-1,2}+\dots+\beta_{n-1}x_{n-1,n-1}+\epsilon_{n-1}.\\
\end{align*}
\end{split}\]</div>
<p>As we noted above, we stayed with a system with the design matrix
<span class="math notranslate nohighlight">\(\boldsymbol{X}\in {\mathbb{R}}^{n\times n}\)</span>, that is we have <span class="math notranslate nohighlight">\(p=n\)</span>. For reasons to come later (algorithmic arguments) we will hereafter define
our matrix as <span class="math notranslate nohighlight">\(\boldsymbol{X}\in {\mathbb{R}}^{n\times p}\)</span>, with the predictors refering to the column numbers and the entries <span class="math notranslate nohighlight">\(n\)</span> being the row elements.</p>
</div>
<div class="section" id="our-model-for-the-nuclear-binding-energies">
<h2>Our model for the nuclear binding energies<a class="headerlink" href="#our-model-for-the-nuclear-binding-energies" title="Permalink to this headline"></a></h2>
<p>In our <a class="reference external" href="https://compphysics.github.io/MachineLearning/doc/pub/How2ReadData/html/How2ReadData.html">introductory notes</a> we looked at the so-called <a class="reference external" href="https://en.wikipedia.org/wiki/Semi-empirical_mass_formula">liquid drop model</a>. Let us remind ourselves about what we did by looking at the code.</p>
<p>We restate the parts of the code we are most interested in.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Common imports</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">IPython.display</span> <span class="kn">import</span> <span class="n">display</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="c1"># Where to save the figures and data files</span>
<span class="n">PROJECT_ROOT_DIR</span> <span class="o">=</span> <span class="s2">&quot;Results&quot;</span>
<span class="n">FIGURE_ID</span> <span class="o">=</span> <span class="s2">&quot;Results/FigureFiles&quot;</span>
<span class="n">DATA_ID</span> <span class="o">=</span> <span class="s2">&quot;DataFiles/&quot;</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">mkdir</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">,</span> <span class="n">fig_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">data_path</span><span class="p">(</span><span class="n">dat_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">,</span> <span class="n">dat_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">save_fig</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="n">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;.png&quot;</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">&#39;png&#39;</span><span class="p">)</span>
<span class="n">infile</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">data_path</span><span class="p">(</span><span class="s2">&quot;MassEval2016.dat&quot;</span><span class="p">),</span><span class="s1">&#39;r&#39;</span><span class="p">)</span>
<span class="c1"># Read the experimental data with Pandas</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">read_fwf</span><span class="p">(</span><span class="n">infile</span><span class="p">,</span> <span class="n">usecols</span><span class="o">=</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">6</span><span class="p">,</span><span class="mi">11</span><span class="p">),</span>
<span class="n">names</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;N&#39;</span><span class="p">,</span> <span class="s1">&#39;Z&#39;</span><span class="p">,</span> <span class="s1">&#39;A&#39;</span><span class="p">,</span> <span class="s1">&#39;Element&#39;</span><span class="p">,</span> <span class="s1">&#39;Ebinding&#39;</span><span class="p">),</span>
<span class="n">widths</span><span class="o">=</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">5</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">13</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">9</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">9</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">12</span><span class="p">,</span><span class="mi">11</span><span class="p">,</span><span class="mi">1</span><span class="p">),</span>
<span class="n">header</span><span class="o">=</span><span class="mi">39</span><span class="p">,</span>
<span class="n">index_col</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span>
<span class="c1"># Extrapolated values are indicated by &#39;#&#39; in place of the decimal place, so</span>
<span class="c1"># the Ebinding column won&#39;t be numeric. Coerce to float and drop these entries.</span>
<span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">to_numeric</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">],</span> <span class="n">errors</span><span class="o">=</span><span class="s1">&#39;coerce&#39;</span><span class="p">)</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">dropna</span><span class="p">()</span>
<span class="c1"># Convert from keV to MeV.</span>
<span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span> <span class="o">/=</span> <span class="mi">1000</span>
<span class="c1"># Group the DataFrame by nucleon number, A.</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">groupby</span><span class="p">(</span><span class="s1">&#39;A&#39;</span><span class="p">)</span>
<span class="c1"># Find the rows of the grouped DataFrame with the maximum binding energy.</span>
<span class="n">Masses</span> <span class="o">=</span> <span class="n">Masses</span><span class="o">.</span><span class="n">apply</span><span class="p">(</span><span class="k">lambda</span> <span class="n">t</span><span class="p">:</span> <span class="n">t</span><span class="p">[</span><span class="n">t</span><span class="o">.</span><span class="n">Ebinding</span><span class="o">==</span><span class="n">t</span><span class="o">.</span><span class="n">Ebinding</span><span class="o">.</span><span class="n">max</span><span class="p">()])</span>
<span class="n">A</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">]</span>
<span class="n">Z</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Z&#39;</span><span class="p">]</span>
<span class="n">N</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;N&#39;</span><span class="p">]</span>
<span class="n">Element</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Element&#39;</span><span class="p">]</span>
<span class="n">Energies</span> <span class="o">=</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">]</span>
<span class="c1"># Now we set up the design matrix X</span>
<span class="n">X</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">A</span><span class="p">),</span><span class="mi">5</span><span class="p">))</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="mf">2.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="o">-</span><span class="mf">1.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">4</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="o">**</span><span class="p">(</span><span class="o">-</span><span class="mf">1.0</span><span class="p">)</span>
<span class="c1"># Then nice printout using pandas</span>
<span class="n">DesignMatrix</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">DataFrame</span><span class="p">(</span><span class="n">X</span><span class="p">)</span>
<span class="n">DesignMatrix</span><span class="o">.</span><span class="n">index</span> <span class="o">=</span> <span class="n">A</span>
<span class="n">DesignMatrix</span><span class="o">.</span><span class="n">columns</span> <span class="o">=</span> <span class="p">[</span><span class="s1">&#39;1&#39;</span><span class="p">,</span> <span class="s1">&#39;A&#39;</span><span class="p">,</span> <span class="s1">&#39;A^(2/3)&#39;</span><span class="p">,</span> <span class="s1">&#39;A^(-1/3)&#39;</span><span class="p">,</span> <span class="s1">&#39;1/A&#39;</span><span class="p">]</span>
<span class="n">display</span><span class="p">(</span><span class="n">DesignMatrix</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>With <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\in {\mathbb{R}}^{p\times 1}\)</span>, it means that we will hereafter write our equations for the approximation as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\tilde{y}}= \boldsymbol{X}\boldsymbol{\beta},
\]</div>
<p>throughout these lectures.</p>
</div>
<div class="section" id="optimizing-our-parameters-more-details">
<h2>Optimizing our parameters, more details<a class="headerlink" href="#optimizing-our-parameters-more-details" title="Permalink to this headline"></a></h2>
<p>With the above we use the design matrix to define the approximation <span class="math notranslate nohighlight">\(\boldsymbol{\tilde{y}}\)</span> via the unknown quantity <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span> as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\tilde{y}}= \boldsymbol{X}\boldsymbol{\beta},
\]</div>
<p>and in order to find the optimal parameters <span class="math notranslate nohighlight">\(\beta_i\)</span> instead of solving the above linear algebra problem, we define a function which gives a measure of the spread between the values <span class="math notranslate nohighlight">\(y_i\)</span> (which represent hopefully the exact values) and the parameterized values <span class="math notranslate nohighlight">\(\tilde{y}_i\)</span>, namely</p>
<div class="math notranslate nohighlight">
\[
C(\boldsymbol{\beta})=\frac{1}{n}\sum_{i=0}^{n-1}\left(y_i-\tilde{y}_i\right)^2=\frac{1}{n}\left\{\left(\boldsymbol{y}-\boldsymbol{\tilde{y}}\right)^T\left(\boldsymbol{y}-\boldsymbol{\tilde{y}}\right)\right\},
\]</div>
<p>or using the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> and in a more compact matrix-vector notation as</p>
<div class="math notranslate nohighlight">
\[
C(\boldsymbol{\beta})=\frac{1}{n}\left\{\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)^T\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)\right\}.
\]</div>
<p>This function is one possible way to define the so-called cost function.</p>
<p>It is also common to define
the function <span class="math notranslate nohighlight">\(C\)</span> as</p>
<div class="math notranslate nohighlight">
\[
C(\boldsymbol{\beta})=\frac{1}{2n}\sum_{i=0}^{n-1}\left(y_i-\tilde{y}_i\right)^2,
\]</div>
<p>since when taking the first derivative with respect to the unknown parameters <span class="math notranslate nohighlight">\(\beta\)</span>, the factor of <span class="math notranslate nohighlight">\(2\)</span> cancels out.</p>
</div>
<div class="section" id="interpretations-and-optimizing-our-parameters">
<h2>Interpretations and optimizing our parameters<a class="headerlink" href="#interpretations-and-optimizing-our-parameters" title="Permalink to this headline"></a></h2>
<p>The function</p>
<div class="math notranslate nohighlight">
\[
C(\boldsymbol{\beta})=\frac{1}{n}\left\{\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)^T\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)\right\},
\]</div>
<p>can be linked to the variance of the quantity <span class="math notranslate nohighlight">\(y_i\)</span> if we interpret the latter as the mean value.
When linking (see the discussion below) with the maximum likelihood approach below, we will indeed interpret <span class="math notranslate nohighlight">\(y_i\)</span> as a mean value</p>
<div class="math notranslate nohighlight">
\[
y_{i}=\langle y_i \rangle = \beta_0x_{i,0}+\beta_1x_{i,1}+\beta_2x_{i,2}+\dots+\beta_{n-1}x_{i,n-1}+\epsilon_i,
\]</div>
<p>where <span class="math notranslate nohighlight">\(\langle y_i \rangle\)</span> is the mean value. Keep in mind also that
till now we have treated <span class="math notranslate nohighlight">\(y_i\)</span> as the exact value. Normally, the
response (dependent or outcome) variable <span class="math notranslate nohighlight">\(y_i\)</span> the outcome of a
numerical experiment or another type of experiment and is thus only an
approximation to the true value. It is then always accompanied by an
error estimate, often limited to a statistical error estimate given by
the standard deviation discussed earlier. In the discussion here we
will treat <span class="math notranslate nohighlight">\(y_i\)</span> as our exact value for the response variable.</p>
<p>In order to find the parameters <span class="math notranslate nohighlight">\(\beta_i\)</span> we will then minimize the spread of <span class="math notranslate nohighlight">\(C(\boldsymbol{\beta})\)</span>, that is we are going to solve the problem</p>
<div class="math notranslate nohighlight">
\[
{\displaystyle \min_{\boldsymbol{\beta}\in
{\mathbb{R}}^{p}}}\frac{1}{n}\left\{\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)^T\left(\boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)\right\}.
\]</div>
<p>In practical terms it means we will require</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial C(\boldsymbol{\beta})}{\partial \beta_j} = \frac{\partial }{\partial \beta_j}\left[ \frac{1}{n}\sum_{i=0}^{n-1}\left(y_i-\beta_0x_{i,0}-\beta_1x_{i,1}-\beta_2x_{i,2}-\dots-\beta_{n-1}x_{i,n-1}\right)^2\right]=0,
\]</div>
<p>which results in</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial C(\boldsymbol{\beta})}{\partial \beta_j} = -\frac{2}{n}\left[ \sum_{i=0}^{n-1}x_{ij}\left(y_i-\beta_0x_{i,0}-\beta_1x_{i,1}-\beta_2x_{i,2}-\dots-\beta_{n-1}x_{i,n-1}\right)\right]=0,
\]</div>
<p>or in a matrix-vector form as</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial C(\boldsymbol{\beta})}{\partial \boldsymbol{\beta}} = 0 = \boldsymbol{X}^T\left( \boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right).
\]</div>
</div>
<div class="section" id="id2">
<h2>Interpretations and optimizing our parameters<a class="headerlink" href="#id2" title="Permalink to this headline"></a></h2>
<p>We can rewrite</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial C(\boldsymbol{\beta})}{\partial \boldsymbol{\beta}} = 0 = \boldsymbol{X}^T\left( \boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right),
\]</div>
<p>as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{X}^T\boldsymbol{y} = \boldsymbol{X}^T\boldsymbol{X}\boldsymbol{\beta},
\]</div>
<p>and if the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}^T\boldsymbol{X}\)</span> is invertible we have the solution</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\beta} =\left(\boldsymbol{X}^T\boldsymbol{X}\right)^{-1}\boldsymbol{X}^T\boldsymbol{y}.
\]</div>
<p>We note also that since our design matrix is defined as <span class="math notranslate nohighlight">\(\boldsymbol{X}\in
{\mathbb{R}}^{n\times p}\)</span>, the product <span class="math notranslate nohighlight">\(\boldsymbol{X}^T\boldsymbol{X} \in
{\mathbb{R}}^{p\times p}\)</span>. In the above case we have that <span class="math notranslate nohighlight">\(p \ll n\)</span>,
in our case <span class="math notranslate nohighlight">\(p=5\)</span> meaning that we end up with inverting a small
<span class="math notranslate nohighlight">\(5\times 5\)</span> matrix. This is a rather common situation, in many cases we end up with low-dimensional
matrices to invert. The methods discussed here and for many other
supervised learning algorithms like classification with logistic
regression or support vector machines, exhibit dimensionalities which
allow for the usage of direct linear algebra methods such as <strong>LU</strong> decomposition or <strong>Singular Value Decomposition</strong> (SVD) for finding the inverse of the matrix
<span class="math notranslate nohighlight">\(\boldsymbol{X}^T\boldsymbol{X}\)</span>.</p>
<p><strong>Small question</strong>: Do you think the example we have at hand here (the nuclear binding energies) can lead to problems in inverting the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}^T\boldsymbol{X}\)</span>? What kind of problems can we expect?</p>
</div>
<div class="section" id="id3">
<h2>Interpretations and optimizing our parameters<a class="headerlink" href="#id3" title="Permalink to this headline"></a></h2>
<p>The residuals <span class="math notranslate nohighlight">\(\boldsymbol{\epsilon}\)</span> are in turn given by</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\epsilon} = \boldsymbol{y}-\boldsymbol{\tilde{y}} = \boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta},
\]</div>
<p>and with</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{X}^T\left( \boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)= 0,
\]</div>
<p>we have</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{X}^T\boldsymbol{\epsilon}=\boldsymbol{X}^T\left( \boldsymbol{y}-\boldsymbol{X}\boldsymbol{\beta}\right)= 0,
\]</div>
<p>meaning that the solution for <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span> is the one which minimizes the residuals. Later we will link this with the maximum likelihood approach.</p>
<p>Let us now return to our nuclear binding energies and simply code the above equations.</p>
</div>
<div class="section" id="own-code-for-ordinary-least-squares">
<h2>Own code for Ordinary Least Squares<a class="headerlink" href="#own-code-for-ordinary-least-squares" title="Permalink to this headline"></a></h2>
<p>It is rather straightforward to implement the matrix inversion and obtain the parameters <span class="math notranslate nohighlight">\(\boldsymbol{\beta}\)</span>. After having defined the matrix <span class="math notranslate nohighlight">\(\boldsymbol{X}\)</span> we simply need to
write</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># matrix inversion to find beta</span>
<span class="n">beta</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linalg</span><span class="o">.</span><span class="n">inv</span><span class="p">(</span><span class="n">X</span><span class="o">.</span><span class="n">T</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">X</span><span class="p">))</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">X</span><span class="o">.</span><span class="n">T</span><span class="p">)</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">Energies</span><span class="p">)</span>
<span class="c1"># and then make the prediction</span>
<span class="n">ytilde</span> <span class="o">=</span> <span class="n">X</span> <span class="o">@</span> <span class="n">beta</span>
</pre></div>
</div>
</div>
</div>
<p>Alternatively, you can use the least squares functionality in <strong>Numpy</strong> as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">fit</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linalg</span><span class="o">.</span><span class="n">lstsq</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">Energies</span><span class="p">,</span> <span class="n">rcond</span> <span class="o">=</span><span class="kc">None</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span>
<span class="n">ytildenp</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">fit</span><span class="p">,</span><span class="n">X</span><span class="o">.</span><span class="n">T</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>And finally we plot our fit with and compare with data</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Eapprox&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">ytilde</span>
<span class="c1"># Generate a plot comparing the experimental with the fitted values values.</span>
<span class="n">fig</span><span class="p">,</span> <span class="n">ax</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">subplots</span><span class="p">()</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$A = N + Z$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$E_\mathrm</span><span class="si">{bind}</span><span class="s1">\,/\mathrm</span><span class="si">{MeV}</span><span class="s1">$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">],</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Ebinding&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Ame2016&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;A&#39;</span><span class="p">],</span> <span class="n">Masses</span><span class="p">[</span><span class="s1">&#39;Eapprox&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">c</span><span class="o">=</span><span class="s1">&#39;m&#39;</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Fit&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<span class="n">save_fig</span><span class="p">(</span><span class="s2">&quot;Masses2016OLS&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="adding-error-analysis-and-training-set-up">
<h2>Adding error analysis and training set up<a class="headerlink" href="#adding-error-analysis-and-training-set-up" title="Permalink to this headline"></a></h2>
<p>We can easily test our fit by computing the <span class="math notranslate nohighlight">\(R2\)</span> score that we discussed in connection with the functionality of <strong>Scikit-Learn</strong> in the introductory slides.
Since we are not using <strong>Scikit-Learn</strong> here we can define our own <span class="math notranslate nohighlight">\(R2\)</span> function as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">R2</span><span class="p">(</span><span class="n">y_data</span><span class="p">,</span> <span class="n">y_model</span><span class="p">):</span>
<span class="k">return</span> <span class="mi">1</span> <span class="o">-</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span> <span class="o">-</span> <span class="n">y_model</span><span class="p">)</span> <span class="o">**</span> <span class="mi">2</span><span class="p">)</span> <span class="o">/</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span> <span class="o">-</span> <span class="n">np</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">y_data</span><span class="p">))</span> <span class="o">**</span> <span class="mi">2</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>and we would be using it as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span><span class="p">(</span><span class="n">R2</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span><span class="n">ytilde</span><span class="p">))</span>
</pre></div>
</div>
</div>
</div>
<p>We can easily add our <strong>MSE</strong> score as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">MSE</span><span class="p">(</span><span class="n">y_data</span><span class="p">,</span><span class="n">y_model</span><span class="p">):</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">size</span><span class="p">(</span><span class="n">y_model</span><span class="p">)</span>
<span class="k">return</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span><span class="o">-</span><span class="n">y_model</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span><span class="o">/</span><span class="n">n</span>
<span class="nb">print</span><span class="p">(</span><span class="n">MSE</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span><span class="n">ytilde</span><span class="p">))</span>
</pre></div>
</div>
</div>
</div>
<p>and finally the relative error as</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">RelativeError</span><span class="p">(</span><span class="n">y_data</span><span class="p">,</span><span class="n">y_model</span><span class="p">):</span>
<span class="k">return</span> <span class="nb">abs</span><span class="p">((</span><span class="n">y_data</span><span class="o">-</span><span class="n">y_model</span><span class="p">)</span><span class="o">/</span><span class="n">y_data</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">RelativeError</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">ytilde</span><span class="p">))</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="the-chi-2-function">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#the-chi-2-function" title="Permalink to this headline"></a></h2>
<p>Normally, the response (dependent or outcome) variable <span class="math notranslate nohighlight">\(y_i\)</span> is the
outcome of a numerical experiment or another type of experiment and is
thus only an approximation to the true value. It is then always
accompanied by an error estimate, often limited to a statistical error
estimate given by the standard deviation discussed earlier. In the
discussion here we will treat <span class="math notranslate nohighlight">\(y_i\)</span> as our exact value for the
response variable.</p>
<p>Introducing the standard deviation <span class="math notranslate nohighlight">\(\sigma_i\)</span> for each measurement
<span class="math notranslate nohighlight">\(y_i\)</span>, we define now the <span class="math notranslate nohighlight">\(\chi^2\)</span> function (omitting the <span class="math notranslate nohighlight">\(1/n\)</span> term)
as</p>
<div class="math notranslate nohighlight">
\[
\chi^2(\boldsymbol{\beta})=\frac{1}{n}\sum_{i=0}^{n-1}\frac{\left(y_i-\tilde{y}_i\right)^2}{\sigma_i^2}=\frac{1}{n}\left\{\left(\boldsymbol{y}-\boldsymbol{\tilde{y}}\right)^T\frac{1}{\boldsymbol{\Sigma^2}}\left(\boldsymbol{y}-\boldsymbol{\tilde{y}}\right)\right\},
\]</div>
<p>where the matrix <span class="math notranslate nohighlight">\(\boldsymbol{\Sigma}\)</span> is a diagonal matrix with <span class="math notranslate nohighlight">\(\sigma_i\)</span> as matrix elements.</p>
</div>
<div class="section" id="id4">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#id4" title="Permalink to this headline"></a></h2>
<p>In order to find the parameters <span class="math notranslate nohighlight">\(\beta_i\)</span> we will then minimize the spread of <span class="math notranslate nohighlight">\(\chi^2(\boldsymbol{\beta})\)</span> by requiring</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \beta_j} = \frac{\partial }{\partial \beta_j}\left[ \frac{1}{n}\sum_{i=0}^{n-1}\left(\frac{y_i-\beta_0x_{i,0}-\beta_1x_{i,1}-\beta_2x_{i,2}-\dots-\beta_{n-1}x_{i,n-1}}{\sigma_i}\right)^2\right]=0,
\]</div>
<p>which results in</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \beta_j} = -\frac{2}{n}\left[ \sum_{i=0}^{n-1}\frac{x_{ij}}{\sigma_i}\left(\frac{y_i-\beta_0x_{i,0}-\beta_1x_{i,1}-\beta_2x_{i,2}-\dots-\beta_{n-1}x_{i,n-1}}{\sigma_i}\right)\right]=0,
\]</div>
<p>or in a matrix-vector form as</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \boldsymbol{\beta}} = 0 = \boldsymbol{A}^T\left( \boldsymbol{b}-\boldsymbol{A}\boldsymbol{\beta}\right).
\]</div>
<p>where we have defined the matrix <span class="math notranslate nohighlight">\(\boldsymbol{A} =\boldsymbol{X}/\boldsymbol{\Sigma}\)</span> with matrix elements <span class="math notranslate nohighlight">\(a_{ij} = x_{ij}/\sigma_i\)</span> and the vector <span class="math notranslate nohighlight">\(\boldsymbol{b}\)</span> with elements <span class="math notranslate nohighlight">\(b_i = y_i/\sigma_i\)</span>.</p>
</div>
<div class="section" id="id5">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#id5" title="Permalink to this headline"></a></h2>
<p>We can rewrite</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \boldsymbol{\beta}} = 0 = \boldsymbol{A}^T\left( \boldsymbol{b}-\boldsymbol{A}\boldsymbol{\beta}\right),
\]</div>
<p>as</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{A}^T\boldsymbol{b} = \boldsymbol{A}^T\boldsymbol{A}\boldsymbol{\beta},
\]</div>
<p>and if the matrix <span class="math notranslate nohighlight">\(\boldsymbol{A}^T\boldsymbol{A}\)</span> is invertible we have the solution</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{\beta} =\left(\boldsymbol{A}^T\boldsymbol{A}\right)^{-1}\boldsymbol{A}^T\boldsymbol{b}.
\]</div>
</div>
<div class="section" id="id6">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#id6" title="Permalink to this headline"></a></h2>
<p>If we then introduce the matrix</p>
<div class="math notranslate nohighlight">
\[
\boldsymbol{H} = \left(\boldsymbol{A}^T\boldsymbol{A}\right)^{-1},
\]</div>
<p>we have then the following expression for the parameters <span class="math notranslate nohighlight">\(\beta_j\)</span> (the matrix elements of <span class="math notranslate nohighlight">\(\boldsymbol{H}\)</span> are <span class="math notranslate nohighlight">\(h_{ij}\)</span>)</p>
<div class="math notranslate nohighlight">
\[
\beta_j = \sum_{k=0}^{p-1}h_{jk}\sum_{i=0}^{n-1}\frac{y_i}{\sigma_i}\frac{x_{ik}}{\sigma_i} = \sum_{k=0}^{p-1}h_{jk}\sum_{i=0}^{n-1}b_ia_{ik}
\]</div>
<p>We state without proof the expression for the uncertainty in the parameters <span class="math notranslate nohighlight">\(\beta_j\)</span> as (we leave this as an exercise)</p>
<div class="math notranslate nohighlight">
\[
\sigma^2(\beta_j) = \sum_{i=0}^{n-1}\sigma_i^2\left( \frac{\partial \beta_j}{\partial y_i}\right)^2,
\]</div>
<p>resulting in</p>
<div class="math notranslate nohighlight">
\[
\sigma^2(\beta_j) = \left(\sum_{k=0}^{p-1}h_{jk}\sum_{i=0}^{n-1}a_{ik}\right)\left(\sum_{l=0}^{p-1}h_{jl}\sum_{m=0}^{n-1}a_{ml}\right) = h_{jj}!
\]</div>
</div>
<div class="section" id="id7">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#id7" title="Permalink to this headline"></a></h2>
<p>The first step here is to approximate the function <span class="math notranslate nohighlight">\(y\)</span> with a first-order polynomial, that is we write</p>
<div class="math notranslate nohighlight">
\[
y=y(x) \rightarrow y(x_i) \approx \beta_0+\beta_1 x_i.
\]</div>
<p>By computing the derivatives of <span class="math notranslate nohighlight">\(\chi^2\)</span> with respect to <span class="math notranslate nohighlight">\(\beta_0\)</span> and <span class="math notranslate nohighlight">\(\beta_1\)</span> show that these are given by</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \beta_0} = -2\left[ \frac{1}{n}\sum_{i=0}^{n-1}\left(\frac{y_i-\beta_0-\beta_1x_{i}}{\sigma_i^2}\right)\right]=0,
\]</div>
<p>and</p>
<div class="math notranslate nohighlight">
\[
\frac{\partial \chi^2(\boldsymbol{\beta})}{\partial \beta_1} = -\frac{2}{n}\left[ \sum_{i=0}^{n-1}x_i\left(\frac{y_i-\beta_0-\beta_1x_{i}}{\sigma_i^2}\right)\right]=0.
\]</div>
</div>
<div class="section" id="id8">
<h2>The <span class="math notranslate nohighlight">\(\chi^2\)</span> function<a class="headerlink" href="#id8" title="Permalink to this headline"></a></h2>
<p>For a linear fit (a first-order polynomial) we don’t need to invert a matrix!!<br />
Defining</p>
<div class="math notranslate nohighlight">
\[
\gamma = \sum_{i=0}^{n-1}\frac{1}{\sigma_i^2},
\]</div>
<div class="math notranslate nohighlight">
\[
\gamma_x = \sum_{i=0}^{n-1}\frac{x_{i}}{\sigma_i^2},
\]</div>
<div class="math notranslate nohighlight">
\[
\gamma_y = \sum_{i=0}^{n-1}\left(\frac{y_i}{\sigma_i^2}\right),
\]</div>
<div class="math notranslate nohighlight">
\[
\gamma_{xx} = \sum_{i=0}^{n-1}\frac{x_ix_{i}}{\sigma_i^2},
\]</div>
<div class="math notranslate nohighlight">
\[
\gamma_{xy} = \sum_{i=0}^{n-1}\frac{y_ix_{i}}{\sigma_i^2},
\]</div>
<p>we obtain</p>
<div class="math notranslate nohighlight">
\[
\beta_0 = \frac{\gamma_{xx}\gamma_y-\gamma_x\gamma_y}{\gamma\gamma_{xx}-\gamma_x^2},
\]</div>
<div class="math notranslate nohighlight">
\[
\beta_1 = \frac{\gamma_{xy}\gamma-\gamma_x\gamma_y}{\gamma\gamma_{xx}-\gamma_x^2}.
\]</div>
<p>This approach (different linear and non-linear regression) suffers
often from both being underdetermined and overdetermined in the
unknown coefficients <span class="math notranslate nohighlight">\(\beta_i\)</span>. A better approach is to use the
Singular Value Decomposition (SVD) method discussed next week.</p>
</div>
<div class="section" id="fitting-an-equation-of-state-for-dense-nuclear-matter">
<h2>Fitting an Equation of State for Dense Nuclear Matter<a class="headerlink" href="#fitting-an-equation-of-state-for-dense-nuclear-matter" title="Permalink to this headline"></a></h2>
<p>Before we continue, let us introduce yet another example. We are going to fit the
nuclear equation of state using results from many-body calculations.
The equation of state we have made available here, as function of
density, has been derived using modern nucleon-nucleon potentials with
<a class="reference external" href="https://www.sciencedirect.com/science/article/pii/S0370157399001106">the addition of three-body
forces</a>. This
time the file is presented as a standard <strong>csv</strong> file.</p>
<p>The beginning of the Python code here is similar to what you have seen
before, with the same initializations and declarations. We use also
<strong>pandas</strong> again, rather extensively in order to organize our data.</p>
<p>The difference now is that we use <strong>Scikit-Learn’s</strong> regression tools
instead of our own matrix inversion implementation. Furthermore, we
sneak in <strong>Ridge</strong> regression (to be discussed below) which includes a
hyperparameter <span class="math notranslate nohighlight">\(\lambda\)</span>, also to be explained below.</p>
</div>
<div class="section" id="the-code">
<h2>The code<a class="headerlink" href="#the-code" title="Permalink to this headline"></a></h2>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Common imports</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">import</span> <span class="nn">sklearn.linear_model</span> <span class="k">as</span> <span class="nn">skl</span>
<span class="kn">from</span> <span class="nn">sklearn.metrics</span> <span class="kn">import</span> <span class="n">mean_squared_error</span><span class="p">,</span> <span class="n">r2_score</span><span class="p">,</span> <span class="n">mean_absolute_error</span>
<span class="c1"># Where to save the figures and data files</span>
<span class="n">PROJECT_ROOT_DIR</span> <span class="o">=</span> <span class="s2">&quot;Results&quot;</span>
<span class="n">FIGURE_ID</span> <span class="o">=</span> <span class="s2">&quot;Results/FigureFiles&quot;</span>
<span class="n">DATA_ID</span> <span class="o">=</span> <span class="s2">&quot;DataFiles/&quot;</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">mkdir</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">,</span> <span class="n">fig_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">data_path</span><span class="p">(</span><span class="n">dat_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">,</span> <span class="n">dat_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">save_fig</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="n">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;.png&quot;</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">&#39;png&#39;</span><span class="p">)</span>
<span class="n">infile</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">data_path</span><span class="p">(</span><span class="s2">&quot;EoS.csv&quot;</span><span class="p">),</span><span class="s1">&#39;r&#39;</span><span class="p">)</span>
<span class="c1"># Read the EoS data as csv file and organize the data into two arrays with density and energies</span>
<span class="n">EoS</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">read_csv</span><span class="p">(</span><span class="n">infile</span><span class="p">,</span> <span class="n">names</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;Density&#39;</span><span class="p">,</span> <span class="s1">&#39;Energy&#39;</span><span class="p">))</span>
<span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">to_numeric</span><span class="p">(</span><span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">],</span> <span class="n">errors</span><span class="o">=</span><span class="s1">&#39;coerce&#39;</span><span class="p">)</span>
<span class="n">EoS</span> <span class="o">=</span> <span class="n">EoS</span><span class="o">.</span><span class="n">dropna</span><span class="p">()</span>
<span class="n">Energies</span> <span class="o">=</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">]</span>
<span class="n">Density</span> <span class="o">=</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Density&#39;</span><span class="p">]</span>
<span class="c1"># The design matrix now as function of various polytrops</span>
<span class="n">X</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">Density</span><span class="p">),</span><span class="mi">4</span><span class="p">))</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span><span class="o">**</span><span class="p">(</span><span class="mf">4.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span><span class="o">**</span><span class="p">(</span><span class="mf">2.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="c1"># We use now Scikit-Learn&#39;s linear regressor and ridge regressor</span>
<span class="c1"># OLS part</span>
<span class="n">clf</span> <span class="o">=</span> <span class="n">skl</span><span class="o">.</span><span class="n">LinearRegression</span><span class="p">()</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">Energies</span><span class="p">)</span>
<span class="n">ytilde</span> <span class="o">=</span> <span class="n">clf</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">X</span><span class="p">)</span>
<span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Eols&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">ytilde</span>
<span class="c1"># The mean squared error </span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Mean squared error: </span><span class="si">%.2f</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">ytilde</span><span class="p">))</span>
<span class="c1"># Explained variance score: 1 is perfect prediction </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Variance score: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">r2_score</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">ytilde</span><span class="p">))</span>
<span class="c1"># Mean absolute error </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Mean absolute error: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">mean_absolute_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">ytilde</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">clf</span><span class="o">.</span><span class="n">coef_</span><span class="p">,</span> <span class="n">clf</span><span class="o">.</span><span class="n">intercept_</span><span class="p">)</span>
<span class="c1"># The Ridge regression with a hyperparameter lambda = 0.1</span>
<span class="n">_lambda</span> <span class="o">=</span> <span class="mf">0.1</span>
<span class="n">clf_ridge</span> <span class="o">=</span> <span class="n">skl</span><span class="o">.</span><span class="n">Ridge</span><span class="p">(</span><span class="n">alpha</span><span class="o">=</span><span class="n">_lambda</span><span class="p">)</span><span class="o">.</span><span class="n">fit</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">Energies</span><span class="p">)</span>
<span class="n">yridge</span> <span class="o">=</span> <span class="n">clf_ridge</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">X</span><span class="p">)</span>
<span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Eridge&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">yridge</span>
<span class="c1"># The mean squared error </span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Mean squared error: </span><span class="si">%.2f</span><span class="s2">&quot;</span> <span class="o">%</span> <span class="n">mean_squared_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">yridge</span><span class="p">))</span>
<span class="c1"># Explained variance score: 1 is perfect prediction </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Variance score: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">r2_score</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">yridge</span><span class="p">))</span>
<span class="c1"># Mean absolute error </span>
<span class="nb">print</span><span class="p">(</span><span class="s1">&#39;Mean absolute error: </span><span class="si">%.2f</span><span class="s1">&#39;</span> <span class="o">%</span> <span class="n">mean_absolute_error</span><span class="p">(</span><span class="n">Energies</span><span class="p">,</span> <span class="n">yridge</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="n">clf_ridge</span><span class="o">.</span><span class="n">coef_</span><span class="p">,</span> <span class="n">clf_ridge</span><span class="o">.</span><span class="n">intercept_</span><span class="p">)</span>
<span class="n">fig</span><span class="p">,</span> <span class="n">ax</span> <span class="o">=</span> <span class="n">plt</span><span class="o">.</span><span class="n">subplots</span><span class="p">()</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_xlabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;$\rho[\mathrm</span><span class="si">{fm}</span><span class="s1">^{-3}]$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">set_ylabel</span><span class="p">(</span><span class="sa">r</span><span class="s1">&#39;Energy per particle&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Density&#39;</span><span class="p">],</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Theoretical data&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Density&#39;</span><span class="p">],</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Eols&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">c</span><span class="o">=</span><span class="s1">&#39;m&#39;</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;OLS&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">plot</span><span class="p">(</span><span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Density&#39;</span><span class="p">],</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Eridge&#39;</span><span class="p">],</span> <span class="n">alpha</span><span class="o">=</span><span class="mf">0.7</span><span class="p">,</span> <span class="n">lw</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">c</span><span class="o">=</span><span class="s1">&#39;g&#39;</span><span class="p">,</span>
<span class="n">label</span><span class="o">=</span><span class="s1">&#39;Ridge $\lambda = 0.1$&#39;</span><span class="p">)</span>
<span class="n">ax</span><span class="o">.</span><span class="n">legend</span><span class="p">()</span>
<span class="n">save_fig</span><span class="p">(</span><span class="s2">&quot;EoSfitting&quot;</span><span class="p">)</span>
<span class="n">plt</span><span class="o">.</span><span class="n">show</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<p>The above simple polynomial in density <span class="math notranslate nohighlight">\(\rho\)</span> gives an excellent fit
to the data.</p>
<p>We note also that there is a small deviation between the
standard OLS and the Ridge regression at higher densities. We discuss this in more detail
below.</p>
</div>
<div class="section" id="splitting-our-data-in-training-and-test-data">
<h2>Splitting our Data in Training and Test data<a class="headerlink" href="#splitting-our-data-in-training-and-test-data" title="Permalink to this headline"></a></h2>
<p>It is normal in essentially all Machine Learning studies to split the
data in a training set and a test set (sometimes also an additional
validation set). <strong>Scikit-Learn</strong> has an own function for this. There
is no explicit recipe for how much data should be included as training
data and say test data. An accepted rule of thumb is to use
approximately <span class="math notranslate nohighlight">\(2/3\)</span> to <span class="math notranslate nohighlight">\(4/5\)</span> of the data as training data. We will
postpone a discussion of this splitting to the end of these notes and
our discussion of the so-called <strong>bias-variance</strong> tradeoff. Here we
limit ourselves to repeat the above equation of state fitting example
but now splitting the data into a training set and a test set.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="kn">import</span> <span class="nn">pandas</span> <span class="k">as</span> <span class="nn">pd</span>
<span class="kn">import</span> <span class="nn">matplotlib.pyplot</span> <span class="k">as</span> <span class="nn">plt</span>
<span class="kn">from</span> <span class="nn">sklearn.model_selection</span> <span class="kn">import</span> <span class="n">train_test_split</span>
<span class="c1"># Where to save the figures and data files</span>
<span class="n">PROJECT_ROOT_DIR</span> <span class="o">=</span> <span class="s2">&quot;Results&quot;</span>
<span class="n">FIGURE_ID</span> <span class="o">=</span> <span class="s2">&quot;Results/FigureFiles&quot;</span>
<span class="n">DATA_ID</span> <span class="o">=</span> <span class="s2">&quot;DataFiles/&quot;</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">mkdir</span><span class="p">(</span><span class="n">PROJECT_ROOT_DIR</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">)</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">):</span>
<span class="n">os</span><span class="o">.</span><span class="n">makedirs</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">FIGURE_ID</span><span class="p">,</span> <span class="n">fig_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">data_path</span><span class="p">(</span><span class="n">dat_id</span><span class="p">):</span>
<span class="k">return</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">join</span><span class="p">(</span><span class="n">DATA_ID</span><span class="p">,</span> <span class="n">dat_id</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">save_fig</span><span class="p">(</span><span class="n">fig_id</span><span class="p">):</span>
<span class="n">plt</span><span class="o">.</span><span class="n">savefig</span><span class="p">(</span><span class="n">image_path</span><span class="p">(</span><span class="n">fig_id</span><span class="p">)</span> <span class="o">+</span> <span class="s2">&quot;.png&quot;</span><span class="p">,</span> <span class="nb">format</span><span class="o">=</span><span class="s1">&#39;png&#39;</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">R2</span><span class="p">(</span><span class="n">y_data</span><span class="p">,</span> <span class="n">y_model</span><span class="p">):</span>
<span class="k">return</span> <span class="mi">1</span> <span class="o">-</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span> <span class="o">-</span> <span class="n">y_model</span><span class="p">)</span> <span class="o">**</span> <span class="mi">2</span><span class="p">)</span> <span class="o">/</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span> <span class="o">-</span> <span class="n">np</span><span class="o">.</span><span class="n">mean</span><span class="p">(</span><span class="n">y_data</span><span class="p">))</span> <span class="o">**</span> <span class="mi">2</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">MSE</span><span class="p">(</span><span class="n">y_data</span><span class="p">,</span><span class="n">y_model</span><span class="p">):</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">size</span><span class="p">(</span><span class="n">y_model</span><span class="p">)</span>
<span class="k">return</span> <span class="n">np</span><span class="o">.</span><span class="n">sum</span><span class="p">((</span><span class="n">y_data</span><span class="o">-</span><span class="n">y_model</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span><span class="o">/</span><span class="n">n</span>
<span class="n">infile</span> <span class="o">=</span> <span class="nb">open</span><span class="p">(</span><span class="n">data_path</span><span class="p">(</span><span class="s2">&quot;EoS.csv&quot;</span><span class="p">),</span><span class="s1">&#39;r&#39;</span><span class="p">)</span>
<span class="c1"># Read the EoS data as csv file and organized into two arrays with density and energies</span>
<span class="n">EoS</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">read_csv</span><span class="p">(</span><span class="n">infile</span><span class="p">,</span> <span class="n">names</span><span class="o">=</span><span class="p">(</span><span class="s1">&#39;Density&#39;</span><span class="p">,</span> <span class="s1">&#39;Energy&#39;</span><span class="p">))</span>
<span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">]</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">to_numeric</span><span class="p">(</span><span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">],</span> <span class="n">errors</span><span class="o">=</span><span class="s1">&#39;coerce&#39;</span><span class="p">)</span>
<span class="n">EoS</span> <span class="o">=</span> <span class="n">EoS</span><span class="o">.</span><span class="n">dropna</span><span class="p">()</span>
<span class="n">Energies</span> <span class="o">=</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Energy&#39;</span><span class="p">]</span>
<span class="n">Density</span> <span class="o">=</span> <span class="n">EoS</span><span class="p">[</span><span class="s1">&#39;Density&#39;</span><span class="p">]</span>
<span class="c1"># The design matrix now as function of various polytrops</span>
<span class="n">X</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">((</span><span class="nb">len</span><span class="p">(</span><span class="n">Density</span><span class="p">),</span><span class="mi">5</span><span class="p">))</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span><span class="o">**</span><span class="p">(</span><span class="mf">2.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">2</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span><span class="o">**</span><span class="p">(</span><span class="mf">4.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="n">X</span><span class="p">[:,</span><span class="mi">4</span><span class="p">]</span> <span class="o">=</span> <span class="n">Density</span><span class="o">**</span><span class="p">(</span><span class="mf">5.0</span><span class="o">/</span><span class="mf">3.0</span><span class="p">)</span>
<span class="c1"># We split the data in test and training data</span>
<span class="n">X_train</span><span class="p">,</span> <span class="n">X_test</span><span class="p">,</span> <span class="n">y_train</span><span class="p">,</span> <span class="n">y_test</span> <span class="o">=</span> <span class="n">train_test_split</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">Energies</span><span class="p">,</span> <span class="n">test_size</span><span class="o">=</span><span class="mf">0.2</span><span class="p">)</span>
<span class="c1"># matrix inversion to find beta</span>
<span class="n">beta</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linalg</span><span class="o">.</span><span class="n">inv</span><span class="p">(</span><span class="n">X_train</span><span class="o">.</span><span class="n">T</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">X_train</span><span class="p">))</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">X_train</span><span class="o">.</span><span class="n">T</span><span class="p">)</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">y_train</span><span class="p">)</span>
<span class="c1"># and then make the prediction</span>
<span class="n">ytilde</span> <span class="o">=</span> <span class="n">X_train</span> <span class="o">@</span> <span class="n">beta</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Training R2&quot;</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">R2</span><span class="p">(</span><span class="n">y_train</span><span class="p">,</span><span class="n">ytilde</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Training MSE&quot;</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">MSE</span><span class="p">(</span><span class="n">y_train</span><span class="p">,</span><span class="n">ytilde</span><span class="p">))</span>
<span class="n">ypredict</span> <span class="o">=</span> <span class="n">X_test</span> <span class="o">@</span> <span class="n">beta</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Test R2&quot;</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">R2</span><span class="p">(</span><span class="n">y_test</span><span class="p">,</span><span class="n">ypredict</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Test MSE&quot;</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">MSE</span><span class="p">(</span><span class="n">y_test</span><span class="p">,</span><span class="n">ypredict</span><span class="p">))</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="exercises">
<h2>Exercises<a class="headerlink" href="#exercises" title="Permalink to this headline"></a></h2>
<p>Here are three possible exercises for week 34</p>
</div>
<div class="section" id="exercise-1-setting-up-various-python-environments">
<h2>Exercise 1: Setting up various Python environments<a class="headerlink" href="#exercise-1-setting-up-various-python-environments" title="Permalink to this headline"></a></h2>
<p>The first exercise here is of a mere technical art. We want you to have</p>
<ul class="simple">
<li><p>git as a version control software and to establish a user account on a provider like GitHub. Other providers like GitLab etc are equally fine. You can also use the University of Oslo <a class="reference external" href="https://www.uio.no/tjenester/it/maskin/filer/versjonskontroll/github.html">GitHub facilities</a>.</p></li>
<li><p>Install various Python packages</p></li>
</ul>
<p>We will make extensive use of Python as programming language and its
myriad of available libraries. You will find
IPython/Jupyter notebooks invaluable in your work. You can run <strong>R</strong>
codes in the Jupyter/IPython notebooks, with the immediate benefit of
visualizing your data. You can also use compiled languages like C++,
Rust, Fortran etc if you prefer. The focus in these lectures will be
on Python.</p>
<p>If you have Python installed (we recommend Python3) and you feel
pretty familiar with installing different packages, we recommend that
you install the following Python packages via <strong>pip</strong> as</p>
<ol class="simple">
<li><p>pip install numpy scipy matplotlib ipython scikit-learn sympy pandas pillow</p></li>
</ol>
<p>For <strong>Tensorflow</strong>, we recommend following the instructions in the text of
<a class="reference external" href="http://shop.oreilly.com/product/0636920052289.do">Aurelien Geron, Hands‑On Machine Learning with Scikit‑Learn and TensorFlow, O’Reilly</a></p>
<p>We will come back to <strong>tensorflow</strong> later.</p>
<p>For Python3, replace <strong>pip</strong> with <strong>pip3</strong>.</p>
<p>For OSX users we recommend, after having installed Xcode, to
install <strong>brew</strong>. Brew allows for a seamless installation of additional
software via for example</p>
<ol class="simple">
<li><p>brew install python3</p></li>
</ol>
<p>For Linux users, with its variety of distributions like for example the widely popular Ubuntu distribution,
you can use <strong>pip</strong> as well and simply install Python as</p>
<ol class="simple">
<li><p>sudo apt-get install python3 (or python for Python2.7)</p></li>
</ol>
<p>If you don’t want to perform these operations separately and venture
into the hassle of exploring how to set up dependencies and paths, we
recommend two widely used distrubutions which set up all relevant
dependencies for Python, namely</p>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.anaconda.com/">Anaconda</a>,</p></li>
</ul>
<p>which is an open source
distribution of the Python and R programming languages for large-scale
data processing, predictive analytics, and scientific computing, that
aims to simplify package management and deployment. Package versions
are managed by the package management system <strong>conda</strong>.</p>
<ul class="simple">
<li><p><a class="reference external" href="https://www.enthought.com/product/canopy/">Enthought canopy</a></p></li>
</ul>
<p>is a Python
distribution for scientific and analytic computing distribution and
analysis environment, available for free and under a commercial
license.</p>
<p>We recommend using <strong>Anaconda</strong> if you are not too familiar with setting paths in a terminal environment.</p>
</div>
<div class="section" id="exercise-2-making-your-own-data-and-exploring-scikit-learn">
<h2>Exercise 2: making your own data and exploring scikit-learn<a class="headerlink" href="#exercise-2-making-your-own-data-and-exploring-scikit-learn" title="Permalink to this headline"></a></h2>
<p>We will generate our own dataset for a function <span class="math notranslate nohighlight">\(y(x)\)</span> where <span class="math notranslate nohighlight">\(x \in [0,1]\)</span> and defined by random numbers computed with the uniform distribution. The function <span class="math notranslate nohighlight">\(y\)</span> is a quadratic polynomial in <span class="math notranslate nohighlight">\(x\)</span> with added stochastic noise according to the normal distribution <span class="math notranslate nohighlight">\(\cal {N}(0,1)\)</span>.
The following simple Python instructions define our <span class="math notranslate nohighlight">\(x\)</span> and <span class="math notranslate nohighlight">\(y\)</span> values (with 100 data points).</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">rand</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="mf">2.0</span><span class="o">+</span><span class="mi">5</span><span class="o">*</span><span class="n">x</span><span class="o">*</span><span class="n">x</span><span class="o">+</span><span class="mf">0.1</span><span class="o">*</span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">randn</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">1</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<ol class="simple">
<li><p>Write your own code (following the examples under the <a class="reference external" href="https://compphysics.github.io/MachineLearning/doc/LectureNotes/_build/html/chapter1.html">regression notes</a>) for computing the parametrization of the data set fitting a second-order polynomial.</p></li>
<li><p>Use thereafter <strong>scikit-learn</strong> (see again the examples in the regression slides) and compare with your own code.</p></li>
<li><p>Using scikit-learn, compute also the mean square error, a risk metric corresponding to the expected value of the squared (quadratic) error defined as</p></li>
</ol>
<div class="math notranslate nohighlight">
\[
MSE(\boldsymbol{y},\boldsymbol{\tilde{y}}) = \frac{1}{n}
\sum_{i=0}^{n-1}(y_i-\tilde{y}_i)^2,
\]</div>
<p>and the <span class="math notranslate nohighlight">\(R^2\)</span> score function.
If <span class="math notranslate nohighlight">\(\tilde{\boldsymbol{y}}_i\)</span> is the predicted value of the <span class="math notranslate nohighlight">\(i-th\)</span> sample and <span class="math notranslate nohighlight">\(y_i\)</span> is the corresponding true value, then the score <span class="math notranslate nohighlight">\(R^2\)</span> is defined as</p>
<div class="math notranslate nohighlight">
\[
R^2(\boldsymbol{y}, \tilde{\boldsymbol{y}}) = 1 - \frac{\sum_{i=0}^{n - 1} (y_i - \tilde{y}_i)^2}{\sum_{i=0}^{n - 1} (y_i - \bar{y})^2},
\]</div>
<p>where we have defined the mean value of <span class="math notranslate nohighlight">\(\boldsymbol{y}\)</span> as</p>
<div class="math notranslate nohighlight">
\[
\bar{y} = \frac{1}{n} \sum_{i=0}^{n - 1} y_i.
\]</div>
<p>You can use the functionality included in scikit-learn. If you feel for it, you can use your own program and define functions which compute the above two functions.
Discuss the meaning of these results. Try also to vary the coefficient in front of the added stochastic noise term and discuss the quality of the fits.</p>
</div>
<div class="section" id="exercise-3-split-data-in-test-and-training-data">
<h2>Exercise 3: Split data in test and training data<a class="headerlink" href="#exercise-3-split-data-in-test-and-training-data" title="Permalink to this headline"></a></h2>
<p>In this exercise we want you to to compute the MSE for the training
data and the test data as function of the complexity of a polynomial,
that is the degree of a given polynomial.</p>
<p>The aim is to reproduce Figure 2.11 of <a class="reference external" href="https://github.com/CompPhysics/MLErasmus/blob/master/doc/Textbooks/elementsstat.pdf">Hastie et al</a>.</p>
<p>Our data is defined by <span class="math notranslate nohighlight">\(x\in [-3,3]\)</span> with a total of for example <span class="math notranslate nohighlight">\(n=100\)</span> data points. You should try to vary the number of data points <span class="math notranslate nohighlight">\(n\)</span> in your analysis.</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">seed</span><span class="p">()</span>
<span class="n">n</span> <span class="o">=</span> <span class="mi">100</span>
<span class="c1"># Make data set.</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">linspace</span><span class="p">(</span><span class="o">-</span><span class="mi">3</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="n">n</span><span class="p">)</span><span class="o">.</span><span class="n">reshape</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="o">-</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span> <span class="o">+</span> <span class="mf">1.5</span> <span class="o">*</span> <span class="n">np</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="o">-</span><span class="p">(</span><span class="n">x</span><span class="o">-</span><span class="mi">2</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span><span class="o">+</span> <span class="n">np</span><span class="o">.</span><span class="n">random</span><span class="o">.</span><span class="n">normal</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">,</span> <span class="n">x</span><span class="o">.</span><span class="n">shape</span><span class="p">)</span>
</pre></div>
</div>
</div>
</div>
<p>where <span class="math notranslate nohighlight">\(y\)</span> is the function we want to fit with a given polynomial.</p>
<p><strong>a)</strong>
Write a first code which sets up a design matrix <span class="math notranslate nohighlight">\(X\)</span> defined by a fifth-order polynomial and split your data set in training and test data.</p>
<p><strong>b)</strong>
Write thereafter (using either <strong>scikit-learn</strong> or your matrix inversion code using for example <strong>numpy</strong>)
and perform an ordinary least squares fitting and compute the mean squared error for the training data and the test data. These calculations should apply to a model given by a fifth-order polynomial.</p>
<p><strong>c)</strong>
Add now a model which allows you to make polynomials up to degree <span class="math notranslate nohighlight">\(15\)</span>. Perform a standard OLS fitting of the training data and compute the MSE for the training and test data and plot both test and training data MSE as functions of the polynomial degree. Compare what you see with Figure 2.11 of Hastie et al. Comment your results. For which polynomial degree do you find an optimal MSE (smallest value)?</p>
</div>
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