Count letter differences of two strings

Question

This is the behaviour I want:

a: IGADKYFHARGNYDAA
c: KGADKYFHARGNYEAA
2 difference(s).

Answer 1

The Theory

1. Iterate over both strings simultaneously and compare the characters.
2. Store the result with a new string by adding either a spacebar or a | character to it, respectively. Also, increase a integer-value starting from zero for each different character.
3. Output the result.

Implementation

You can use the built-in zip function or itertools.izip to simultaneously iterate over both strings, while the latter is a little more performant in case of huge input. If the strings are not of the same size, iteration will only happen for the shorter-part. If this is the case, you can fill up the rest with the no-match indicating character.

import itertools

def compare(string1, string2, no_match_c=' ', match_c='|'):
    if len(string2) < len(string1):
        string1, string2 = string2, string1
    result = ''
    n_diff = 0
    for c1, c2 in itertools.izip(string1, string2):
        if c1 == c2:
            result += match_c
        else:
            result += no_match_c
            n_diff += 1
    delta = len(string2) - len(string1)
    result += delta * no_match_c
    n_diff += delta
    return (result, n_diff)

Example

Here's a simple test, with slightly different options than from your example above. Note that I have used an underscore for indicating non-matching characters to better demonstrate how the resulting string is expanded to the size of the longer string.

def main():
    string1 = 'IGADKYFHARGNYDAA AWOOH'
    string2 = 'KGADKYFHARGNYEAA  W'
    result, n_diff = compare(string1, string2, no_match_c='_')

    print "%d difference(s)." % n_diff  
    print string1
    print result
    print string2

main()

Output:

niklas@saphire:~/Desktop$ python foo.py 
6 difference(s).
IGADKYFHARGNYDAA AWOOH
_||||||||||||_|||_|___
KGADKYFHARGNYEAA  W