CMSC 25400/STAT 27725 Homework 6


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In this assignment you will code a Long-Short Term Memory (LSTM) recurrent network in PyTorch
to predict words in English language sentences taken from a story (some words, such as names, in
the story have been modified). The network should have a single LSTM layer, and by default the
ht hidden vectors should be of dimension 200 (you may experiment with shorter or longer vectors).
The ct cell vectors must be of the same dimension.
You can set the input and output vectors to be also 200 dimensional, however size of the vocabulary, nvocab is of course much greater than 200. To bridge this gap, try two different strategies:
1. Use a random word embedding, i.e., just assign a uniformly distributed random vector vi ∈
[0, 1]200 to each word in the vocabulary.
2. Use a pre-computed embedding, as described on Mart´ın Pellarolo’s blog.
Please compare the accuracy of these two approaches.
The data is provided in four files:
◦ bobsue.seq2seq.train.tsv is the training file consisting of 6036 pairs of consecutive sentences
from the story separated by a tab.
◦ is a validation set consisting of similar paired sentences that you can
use to set parameters of the model and decide when to stop training (to avoid overfitting).
◦ bobsue.seq2seq.test.tsv is a test set of similar pairs of sentences.
◦ bobsue.voc.txt contains the vocabulary of the above files, with one word per line.
Each sentence in the training/validation/testing sets is enclosed between tokens.
Your task is to learn to predict the second sentence of each pair, word by word, from the first
sentence. In training, define the loss on each word as the squared distance ∥ v −vb∥
between the
predicted word vector and the vector corresponding to the correct word in the embedding space.
At test time, just predict the word in the vocabulary whose vector is closest to the predicted word
vector. You can ignore the token, but please treat the token as a word (or maybe an
extra dimension of your output space), since predicting where the second sentence ends is highly
Report the accuracy of your network on the test set as a function of various design parameters,
and also plot how the loss on the training and validation sets changes as a function of training
epochs. Report the 20 words that your network most often got right and the 20 words that
it least often got right. Also include a sample of sentences produced by your network and the
corresponding ground truth sentences. Submit the predictions of your network in a text file similar
to the training/validation/testing files.
In this assignment you are expected to use Pytorch’s automatic differentiation functionality to
compute all the gradients for you. You may use one of PyTorch’s built-in training algorithms too
(sugh as SGD etc) to update the weights. However, please do not use the pre-made LSTM class.
An excellent tutrial on LSTMs can be found on Chris Olah’s blog.