Description
1 Theoretical Part (40 points)
For the following, please show all steps of your derivation and list any assumptions that you make. You can submit typed or legible hand-written solutions.
If the TA cannot read your handwriting, no credit will be given.
1.1 Revisiting Backpropagation Algorithm
In class we had derived the backpropagation algorithm for the case where each
of the hidden and output layer neurons used the sigmoid activation function:
σ(x) = 1
1 + e−x
Revise the backpropagation algorithm for the case where each hidden and output
layer neuron uses the
a. tanh activation function
tanh(x) = ex − e−x
ex + e−x
b. ReLu activation function:
ReLu(x) = max(0, x)
Show all steps of your derivation and the final equation for output layer and
hidden layers.
1.2 Gradient Descent
Derive a gradient descent training rule for a single unit neuron with output o,
defined as:
o = w0 + w1(x1 + x2
1) + · · · + wn(xn + x2
n)
where x1, x2, . . . , xn are the inputs, w1, w2, . . . , wn are the corresponding weights,
and w0 is the bias weight. You can assume an identity activation function i.e.
f(x) = x. Show all steps of your derivation and the final result for weight
update. You can assume a learning rate of η.
1.3 Comparing Activation Function
Consider a neural net with 2 input layer neurons, one hidden layer with 2 neurons, and 1 output layer neuron as shown in Figure 1. Assume that the input
layer uses the identity activation function i.e. f(x) = x, and each of the hidden
layers and output layer use an activation function h(x). The weights of each of
Machine Learning Assignment 2
the connections are marked in the figure.
Figure 1: A neural net with 1 hidden layer having 2 neurons
a. Write down the output of the neural net y5 in terms of weights, inputs, and
a general activation function h(x).
b. Now suppose we use vector notation, with symbols defined as below:
X =
!
x1
x2
”
W(1) =
!
w3,1 w3,2
w4,1 w4,2
”
W(2) = #
w5,3 w5,4
$
Write down the output of the neural net in vector format using above vectors.
c. Now suppose that you have two choices for activation function h(x), as shown
below:
Sigmoid:
hs(x) = 1
1 + e−x
Tanh:
ht(x) = ex − e−x
ex + e−x
Show that neural nets created using the above two activation functions can generate the same function.
Hint: First compute the relationship between hs(x) and ht(x) and then show
that the output functions are same, with the parameters differing only by linear
transformations and constants.
1.4 Gradient Descent with a Weight Penalty
Go through Chapter 4 of Tom Mitchell’s Machine Learning textbook, which is
available at the following link:
https://users.cs.northwestern.edu/~pardo/courses/eecs349/readings/chapter4-ml.pdf
Solve question 4.10 from the book. Show all steps of derivation and clearly state
the final update rule for output as well as hidden layers.
2 Programming Part (60 points)
In this part, you will code a neural network (NN) having at least one hidden
layers, besides the input and output layers. You are required to pre-process the
data and then run the processed data through your neural net. Below are the
requirements and suggested steps of the program
• The programming language for this assignment will be Python 3.x
• You cannot use any libraries for neural net creation. You are free
to use any other libraries for data loading, pre-processing, splitting, model
evaluation, plotting, etc.
• As the first step, pre-process and clean your dataset. There should be a
method that does this.
• Split the pre-processed dataset into training and testing parts. You are
free to choose any reasonable value for the train/test ratio, but be sure to
mention it in the README file.
• Code a neural net having at least one hidden layer. You are free to
select the number of neurons in each layer. Each neuron in the hidden
and output layers should have a bias connection.
• You are required to add an optimizer on top of the basic backpropagation
algorithm. This could be the one you selected in the previous assigment
or a new one. Some good resources for gradient descent optimizers are:
https://arxiv.org/pdf/1609.04747.pdf
https://ruder.io/optimizing-gradient-descent/
https://towardsdatascience.com/10-gradient-descent-optimisation-algorithms-86989510b5e9
• Your code should be in the form of a Python class with methods like
pre-process, train, test within the class. I leave the other details up to
you.
• You are required to code three different activation functions:
1. Sigmoid
2. Tanh
3. ReLu
The earlier part of this assignment may prove useful for this stage. The
activation function should be a parameter in your code.
• Code a method for creating a neural net model from the training part of
the dataset. Report the training accuracy.
• Apply the trained model on the test part of the dataset. Report the test
accuracy.
• You have to tune model parameters like learning rate, activation functions,
etc. Report your results in a tabular format, with a column indicating the
parameters used, a column for training accuracy, and one for test accuracy.
Dataset
You can use any one dataset from the UCI ML repository:
https://archive.ics.uci.edu/ml/datasets.php
Note: If the above direct link does not work, you can just Google the UCI ML
repository.
What to submit:
You need to submit the following for the programming part:
• Link to the dataset used. Please do not include the data as part of you
submission.
• Your source code and a README file indicating how to run your code.
Do not hardcode any paths to your local computer. It is fine to code any
public paths, such as AWS S3.
• Output for your dataset summarized in a tabular format for different
combination of parameters
• A brief report summarizing your results. For example, which activation
function performed the best and why do you think so.
• Any assumptions that you made.
