Assignment 4 Hash Tables


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Project Assignment 4

In this assignment, you are going to actually build something useful using the data structures
you have learned in this course, particularly Hash Tables. Namely, you are going to build a
search engine for documents, which finds documents relevant to query terms given by your user
among documents stored in a directory on your computer.
You are going to use one of the hash tables you have implemented in labs (Pick the one you
think is the most reliable). Make sure they work. Fix them if there are some issues. We are also
going to use the import_stopwords function from lab 8.
You need to add keys() method, which returns a list of keys in the hash table, in your class for
the hash table.
Concepts and Terminologies
Inverted Index
An index storing a mapping from content, such as words, to its location in a document. In this
assignment, we cut corners and just build an inverted index from terms to documents.
In this project, our inverted index, called term_freqs, has the following structure:
Term – File Name with its path – frequency
Term Frequency (TF)
The frequency of a term in a document.
Document Frequency (DF)
The frequency of documents that contain a particular term or terms.
TF * IDF, where IDF is inverted DF.
CPE202 Spring2020
The idea is to penalize the term frequency by the number of documents containing the term
because common terms across multiple documents are believed to have small discriminative
power. In this assignment, we cut corners again and we use weighted frequencies, which can
be computed more efficiently, instead.
Program Structure
in operator (except for with your hash table), dict (dictionary. use your hash table instead!),
sort, sorted, and built-in list functions such as count() except for append().
SearchEngine class
Builds and maintains an inverted index of documents stored in a specified directory and
provides a functionality to search documents with query terms.
main function
The entry point of your program. It takes a directory name as its command line argument.
It will create an instance of SearchEngine class by passing the directory name.
Then, it will get into an infinite loop.
Inside the loop it will present a prompt to the user and waits for inputs (use input() function).
If the user types q, it will exit from the loop and end the session.
If the user types s: followed by some space separated terms (ex. s:computer science), it will
search for relevant documents and print a list of file names in the descending order of the
relevancy. All search query terms need to be converted into lower cases in your search engine.
Do not forget if __name__ == ‘__main__’ at the bottom of the file.
Attributes of SearchEngine class
● directory (str) : a directory name
● stopwords (HashMap) : a hash table containing stopwords
● doc_length (HashMap) : a hash table containing the total number of words in each
CPE202 Spring2020
● term_freqs (HashMap) : a hash table of hash tables for each term. Each hash table
contains the frequency of the term in documents (document names are the keys and the
frequencies are the values)
Constructor for SearchEngine class
Take a directory name and a hash table containing stopwords as arguments.
Initialize doc_length, term_freqs with empty hash tables. Assign the stopwords hash table to
stopwords. Call self.index_files(directory) and index files contained in the directory.
def __init__(self, directory, stopwords):
#Replace HashMap() with your hash table.
self.doc_length = HashMap() # The key is file_path_name,
# the value is the number of words
self.term_freqs = HashMap() # The key is word, the value is another HashMap
self.stopwords = stopwords
Methods in SearchEngine class
Create a function read_file() in SearchEngine class to read a text file. The function needs to
read all words contained in the file except for stop words. (from now on we are going to call a
function defined in a class as a method)
Use “with open() as infile:” so that the opened file will be automatically closed when you get out
of the with block. Read more about the with here:
And here:
def read_file(self, infile):
“””A helper function to read a file
infile (str) : the path to a file
list : a list of str read from a file
CPE202 Spring2020
Create a method parse_words() in SearchEngine class. It takes a list of strings as an argument.
Each string contains multiple words. Split each string into words at spaces. Convert all words to
lower cases and remove new line chars.
def parse_words(self, lines):
“””splits strings into words by spaces.
Converts words to lower cases,
and removes newline chars, parentheses, brackets such as “[“, “]”, “{“, “}”
and punctuations such as “,”, “.”, “?”, “!”.
You may use replace() and other builtin string functions.
Excludes stopwords.
lines (list) : a list of strings
list : a list of words
It is recommended that you create a helper function for the parse_words function to exclude
stop words.
def exclude_stopwords(self, terms):
“””exclude stopwords from the list of terms
terms (list) :
list : a list of str with stopwords removed
Create a method count_words() in SearchEngine class.
def count_words(self, file_path_name, words):
“””count words in a file and store the frequency of each
word in the term_freqs hash table. The keys of the term_freqs hash table shall be
words. The values of the term_freqs hash table shall be hash tables (term_freqs
is a hash table of hash tables). The keys of the hash tables (inner hash table) stored
in the term_freqs shall be file names. The values of the inner hash tables shall be
the frequencies of words. For example, self.term_freqs[word][file_path_name] += 1;
Words should not contain stopwords.
Also store the total count of words contained in the file in the doc_length hash table.
file_path_name (str) : the file name
words (list) : a list of words
CPE202 Spring2020
Create a method index_files, which process all text files in a specified directory and build an
inverted index. Use python builtin os.listdir(directory) function to get a list of files in a directory.
Use os.path.join(directory, item) to construct the full path of each file. Use os.path.isfile(item) to
check if the item is a file. If it is not a file, i.e. directory, skip it. Use os.path.splitext(item) to split a
path into a file extension and the rest. Check if parts[1] == ‘.txt’. Only process text files. For each
text file, process it with read_file(), parse_words(), and count_words() functions. This function
needs to be called in the constructor __init__.
def index_files(self, directory):
“””index all text files in a given directory
directory (str) : the path of a directory
Create a method get_wf() in SearchEngine class.
def get_wf(self, tf):
“””comptes the weighted frequency
tf (float) : term frequency
float : the weighted frequency
if tf 0:
wf = 1 + math.log(tf)
wf = 0
return wf
Create a method get_scores() in SearchEngine class. Use term_freqs and doc_length hash
tables. Create a hash table called scores to store scores for each file. For each term in a query
compute the score of each document using the formula: score = weighted frequency. Add it to
the score stored in the scores hash table for the document. After the score has been
accumulated, normalize the score for each document by dividing it by the total word count in the
file (we exclude stopwords from the count).
Use the keys() function of your hash table to get keys in the table to extract all key value pairs
stored in the hash table. Files with scores being 0 will be ignored.
def get_scores(self, terms):
CPE202 Spring2020
“””creates a list of scores for each file in corpus
The score = weighted frequency / the total word count in the file.
Compute this score for each term in a query and sum all the scores.
terms (list) : a list of str
list : a list of tuples, each containing the file_path_name and its relevancy score.
#The code listed below is pseudo code
scores = HashMap()
For each query term t
Fetch a hash table of t from self.term_freqs
For each file in the hash table, add wf to scores[file]
For each file in scores, do scores[file] /= self.doc_length[file]
Return scores
Create a method rank() in SearchEngine class. You are not allowed to use python builtin sorting
functionalities. You need to write your own function to sort items. You need to sort the
(file_path_name, score) pairs in descending order of the relevancy score.
def rank(self, scores):
“””ranks files in the descending order of relevancy
scores(list) : a list of tuples: (file_path_name, score)
list : a list of tuples: (file_path_name, score) sorted in descending order of
Create a method search() in SearchEngine class. It takes a query string user typed as an
argument called query, and returns a list of tuples (filen_path_name, score). The search()
method will call parse_words(), get_scores(), and rank() methods. The query string needs to
be parsed into words, being converted to lowercase with stop words removed using
parse_words() method. The parsed query terms will be passed to get_scores() method, which
calls get_wf(). Remove duplicate words using a hash table before passing the words to
get_scores(). The return value of get_scores(), a list of tuples of file_path_name and score, will
be passed to rank() method, which will sort the list in descending order of the relevancy score.
def search(self, query):
“”” search for the query terms in files.
query (str) : query input: e.g. “computer science”
CPE202 Spring2020
list : a list of tuples: (file_path_name, score) sorted in descending order or
excluding files whose relevancy score is 0.
def main():
“”” The driver of the program.
1. It asks the user to input the path of the directory containing documents.
2. It creates an object of SearchEngine importing stop words, and builds an
inverted index on the documents.
3. It asks the user to input a search query (keywords separated by space: e.g.
computer science).
4. The user must prepend “s:” to a query indicating that the user wants to do
search. The user must type “:q” if he/she wants to quit.
5. It searches for documents containing any of the keywords using the search()
method. The method is going to return a list of tuples(file_path_name, score).
6. It shows a list of files (including their paths) containing the keywords in
descending order of relevancy. You may create a helper function / method for
printing the list of files nicely on the screen.
7. Until the user types “q:” for quitting, it keeps asking the user to enter a new
Helper Functions
You are free to create additional functions, classes, and methods, but you may not change the
interfaces of required functions / methods.
Download a zip file containing test files to be indexed by your search engine.
Assuming that the documents are in a directory “docs”, for a query “Computer Science”, your
search engine shall output the following lines on a screen:
The score of test.txt should be 1.0.
For a query “hash table”, your search engine shall output the following lines on a screen:
CPE202 Spring2020
The score of hash_table.txt should be around 0.06.
For a query “ADT”, your search engine shall output the following lines on a screen:
The score of this document should be around 0.017.
Create, and test methods and functions that return some values.
Further Development
The search engine presented here is very primitive. For example, the engine does not account
for different forms of the same word, nor synonyms, misspelled words. Also, it can not do
proximity search, in which the engine searches for two or more words that occur within a certain
number of words from each other. You can extend the engine to do those things. Also, you can
extend the engine so that it outputs the location where query terms occur in documents.
Submission and demo
Zip, and all the files required to run your program into one zip file,
but do not put Python (.py) files in a directory. Gradzilla will mainly test the search() method
comparing the returned value to an expected value. Submit your zipped file to Gradzilla, then to

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