Selected Programming Pearls Excercises

phone_number_map.py

from optparse import OptionParser
import re

# The book Programming Pearls has an excercise to code
# a simple algorithm for taking numbers pressed on a
# phone keypad and returning the names from a directory
# that the caller may have been looking for.  The book
# is really aimed at C and C++, but I'm doing some of these
# in Python since that's what I mostly work in these days
# and am building my skills in.

# Could have skipped the first two empty array elements
# and just remembered to subtract 2 to get the correct
# array element later.
phone_key_letters = [[],[],
                     ['a','b','c'],
                     ['d','e','f'],
                     ['g','h','i'],
                     ['j','k','l'],
                     ['m','n','o'],
                     ['p','q','r','s'],
                     ['t','u','v'],
                     ['w','x','y','z']]


def lookup_number(letter):
    for i in range(2, 10):
        if letter in phone_key_letters[i]:
            return i

def generate_word_map(wordfile, word_map):
    """Map words to the number combo that can create them"""
    f = open(wordfile, 'r')
    for word in f.readlines():
        word_number = ''
        i = 0
        for letter in word.strip().lower():
            word_number += str(lookup_number(letter))
            i += 1
            # Make this however many numbers the user would have entered
            if i == 3:
                break

        if word_number not in word_map:
            word_map[word_number] = []

        word_map[word_number].append(word.strip())

def lookup_words(word_map, phone_number):
    phone = re.sub(r'\D', '', phone_number)
    return word_map.get(phone, [])

if __name__ == '__main__':
    parser = OptionParser()
    parser.add_option("-f", "--file", dest="filename",
                      help="retrieve directory of names or words from FILE", metavar="FILE")
    parser.add_option("-n", "--number",
                      dest="phone_number",
                      help="phone number to lookup words for")

    (options, args) = parser.parse_args()

    # This could also be done with an array since the keys are all numbers.
    # The book uses C and C++ and the original implemenation probably was 
    # an array.  My data was pretty sparse, so I went with the
    # dictionary/hash instead.
    word_map = {}

    # In a real world app obviously this would be done once and the app
    # stays in memory or stores the mapping somewhere such as a db.
    generate_word_map(options.filename, word_map)

    print lookup_words(word_map, options.phone_number)