LIGN 167: Problem Set 5


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LIGN 167: Problem Set 5

Collaboration policy: You may collaborate with up to two other students on this
problem set. You must write up your own answers to the problems; do not just copy and
paste from your collaborators. You must also submit your work individually. If you do not
submit a copy of the problem set under your own name, you will not get credit. When
you submit your work, you must indicate who you worked with, and what each of your
individual contributions were.
Additional dependencies: After you open your LIGN167 environment in the command line, you should type the following command: python -m spacy download en
This will download some dependencies that are necessary for running the code in this problem set.
Submitting your work: In this problem set you won’t be submitting code. Instead,
you’re going to be writing up answers which include some math. When submitting your
problem set, you can either write it up in LaTex/Word, or hand-write and take a photo/scan.
In either case, your submissions must be done electronically through Gradescope.
This problem set will be different from the previous ones that we have had in this course.
We will be providing you with all of the code. Your job will be to explain what the different
parts of the code are doing.
All of the code is provided in This file imports text from sample_corpus.txt, which is a small sample of text from Simple Wikipedia. It then trains a
language model using this corpus.
The code provides an implementation of a simple RNN (an Elman network) from scratch
in PyTorch. You would never write an RNN this way in practice (you would instead call
built-in functions), but this shows how those build-in functions work.
The problem set should not require much knowledge of PyTorch, but the PyTorch tutorial
from the syllabus may be helpful:
Problem 1. The function load_corpus loads the text from sample_corpus.txt, and returns
it as a string.
The function segment_and_tokenize takes a single argument, corpus, which is assumed
to be a string containing the entire corpus. Test this code on some simple examples, or on
the corpus that is loaded by load_corpus. What does the function do to the string that it
Problem 2. The function get_data first loads the corpus, then creates a variable sents
by calling segment_and_tokenize. It then passes sents to a function make_word_to_ix.
The variable sents is assumed to be a list of sentences, where each sentence is itself a list
of words.
Explain what make_word_to_ix is doing.
Hint: Try this on some simple examples. For example, set sents equal to:
Problem 3. After the function get_data calls make_word_to_ix, it then calls a function
vectorize_sents with arguments sents and word_to_ix.
vectorize_sents turns the sentences into one-hot vectors. Explain how it does this.
Problem 4. The RNN defined in this code is being used for language modeling. As discussed in class, a language model is a probability distribution over sentences. If a sentence
s consists of a sequence of words w0,…,wn−1, then the probability of the sentence is defined
P(s) = P(< start >, w0, …, wn−1, < end >)
= P(w0| < start >) · P(w1| < start >, w0) · … · P(< end > | < start >, w0, …, wn−1)
We will walk through the steps that the RNN is using to define a language model.
The main function in this part of the code is network_forward. This function takes two
arguments: sent and param_dict. The argument sent is a list of one-hot vectors, each
representing a single word in the sentence. The argument param_dict is a dictionary
containing all of the parameters needed to define the RNN. These are the parameters that
will be learned later on.
One of the first things that this function does is call embed_word, given a word (current_word)
from the sentence and the weight matrix We. The function embed_word returns a word embedding for the word. How does embed_word generate a word embedding for current_word?
Problem 5. After generating the word embedding for current_word, the function network_forward
then calls the function elman_unit. This function will apply an Elman unit (as discussed in class) to two inputs: current_word_embedding and h_previous (the hidden
state at the previous time point). The function elman_unit takes five arguments in total:
current_word_embedding, h_previous, and three weight matrices.
Describe mathematically what function elman_unit computes on its inputs.
Hint: torch.matmul performs matrix multiplication: it multiplies a matrix by a vector.
There is very good documentation for torch.matmul, and all other PyTorch functions, that
can be found through Google.
Problem 6. After calling elman_unit and generating the value h_current, the function network_forward then calls the function single_layer_perceptron. The function
single_layer_perceptron takes two arguments: h_current and the weight matrix Wp. Describe mathematically what the function single_layer_perceptron computes on its inputs.
Problem 7. The previous three problems have gone through the code describing the RNN.
The function train is the code that is used for training the RNN. It first defines the dimensions for the different weight vectors, and then initializes the parameters. It uses PyTorch
code for computing gradients automatically and optimizing the model weights.
When you run the code by calling train, what happens to the loss function over time?
Name one problem that the current implementation of the RNN is likely to have. We
discussed several in class.
Hint: What can go wrong with Elman networks?

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