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Classification with Nearest Neighbours Homework 1

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CSC311
Homework 1

Submission: You need to submit three files through MarkUs1
:
• Your answers to Questions 1, 2, 3, and 4, and outputs requested for Question 1 and 4, as
a PDF file titled hw1_writeup.pdf. You can produce the file however you like (e.g. LATEX,
Microsoft Word, scanner), as long as it is readable.
• Your code for Question 1, as the Python file hw1_q1_code.py. This should contain the
functions load_data and select_knn_model.
• Your code for Question 4, as the Python file hw1_q4_code.py. This should contain the all
functions such as split_data and train_model.
Neatness Point: One point will be given for neatness. You will receive this point as long as we
don’t have a hard time reading your solutions or understanding the structure of your code.
Late Submission: 10% of the marks will be deducted for each day late, up to a maximum of 3
days. After that, no submissions will be accepted.
Computing: To install Python and required libraries, see the instructions on the course web page.
Homeworks are individual work. See the Course Information handout2
for detailed policies.
1. [9pts] Classification with Nearest Neighbours. In this question, you will use the
scikit-learn’s KNN classifer to classify real vs. fake news headlines. The aim of this question is for you to read the scikit-learn API and get comfortable with training/validation
splits.
We will use a dataset of 1298 “fake news” headlines (which mostly include headlines of articles
classified as biased, etc.) and 1968 “real” news headlines, where the “fake news” headlines
are from https://www.kaggle.com/mrisdal/fake-news/data and “real news” headlines are
from https://www.kaggle.com/therohk/million-headlines. The data were cleaned by removing words from titles not part of the headlines, removing special characters and restricting
real news headlines after October 2016 using the word ”trump”. The cleaned data are available as clean_real.txt and clean_fake.txt in hw1_data.zip on the course webpage. It
is expected that you use these cleaned data sources for this assignment.
You will build a KNN classifier to classify real vs. fake news headlines. Instead of coding the
KNN yourself, you will do what we normally do in practice — use an existing implementation.
You should use the KNeighborsClassifier included in sklearn. Note that figuring out
how to use this implementation, its corresponding attributes and methods is a part of the
assignment.
(a) [2pts] Write a function load_data which loads the data, preprocesses it using a CountVectorizer
(http://scikit-learn.org/stable/modules/classes.html#module-sklearn.feature_
extraction.text), and splits the entire dataset randomly into 70% training, 15% validation, and 15% test examples.
1The MarkUs URL will be announced soon, once the enrolled students are given access.
2
http://www.cs.toronto.edu/~rgrosse/courses/csc311_f20/syllabus.pdf
1
CSC311 Fall 2020 Homework 1
(b) [5pts] Write a function select_knn_model that uses a KNN classifer to classify between
real vs. fake news. Use a range of k values between 1 to 20 and compute both training
and validation errors, leaving other arguments to KNeighborsClassifier at their default
values. Generate a plot showing the training and validation accuracy for each k. Report
the generated plot in your write-up. Choose the model with the best validation accuracy
and report its accuracy on the test data.
(c) [2pts] Repeat part (b), passing argument metric=‘cosine’ to the KNeighborsClassifier.
You should observe an improvement in accuracy. How does metric=‘cosine’ compute
the distance between data points, and why might this perform better than the Euclidean
metric (default) here? Hint: consider the dataset [‘cat’, ‘bulldozer’, ‘cat cat cat’].
2. [8pts] Regularized Linear Regression. For this problem, we will use the linear regression
model from the lecture:
y =
X
D
j=1
wjxj + b.
In lecture, we saw that regression models with too much capacity can overfit the training data
and fail to generalize. We also saw that one way to improve generalization is regularization:
adding a term to the cost function which favors some explanations over others. For instance,
we might prefer that weights not grow too large in magnitude. We can encourage them to
stay small by adding a penalty:
R(w) = λ
2
w>w =
λ
2
X
D
j=1
w
2
j
to the cost function, for some λ > 0. It is also possible to apply different regularization
penalties in each dimension. The formulation would be:
J
β
reg(w) = 1
2N
X
N
i=1

y
(i) − t
(i)
2
| {z }
=J
+
1
2
X
D
j=1
βjw
2
j
| {z }
=R
,
where i indexes the data points, βj ≥ 0 for all j, and J is the same squared error cost
function from lecture. Note that in this formulation, there is no regularization penalty on
the bias parameter. Also note that when βj = 0, you don’t apply any regularization on j-th
dimension. For this question, show your work in detail as most points are allocated in showing
how you obtained your answer.
(a) [3pts] Determine the gradient descent update rules for the regularized cost function
J
β
reg. Your answer should have the form:
wj ← · · ·
b ← · · ·
This form of regularization is sometimes called “weight decay”. Based on this update
rule, why do you suppose that is?
2
CSC311 Fall 2020 Homework 1
(b) [3pts] It’s also possible to solve the regularized regression problem directly by setting
the partial derivatives equal to zero. In this part, for simplicity, we will drop the bias
term from the model, so our model is:
y =
X
D
j=1
wjxj .
It is possible to derive a system of linear equations of the following form for J
β
reg:
∂J
β
reg
∂wj
=
X
D
j
0=1
Ajj0wj
0 − cj = 0.
Determine formulas for Ajj0 and cj .
(c) [2pts] Based on your answer to part (b), determine formulas for A and c, and derive a
closed-form solution for the parameter w. Note that, as usual, the inputs are organized
into a design matrix X with one row per training example.
3. [4pts] Loss Functions. Suppose we have a prediction problem where the target t corresponds to an angle, measure in radians. A reasonable loss function we might use is:
L(y, t) = 1 − cos(y − t).
Suppose we make predictions using a linear model:
y = w>x + b.
As usual, the cost is the average loss over the training set:
J =
1
N
X
N
i=1
L(y
(i)
, t(i)
).
Derive a sequence of vectorized mathematical expressions for the gradients of the cost with
respect to w and b. Recall that the inputs are organized into a design matrix X with one row
per training example. The expressions should be something you can translate into a Python
program without requiring a for-loop. Your answer should look like:
y = · · ·
∂J
∂y
= · · ·
∂J
∂w
= · · ·
∂J
∂b = · · ·
You can use sin(A) to denote the sin function applied elementwise to A. Remember that
∂J /∂w denotes the gradient vector,
∂J
∂w
=


∂J
∂w1
.
.
.
∂J
∂wD


3
CSC311 Fall 2020 Homework 1
4. [9pts] Cross Validation. In this problem, you will write a function that performs K-fold
cross validation (CV) procedure to tune the penalty parameter λ in Ridge regression. CV
procedure is one of the most commonly used methods for tuning hyperparameters. In this
question, you shouldn’t use the package scikit-learn to perform CV. You should implement
all of the below functions yourself. You may use numpy and scipy for basic math operations
such as linear algebra, sampling etc.
In lecture, we learned training, test, and validation procedures which assumes that you have
enough data and you can set aside a validation set and a test set to use it for assessing the
performance of your machine learning algorithm. However in practice, this may be problematic since we may not have enough data. A remedy to this issue is K-fold cross validation
which uses a part of the available data to fit the model, and a different part to test it. K-fold
CV procedure splits the data into K equal-sized parts; for example, when K = 5, the scenario
looks like this:
Figure 1: credit: Elements of Statistical Learning
(i) We first set aside a test dataset and never use it until the training and parameter tuning
procedures are complete. We will use this data for final evaluation. In this question,
test data is provided to you as a separate dataset.
(ii) CV error estimates the test error of a particular hyperparameter choice. For a particular
hyperparameter value, we split the training data into K blocks (See the figure), and for
k = 1, 2, …, K we use the k-th block for validation and the remaining K −1 blocks are for
training. Therefore, we train and validate our algorithm K times. Our CV estimate for
the test error for that particular hyperparameter choice is given by the average validation
error across these K blocks.
(iii) We repeat the above procedure for several hyperparameter choices and choose the one
that provides us with the smalles CV error (which is an estimate for the test error).
Below, we will code the above procedure for tuning the regularization parameter in linear
regression which is a hyperparameter. Your cross_validation function will rely on 6 short
functions which are defined below along with their variables.
• data is a variable and refers to a (t, X) pair (can be test, training, or validation) where
t is the target (response) vector, and X is the feature matrix.
• model is a variable and refers to the coefficients of the trained model, i.e. w.
• data_shf = shuffle_data(data) is a function and takes data as an argument and
returns its randomly permuted version along the samples. Here, we are considering a
uniformly random permutation of the training data. Note that t and X need to be
permuted the same way preserving the target-feature pairs.
4
CSC311 Fall 2020 Homework 1
• data_fold, data_rest = split_data(data, num_folds, fold) is a function that takes
data, number of partitions as num_folds and the selected partition fold as its arguments
and returns the selected partition (block) fold as data_fold, and the remaining data
as data_rest. If we consider 5-fold cross validation, num_folds=5, and your function
splits the data into 5 blocks and returns the block fold (∈ {1, 2, 3, 4, 5}) as the validation
fold and the remaining 4 blocks as data_rest. Note that data_rest ∪ data_fold =
data, and data_rest ∩ data_fold = ∅.
• model = train_model(data, lambd) is a function that takes data and lambd as its
arguments, and returns the coefficients of ridge regression with penalty level λ. For
simplicity, you may ignore bias parameter for this question. You may use your solution
from question 3.
• predictions = predict(data, model) is a function that takes data and model as its
arguments, and returns the predictions based on data and model.
• error = loss(data, model) is a function which takes data and model as its arguments
and returns the average squared error loss based on model. This means if data is
composed of t ∈ R
N and X ∈ R
N×D, and model is w, then the return value is kXw −
tk
2/2N.
• cv_error = cross_validation(data, num_folds, lambd_seq) is a function that takes
the training data, number of folds num_folds, and a sequence of λ’s as lambd_seq as
its arguments and returns the cross validation error across all λ’s. Take lambd_seq as
evenly spaced 50 numbers over the interval (0.00005, 0.005). This means cv_error will
be a vector of 50 errors corresponding to the values of lambd_seq. Your function will
look like:
data = s h u f f l e d a t a ( data )
for i = 1 , 2 , . . . , l e n g t h ( lambd seq )
lambd = lambd seq ( i )
c v l o s s l m d = 0 .
for f o l d = 1 , 2 , . . . , num f old s
v al c v , t r a i n c v = s p l i t d a t a ( data , num folds , f o l d )
model = t r ai n m o d el ( t r ai n c v , lambd )
c v l o s s l m d += l o s s ( v al c v , model )
c v e r r o r ( i ) = c v l o s s l m d / num f old s
return c v e r r o r
(a) [0pts] Download the dataset from the course webpage hw1_data.zip and place and
extract in your working directory, or note its location file_path. For example, file path
could be /Users/yourname/Desktop/
• In Python:
import numpy a s np
d a t a t r a i n = { ’X ’ : np . gen f r om t x t ( ’ d a t a t r ai n X . c s v ’ , d e l i m i t e r=’ , ’ ) ,
’ t ’ : np . gen f r om t x t ( ’ d a t a t r a i n y . c s v ’ , d e l i m i t e r=’ , ’ )}
d a t a t e s t = { ’X ’ : np . gen f r om t x t ( ’ d a t a t e s t X . c s v ’ , d e l i m i t e r=’ , ’ ) ,
’ t ’ : np . gen f r om t x t ( ’ d a t a t e s t y . c s v ’ , d e l i m i t e r=’ , ’ )}
(b) [5pts] Write the above 6 functions, and identify the correct order and arguments to do
cross validation.
5
CSC311 Fall 2020 Homework 1
(c) [2pts] Report the training and test errors corresponding to each λ in lambd_seq. This
part does not use the cross_validation function but you may find the other functions
helpful.
(d) [2pts] Plot training error, test error, and 5-fold and 10-fold cross validation errors on
the same plot for each value in lambd_seq. What is the value of λ proposed by your
cross validation procedure? Comment on the shapes of the error curves.
6
CSC311 Fall 2020 Homework 1
We have made the following question optional, because some students are taking probability theory concurrently this year and don’t yet have the background to solve this question.
However, you may find it is valuable to try to solve it.
5. [Optional] Nearest Neighbours and the Curse of Dimensionality. In this question,
you will verify the claim from lecture that “most” points in a high-dimensional space are far
away from each other, and also approximately the same distance. There is a very neat proof
of this fact which uses the properties of expectation and variance. You may wish to review
the Tutorial 1 slides3
, or the Metacademy resources4
.
(a) [0pts] Consider two independent univariate random variables X and Y sampled uniformly from the unit interval [0, 1]. Determine the expectation and variance of the
random variable Z, defined as the squared distance Z = (X − Y )
2
.
(b) [0pts] Now suppose we sample two points independently from a unit cube in d dimensions. Observe that each coordinate is sampled independently from [0, 1], i.e. we can view
this as sampling random variables X1, . . . , Xd, Y1, . . . , Yd independently from [0, 1]. The
squared Euclidean distance can be written as R = Z1 + · · · + Zd, where Zi = (Xi − Yi)
2
.
Using the properties of expectation and variance, determine E[R] and Var[R]. You may
give your answer in terms of the dimension d, and E[Z] and Var[Z] (the answers from
part (a)).
(c) [0pts] Based on your answer to part (b), compare the mean and standard deviation of R
to the maximum possible squared Euclidean distance (i.e. the distance between opposite
corners of the cube). Why does this support the claim that in high dimensions, “most
points are far away, and approximately the same distance”?
3
http://www.cs.toronto.edu/~rgrosse/courses/csc311_f20/tutorials/tut01/tut01.pdf
4
https://metacademy.org/graphs/concepts/expectation_and_variance
7

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