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Problems:

1. (30 points) L2-distances through Matrix Operations

Many machine learning algorithms access their input data primarily through pairwise distances. It is therefore important that we have a fast function that computes pairwise (Euclidean) distances of input vectors. Assume we have n data vectors ~x1, . . . , ~xn ∈ R

d and m

vectors ~z1, . . . , zm ∈ R

d

. And let us define two matrices X = [~x1, . . . , ~xn] ∈ R

d×n

, where the

i

th column is a vector ~xi and similarly Z = [~z1, . . . , ~zm]. Our distance function takes as input

the two matrices X and Z and outputs a matrix D ∈ R

n×m, where

Dij =

q

(~xi − ~zj )>(~xi − ~zj ).

The key to efficient programming in Matlab and machine learning in general is to think

about it in terms of mathematics, and not in terms of loops.

(a) Show that the Gram matrix G (aka inner-product matrix) with

Gij = ~x>

i ~zj

can be expressed in terms of a pure matrix multiplication.

(b) Let us define two new matrices S, R ∈ R

n×m

Sij = ~x>

i ~xi

, Rij = ~z>

j ~zj .

Show that the squared-euclidean distance matrix D2 ∈ R

n×m, defined as

D2

ij = (~xi − ~zj )

2

,

can be expressed as a linear combination of the matrix S, G, R. (Hint: It might help

to first express D2

ij in terms of inner-products.) Further, think about and answer the

following questions:

• What mathematical property do all entries of D satisfy?

• What are the diagonal entries of D assuming that X = Z (we want the distance

among all data points in X)?

• What do you need to do to obtain the true Euclidean distance matrix D?

Remember the answers to all of these questions and ensure that your implementation

in the next part satisfies all of them.

(c) Remember the slow function l2distanceSlow.m using nested for loops to compute

the L2-distance. You find it in the hw5 folder in your SVN repository. Read through the

code carefully and make sure you understand it. It is perfectly correct and will produce

the correct result … eventually. To see what is wrong, run the following program in the

MATLAB console:

>> X=rand(100,10000);

>> Z=rand(100,2000);

>> tic;D=l2distanceSlow(X,Z);toc

This code defines some random data in X and Z and computes the corresponding distance matrix D. The tic, toc statements time how long this takes. On my laptop it

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takes over a minute. This is way too slow! If I were to compute the distances between

larger training and testing sets or higher dimensional data points (imagine for instance

images with millions of pixels), it would take days! The problem is that the distance

function uses a programming style that is well suited for C++ or Java, but not MATLAB!

Implement the function l2distance.m (a stub file is in your SVN repository), which

computes the Euclidean distance matrix D without a single loop. Remember all your

answers to the previous parts of this problem and test your implementation to make

sure ot incorporates all of those. Once you are sure that your implementation is correct,

time the distance function again:

>> X=rand(100,10000);

>> Z=rand(100,2000);

>> tic;D=l2distance(X,Z);toc

How much faster is your code now? With your new implementation you should easily be able to compute the distances between many more vectors. Call the function

l2dist_tictoc to see how many distance computations you can perform within one

second. With the loopy implementations, the same computation might have taken you

several days.

Submit your implementation by committing your function l2distance.m, and the

partners.txt file to the hw5 folder in your SVN repository.

2. (20 points) k-Nearest-Neighbor

In this problem, you are going to look at a small dataset to understand various properties of

k-NN better. Suppose there is a set of points on a two-dimensional plane from two different

classes. Below are the coordinates of all points.

Points in class red: (0, 1), (2, 3), (4, 4)

Points in class blue: (2, 0), (5, 2), (6, 3)

(a) Draw the k-nearest-neighbor decision boundary for k = 1. Remember that the decision

boundary is defined as the line where the classification of a test point changes. Use the

standard Euclidean distance between points to determine the nearest neighbors. Start

by plotting the points as a two-dimensional graph. Please use the corresponding colors

for points of each class (e.g, blue and red).

(b) If the y-coordinate of each point was multiplied by 5, what would happen to the k = 1

boundary (draw another picture)? Explain in at most two sentences how this effect

might cause problems when working with real data.

(c) The k-NN decision boundary for k = 3 is shown as below. Suppose now we have a

test point at (1, 2). How would it be classified under 3-NN? Given that you can modify

the 3-NN decision boundary by adding points to the training set in the diagram, what

is the minimum number of points that you need to add to change the classification at

(1, 2)? Show also where you need to add these points.

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(d) What is the testing complexity for one instance, e.g., how long does it take for k-NN

to classify one point? Assume your data has dimensionality d, you have n training

examples and use Euclidean distance. Assume also that you use a quick select implementation which gives you the k smallest elements of a list of length m in O(m).

Suggest two strategies to decrease the testing complexities. Provide the average and

worst case testing complexities for both strategies. At what cost are you gaining this

increased efficiency?

3. (50 points) Build a k-Nearest Neighbor Classifier

In this project, you will build a k-NN classifier for handwritten digits classification and face

recognition. The data for the experiments resides in the files faces.mat and digits.mat

(download them from Piazza resources; do NOT add those to your SVN repositories).

Data description:

xTr are the training vectors with labels yTr.

xTe are the testing vectors with labels yTe.

As a reminder, to predict the label or class of an image in xTe, we will look for the k-nearest

neighbors in xTr and predict a label based on their labels in yTr. For evaluation, we will

compare these labels against the true labels provided in yTe.

Data visualization:

You can visualize one of the faces by running

>> load faces

>> figure(1);

>> clf;

>> imagesc(reshape(xTr(:,1),38,31));

>> colormap gray;

Note that the shape 38 × 31 is the size of the image. We convert it from a flat vector form in

the dataset to a matrix which can be visualized. Note: If your images appear upside down,

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use imagesc(flipud(reshape(xTr(:,1),38,31))).

Implementation:

The following parts will walk you through the project. Your development will work out

best, if you finish these functions in this order. Note that your project will also be graded

based on efficiency. So, avoid tight loops at all cost. Stub files for all functions are in the

hw5 folder in your SVN repository.

(a) Implement the function findknn.m, which should find the k-nearest neighbors of a set

of vectors within a given training data set based on Euclidean distance (Hint: use your

l2distance function here). The call of

>> [I,D]=findknn(xTr,xTe,k);

should result in two matrices I and D, both of dimensions k ×n, where n is the number

of input vectors in xTe. The matrix I(i, j) is the index of the i

th nearest neighbor of

the vector xT e(:, j). So, for example, if we set i=I(1,3), then xTr(:,i) is the first

nearest neighbor of vector xTe(:,3). The second matrix D returns the corresponding

Euclidean distances. So, D(i, j) is the distance of xT e(:, j) to its i

th nearest neighbor.

(b) The function analyze.m should compute various evaluation metrics. The call of

>> result=analyze(kind,truth,preds);

should output the accuracy or absolute loss in variable result. The type of output

required can be specified in the input argument kind as strings “abs” or “acc”. The

input variables truth and pred should contain vectors of true and predicted labels

respectively. For example, the call

>> analyze(“acc”,[1 2 1 2],[1 2 1 1])

should return an accuracy of 0.75. Here, the true labels are 1, 2, 1, 2 and the predicted

labels are 1, 2, 1, 1. So the first three examples are classified correctly, and the last one is

wrong ⇒ 75% accuracy.

Take a look at knneval.m. This script runs the (not yet implemented) k-nearest neighbor classifier over the faces and digits data set and uses analyze to compute the classification accuracy. In its current implementation the results are picked at random. Run

it to test your analyze.m implementation. The faces data set has 40 classes, the digits

data set 10. What classification accuracy would you expect from a random classifier?

(c) Implement the function knnclassifier.m, which should perform k-nearest neighbor

classification on a given test data set. The call

>> preds=knnclassifier(xTr,yTr,xTe,k);

should result in the predictions for the data in xTe. I.e., preds(i) will contain the prediction for xT e(:, i). You can compute the actual classification accuracy (or error) on the

test set by calling

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>> analyze(“acc”,yTe,knnclassifier(xTr,yTr,xTe,3))

Test your function on both data sets with knneval.m. To visualize your classifications,

you can also run

>> knnvisual faces

>> knnvisual digits

Hopefully you will get better results than the ones in the figure below (using the random classifier):

Shuffle the dimensions of the images in random order and verify that this does not

change your classification error. You can use the command randperm and modify

knnvisual.m.

(d) Sometimes a k-NN classifier can result in a draw, when the majority vote is not clearly

defined. Can you improve your accuracy by falling back onto k-NN with lower k (say

k − 1) to break ties? Edit the file knnclassifier.m accordingly.

(e) Edit the function competition.m, which reads in a training and testing set and makes

predictions. Inside this function you are free to use any combination or variation of the

k-nearest neighbor classifier. Can you beat my implementation on our secret training

and testing sets in terms of accuracy? (Hint: My accuracy is around 92% for faces and

95% for digits.)

Submit your implementations by committing your functions findknn.m, analyze.m,

knnclassifier.m, competition.m, and partners.txt to the hw5 folder in your SVN

repository.

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