# CSCI 5525: Machine Learning Homework 1 solution

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1. (15 points) The expected loss of a function f(x) in modeling y using loss function `(f(x), y)
is given by
E(x,y)
[`(f(x), y)] = Z
x
Z
y
`(f(x), y)p(x, y)dydx =
Z
x
Z
y
`(f(x), y)p(y|x)dy
p(x)dx .
(a) (7 points) What is the optimal f(x) when `(f(x), y) = (f(x) − y)
2
.
(b) (8 points) What is the optimal f(x) when `(f(x), y) = |f(x) − y|, where | · | represents
absolute value.
2. (10 points) A generalization of the least squares problem adds an affine function to the least
squares objective,
min
w
kAw − bk
2
2 + c
>w + d
where A ∈ R
m×n
, w ∈ R
n
, b ∈ R
m, c ∈ R
n
, d ∈ R. Assume the columns of A are linearly
independent. This generalized problem can be solved by reducing it to a standard least
squares problem, using a trick called completing the square.
Show that the objective of the problem above can be expressed in the form
kAw − bk
2
2 + c
>w + d = kAw − b + fk
2
2 + g
where f ∈ R
m, g ∈ R. Then solve the generalized least squares problem by finding the w that
minimizes kAw − (b − f)k
2
2
.
Programming assignments: The next two problems involve programming. We will be considering two datasets for these assignments:
(a) Boston: The Boston housing dataset comes prepackaged with scikit-learn. The dataset has
506 data points, 13 features, and 1 target (response) variable. You can find more information
While the original dataset is for a regression problem, we will create two classification datasets
for the homework. Note that you only need to work with the target t to create these classification dataset, the data X should not be changed.
First, load the dataset in with the following commands:
import s kl e a r n a s sk
X, t = sk . d a t a s e t s . l o a d b o s t o n ( r e t u r n X y=True )
Then, create the two following data sets.
i. Boston50: Let τ50 be the median (50th percentile) over all t (response) values. Create a
2-class classification problem such that one class corresponds to label y = 1 if t ≥ τ50 and
the other class corresponds to label y = 0 if t < τ50. By construction, note that the class
priors will be p(y = 1) ≈
1
2
, p(y = 0) ≈
1
2
.
ii. Boston75: Let τ75 be the 75th percentile over all t (response) values. Create a 2-class
classification problem such that one class corresponds to label y = 1 if t ≥ τ75 and the
other class corresponds to label y = 0 if t < τ75. By construction, note that the class priors
will be p(y = 1) ≈
1
4
, p(y = 0) ≈
3
4
.
(b) Digits: The digits dataset comes prepackaged with scikit-learn. The dataset has 1797 data
points, 64 features, and 10 classes corresponding to ten numbers 0, 1, . . . , 9. You can find more
information about the dataset here: https://scikit-learn.org/stable/modules/generated/
3. (35 points) In this problem, we consider Fisher’s linear discriminant analysis (LDA) for
this problem. Implement1
, train, and evaluate the following classifiers using 10-fold crossvalidation:
(i) (15 points) For the Boston50 dataset, apply LDA in the general case, i.e., compute
both the between-class and within-class covariance matrices SB and SW , respectively,
from the training data, project the data onto R (one dimension), and then find a suitable threshold (one that minimizes classification error) to classify the training samples
correctly.
(ii) (20 points) For the Digits dataset, apply LDA in the general case, i.e., compute SB
and SW from the data, project the data to R
2
(two dimensions), then use bi-variate
Gaussian generative modeling to do 10-class classification, i.e., estimate and use class
priors πk and parameters (µk, Σk), k = 1, . . . , 10.
You will have to submit (a) summary of methods and results report and (b) code for
each algorithm:
(a) Summary of methods and results: Briefly describe the approaches in (i) and (ii)
above, along with relevant equations. Also, report the training and test set error rates
and standard deviations from 10-fold cross validation for the methods on the datasets.
(b) Code: For part (i), you will have to submit code for LDA1dThres(num crossval) (main
file). This main file has input: the number of folds for cross-validation, and output: the
training and test set error rates and standard deviations printed to the terminal (stdout).
For part (ii), you will have to submit code for LDA2dGaussGM(num crossval), with all
other guidelines staying the same.
1You must implement all algorithms in this homework from scratch; you cannot use toolboxes like scikit-learn.
4. (40 points) In this problem, the goal is to evaluate the results reported in the paper “On
Discriminative vs. Generative Classifiers: A comparison of logistic regression and naive Bayes”
by A. Ng and M. Jordan2
, using the Boston50, Boston75, and Digits datasets. Implement,
train, and evaluate two classifiers:
(i) (20 points) Logistic regression (LR), and
(ii) (20 points) Naive-Bayes with marginal Gaussian distributions (GNB)
on all three datasets. Evaluation will be done using 10 random class-specific 80-20 traintest splits, i.e., for each class, pick 80% of the data at random for training, train a classifier
using training data from all classes, use the remaining 20% of the data from each class as
testing, and repeat this process 10 times. We will be creating a learning curve, similar to the
You will have to submit (a) summary of methods and results report and (b) code for
each algorithm:
(a) Summary of methods and results: Briefly describe the approaches in (i) and (ii)
above, along with (iterative) equations for parameter estimation. Clearly state which
method you are using for logistic regression. For each dataset and method, create a plot
of the test set error rate illustrating the relative performance of the two methods with
increasing number of training points (see instructions below). The plots will be similar in
spirit to Figure 1 in the Ng-Jordan paper, along with error-bars with standard deviation
of the errors.
Instructions for plots: Your plots will be based on 10 random 80-20 train-test splits.
For each split, we will always evaluate results on the same test set (20% of the data),
while using increasing percentages of the training set (80% of the data) for training. In
particular, we will use the following training set percentages: [10 25 50 75 100], so
that for each 80-20 split, we use 10%, 25%, all the way up to 100% of the training set for
training, and always report results on the same test set. We will repeat the process 10
times, and plot the mean and standard deviation (as error bars) of the test set errors for
different training set percentages.
(b) Code: For logistic regression, you will have to submit code for logisticRegression(num splits,
train percent). This main file has input: the number of 80-20 train-test splits for evaluation, (3) and a vector containing percentages of training data to be used for training
(use [10 25 50 75 100] for the plots), and output: test set error rates for each training
set percent printed to the terminal (stdout). The test set error rates should include both
the error rates for each split for each training set percentage as well as the mean of the
test set error rates across all splits for each training set percentage (print the mean error
rates at the end).
For naive Bayes, you will have to submit code for naiveBayesGaussian(num splits,
train percent), with all other guidelines staying the same.
Additional instructions: Code can only be written in Python 3.6+; no other programming
languages will be accepted. One should be able to execute all programs from the Python command
2
https://ai.stanford.edu/~ang/papers/nips01-discriminativegenerative.pdf
Each function must take the inputs in the order specified in the problem and display the textual
output via the terminal and plots/figures should be included in the report.
For each part, you can submit additional files/functions (as needed) which will be used by the
main file. In your code, you cannot use machine learning libraries such as those available from
scikit-learn for learning the models or for cross-validation. However, you may use libraries for basic
matrix computations. Put comments in your code so that one can follow the key parts and steps
Your code must be runnable on a CSE lab machine (e.g., csel-kh1260-01.cselabs.umn.edu).
One option is to SSH into a machine. Learn about SSH at these links: https://cseit.umn.edu/
and https://cseit.umn.edu/knowledge-help/remote-linux-applications-over-ssh.
Instructions
Follow the rules strictly. If we cannot run your code, you will not get any credit.
• Things to submit
1. hw1.pdf: A document which contains the solutions to Problems 1, 2, 3, and 4, which
including the summary of methods and results.
2. LDA1dThres and LDA2dGaussGM: Code for Problem 3.
3. logisticRegression and naiveBayesGaussian: Code for Problem 4.