How do I perform binary search on a text file to search a keyword in python?

Question

The text file contains two columns- index number(5 spaces) and characters(30 spaces). It is arranged in lexicographic order. I want to perform binary search to search for the keyword.

Answer 1

I wrote a simple Python 3.6+ package that can do this. (See its github page for more information!)

Installation: pip install binary_file_search

Example file:

1,one
2,two_a
2,two_b
3,three

Usage:

from binary_file_search.BinaryFileSearch import BinaryFileSearch
with BinaryFileSearch('example.file', sep=',', string_mode=False) as bfs:
    # assert bfs.is_file_sorted()  # test if the file is sorted.
    print(bfs.search(2))

Result: [[2, 'two_a'], [2, 'two_b']]

Answer 2

Here's an interesting way to do it with Python's built-in bisect module.

import bisect
import os


class Query(object):

    def __init__(self, query, index=5):
        self.query = query
        self.index = index

    def __lt__(self, comparable):
        return self.query < comparable[self.index:]


class FileSearcher(object):

    def __init__(self, file_pointer, record_size=35):
        self.file_pointer = file_pointer
        self.file_pointer.seek(0, os.SEEK_END)
        self.record_size = record_size + len(os.linesep)
        self.num_bytes = self.file_pointer.tell()
        self.file_size = (self.num_bytes // self.record_size)

    def __len__(self):
        return self.file_size

    def __getitem__(self, item):
        self.file_pointer.seek(item * self.record_size)
        return self.file_pointer.read(self.record_size)


if __name__ == '__main__':
    with open('data.dat') as file_to_search:
        query = raw_input('Query: ')
        wrapped_query = Query(query)

        searchable_file = FileSearcher(file_to_search)
        print "Located @ line: ", bisect.bisect(searchable_file, wrapped_query)

Answer 3

Do you need do do a binary search? If not, try converting your flatfile into a cdb (constant database). This will give you very speedy hash lookups to find the index for a given word:

import cdb

# convert the corpus file to a constant database one time
db = cdb.cdbmake('corpus.db', 'corpus.db_temp')
for line in open('largecorpus.txt', 'r'):
    index, word = line.split()
    db.add(word, index)
db.finish()

In a separate script, run queries against it:

import cdb
db = cdb.init('corpus.db')
db.get('chaos')
12345

Answer 4

If you need to find a single keyword in a file:

line_with_keyword = next((line for line in open('file') if keyword in line),None)
if line_with_keyword is not None: 
   print line_with_keyword # found

To find multiple keywords you could use set() as @kriegar suggested:

def extract_keyword(line):
    return line[5:35] # assuming keyword starts on 6 position and has length 30

with open('file') as f:
    keywords = set(extract_keyword(line) for line in f) # O(n) creation
    if keyword in keywords: # O(1) search
       print(keyword)

You could use dict() above instead of set() to preserve index information.

Here's how you could do a binary search on a text file:

import bisect

lines = open('file').readlines() # O(n) list creation
keywords = map(extract_keyword, lines) 
i = bisect.bisect_left(keywords, keyword) # O(log(n)) search
if keyword == keywords[i]:
   print(lines[i]) # found

There is no advantage compared to the set() variant.

Note: all variants except the first one load the whole file in memory. FileSearcher() suggested by @Mahmoud Abdelkader don't require to load the whole file in memory.