# CS416 – HW3 SOLVED

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CS416 – HW3
1 Kernel (10 points)
Suppose x, z ∈ R
d
, and let’s consider the following function
K(x, z) = (x
T
z)
2
Let’s prove K(x, z) is the kernel function corresponding to the feature mapping
φ give by
φ(x) =

x1x1
x1x2
· · ·
x1xd
x2x1
x2x2
· · ·
x2xd
· · ·
xdx1
· · ·
xdxd

The proof is very simple. We merely check that K(x, z) =< φ(x), φ(z) >
K(x, z) = (X
d
i=1
xizi)(X
d
i=1
xj zj ) = X
d
i=1
X
d
j=1
xixj zizj =
X
d
i,j=1
(xixj )(zizj ) =< φ(x), φ(z) >
Please show what feature mapping the following kernel function corresponds
to and prove the kernel function corresponds to the feature mapping. Show
the running time of computing the kernel function of two vectors and that of
computing the inner product of two vectors.
K(x, z) = (x
T
z + 1)3
1
2 SVM (5 points + 10 points)
1. Suppose you are given the following training data
positive: (1,2,3) (1,1,4)
negative: (3,2,-1) (4,3,-2) (3,5,-3)
Write down the SVM optimization for those training data including the optimization objective and the constraints.
2. Suppose you are given the following training data
positive: (1,2) (1,1) (2,1) (0,1)
negative: (3,2) (4,3) (3,5)
Which points are support vectors? What is the decision boundary if you use
SVM? In this problem, you can simply look at the points and decide which
points are support vectors and then calculate the decision boundary.
3 Decision Tree (20 points)
Consider the following dataset consisting of five training examples followed by
three test examples:
x1 x2 x3 y
– + + –
+ + + +
– + – +
– – + –
+ + – +
+ – – ?
– – – ?
+ – + ?
There are three attributes (or features or dimensions), x1, x2 and x3, taking
the values + and -. The label (or class) is given in the last column denoted y;
it also takes the two values + and -.
Simulate each of the following learning algorithms on this dataset. In each case,
show the final hypothesis that is induced, and show how it was computed. Also,
say what its prediction would be on the three test examples.
• The decision tree algorithm discussed in class. For this algorithm, use the
information gain (entropy) impurity measure as a criterion for choosing
an attribute to split on. Grow your tree until all nodes are pure, but do
not attempt to prune the tree.
2
• AdaBoost. For this algorithm, you should interpret label values of + and
– as the real numbers +1 and -1. Use decision stumps as weak hypotheses,
and assume that the weak learner always computes the decision stump
with minimum error on the training set weighted in AdaBoost algorithm.
Note that a decision stump is a one-level decision tree. Run your boosting
algorithm for three rounds and list the intermediate results.
3
Programming: Support Vector Machines (20 points)
In this section, we’ll implement various kernels for the support vector machine
(SVM). This exercise looks long, but in practice you’ll be writing only a few lines
of code. The scikit learn package already includes several SVM implementations;
in this case, we’ll focus on the SVM for classification, sklearn.svm.SVC. Before
starting this assignment, be certain to read through the documentation for SVC,
available at
http://scikit-learn.org/stable/modules/generated/sklearn.svm.SVC.html
While we could certainly create our own SVM implementation, most people
applying SVMs to real problems rely on highly optimized SVM toolboxes, such
as LIBSVM or SVMlight1
. These toolboxes provide highly optimized SVM
implementations that use a variety of optimization techniques to enable them
to scale to extremely large problems. Therefore, we will focus on implementing
custom kernels for the SVMs, which is often essential for applying these SVM
toolboxes to real applications.
Relevant Files in this homework Skeleton2
example_svm.py test_svmPolyKernel.py
example_svmCustomKernel.py data/svmData.dat
* svmKernels.py data/svmTuningData.dat
* test_svm_parameters.ipynb test_svmGaussianKernel.py
1.1 Getting Started
The SVC implementation provided with scikit learn uses the parameter C to
control the penalty for misclassifying training instances. We can think of C as
being similar to the inverse of the regularization parameter 1
λ
that we used before
for linear and logistic regression. C = 0 causes the SVM to incur no penalty
for misclassifications, which will encourage it to fit a larger-margin hyperplane,
even if that hyperplane misclassifies more training instances. As C grows large,
it causes the SVM to try to classify all training examples correctly, and so it
will choose a smaller margin hyperplane if that hyperplane fits the training data
better.
Examine example svm.py, which fits a linear SVM to the data shown below.
Note that most of the positive and negative instances are grouped together,
suggesting a clear separation between the classes, but there is an outlier around
(0.5,6.2). In the first part of this exercise, we will see how this outlier affects
the SVM fit.
1The SVC implementation provided with scikit learn is based on LIBSVM, but is not quite
as efficient.
2* indicates files that you will need to complete; you should not need to modify any of the
other files.
4
(a) A (b) B
Figure 1:
Run example svm.py with C = 0.01, and you can clearly see that the hyperplane
follows the natural separation between most of the data, but misclassifies the
outlier. Try increasing the value of C and observe the effect on the resulting
hyperplane. With C = 1,000, we can see that the decision boundary correctly
classifies all training data, but clearly no longer captures the natural separation
between the data.
1.2 Implementing Custom Kernels
The SVC implementation allows us to define our own kernel functions to learn
non-linear decision surfaces using the SVM. The SVC constructor’s kernel argument can be defined as either a string specifying one of the built-in kernels (e.g.,
’linear’, ’poly’ (polynomial), ’rbf’ (radial basis function), ’sigmoid’, etc.) or it
can take as input a custom kernel function, as we will define in this exercise.
For example, the following code snippet defines a custom kernel and uses it to
train the SVM:
def myCustomKernel (X1 , X2 ) :
“””
Custom kernel :
k(X1, X2) = X1 (3 0) X2.T
(0 2)
Note that X1 and X2 are numpy arrays,
so we must use .dot to multiply them.
“””
M = np.array([[3.0, 0], [0, 2.0]])
return np.dot(np.dot(X1, M), X2.T)
# create SVM with custom kernel and train model
clf = svm.SVC(kernel=myCustomKernel )
clf.fit(X, Y)
5
When the SVM calls the custom kernel function during training, X1 and X2 are
both initialized to be the same as X (i.e., ntrain −by−d numpy arrays); in other
words, they both contain a complete copy of the training instances. The custom
kernel function returns an ntrain − by − ntrain numpy array during the training
step. Later, when it is used for testing, X1 will be the ntest testing instances and
X2 will be the ntrain training instances, and so it will return an ntest−by−ntrain
numpy array. For a complete example, see example svmCustomKernel.py,
which uses the custom kernel above to generate the following figure:
1.3 Implementing the Polynomial Kernel (5 points)
We will start by writing our own implementation of the polynomial kernel and
incorporate it into the SVM.3 Complete the myPolynomialKernel() function in
svmKernels.py to implement the polynomial kernel:
K(v, w) = (< v, w > +1)d
where d is the degree of polynomial and the ”+1” incorporates all lower-order
polynomial terms. In this case, v and w are feature vectors. Vectorize your
implementation to make it fast. Once complete, run test svmPolyKernel.py
to produce a plot of the decision surface. For comparison, it also shows the
decision surface learned using the equivalent built-in polynomial kernel; your
results should be identical.
For the built-in polynomial kernel, the degree is specified in the SVC constructor. However, in our custom kernel we must set the degree via a global variable
polyDegree. Therefore, be sure to use the value of the global variable polyDegree as the degree in your polynomial kernel. The test svmPolyKernel.py script
uses polyDegree for the degree of both your custom kernel and the built-in
polynomial kernel. Vary both C and d and study how the SVM reacts.
3Although scikit learn already defines the polynomial kernel, defining our own version of
it provides an easy way to get started implementing custom kernels.
6
Figure 2: Sample output of test svmPolyKernel.py
1.4 Implementing the Gaussian Radial Basis Function Kernel (5 points)
Next, complete the myGaussianKernel() function in svmKernels.py to implement the Gaussian kernel:
K(v, w) = exp(−
||v − w||2
2

2
)
Be sure to use the gaussSigma for σ in your implementation. For computing
the pairwise squared distances between the points, you must write the method
to compute it yourself; specifically you may not use the helper methods available
7
Figure 3: Sample output of test svmGaussianKernel.py
in sklearn.metrics.pairwise or that come with scipy. You can test your implementation and compare it to the equivalent RBF-kernel provided in sklearn
by running test svmGaussianKernel.py.
Again, vary both C and σ and study how the SVM reacts.
Write a brief paragraph describing how the SVM reacts as both C and d vary
for the polynomial kernel, and as C and σ vary for the Gaussian kernel. Put
1.5 Choosing the Optimal Parameters (10 points)
This exercise will further help you gain further practical skill in applying SVMs
with Gaussian kernels. Choosing the correct values for C and σ can dramatically
affect the quality of the model’s fit to data. Your task is to determine the
optimal values of C and σ for an SVM with your Gaussian kernel as applied to
the data in data/svmTuningData.dat, depicted below. You should search over
the space of possible combinations of C and σ, and determine the optimal fit as
measured by accuracy. The file for this exercise is test svm parameters.ipynb
We recommend that you search over multiplicative steps (e.g., …, 0.01, 0.03,
0.06, 0.1, 0.3, 0.6, 1, 3, 6, 10, 30, 60, 100, …). Once you determine the optimal
parameter values, report those optimal values and the corresponding estimated
accuracy in your test svm parameters.ipynb. For reference, the SVM with the
optimal parameters we found produced the following decision surface.
The resulting decision surface for your optimal parameters may look slightly
different than ours.
To have a good estimate of your model’s performance, you should average the
accuracy over a certain number trials of 10-fold cross-validation over the data
set. Make certain to observe the following details:
• For each trial, split the data randomly into 10 folds, choose one to be
the ”test” fold and train the decision tree classifier on the remaining nine
folds. Then, evaluate the trained model on the held-out ”test” fold to
obtain its performance.
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• Repeat this process until each fold has been the test fold exactly once,
then advance to the next trial.
• Be certain to shuffle the data at the start of each trial, but never within a
trial. Report the mean of the prediction accuracy over all trials of 10-fold
cross validation.
Note: although scikit-learn provides libraries that implement cross-fold validation, you may not use them for this assignment – you must implement cross-fold
validation yourself.
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