CS4395 Homework 4: Language Models solution




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Objective: Use n-gram models for text analysis.
Turn in: your Python programs, zipped (just your 2 programs)
In this homework you will create bigram and unigram dictionaries for English, French, and Italian
using the provided training data where the key is the unigram or bigram text and the value is the
count of that unigram or bigram in the data. Then for the test data, calculate probabilities for
each language and compare against the true labels.
1. Program 1: Build language models for 3 languages as follows.
a. create a function with a filename as argument
b. read in the text and remove newlines
c. tokenize the text
d. use nltk to create a bigrams list
e. create a unigrams list
f. use the bigram list to create a bigram dictionary of bigrams and counts, [‘token1
token2’] -> count
g. use the unigram list to create a unigram dictionary of unigrams and counts,
[‘token’] -> count
h. return the unigram dictionary and bigram dictionary from the function
i. in the main body of code, call the function 3 times for each training file, pickle
the 6 dictionaries and save to files with appropriate names. The reason we are
pickling them in one program and unpickling them in another is that NLTK
ngrams is slow and if you put this all in one program you will waste a lot of time
waiting for ngrams() to finish.
2. Program 2.
a. Read in your pickled dictionaries.
b. For each test file, calculate a probability for each language (see note below) and
write the language with the highest probability to a file.
c. Compute and output your accuracy as the percentage of correctly classified
instances in the test set. The file LangId.sol holds the correct classifications.
d. output your accuracy, as well as the line numbers of the incorrectly classified
Grading Rubric:
Element Points
Python script runs without error 30
Appropriate comments and white space 10
Program 1, 2 30 each
Total 100
Creating the dictionaries in Program 1:
You can use the NLTK ngrams() function to create a bigrams and a unigrams generator object. Then you
can iterate over each to create the dictionary using Python’s .count() string method to extract counts
from the text you read in.
Calculating probabilities in Program 2:
The probabilities will be large enough so that you don’t need to use logs, we will simply multiply the
probabilities together. Each bigram’s probability with Laplace smoothing is: (b + 1) / (u + v) where b is
the bigram count, u is the unigram count of the first word in the bigram, and v is the total vocabulary
size (add the lengths of the 3 unigram dictionaries).