Find the position of difference between two strings

Question

I have two strings of equal length, how can I find all the locations where the strings are different?

For example, "HELPMEPLZ" and "HELPNEPLX" are different at positions 4 and 8.

Answer 1

Try this:

s1 = 'HELPMEPLZ'
s2 = 'HELPNEPLX'
[i for i in xrange(len(s1)) if s1[i] != s2[i]]

It will return:

> [4, 8]

The above solution will return a list with the indexes in sorted order, won't create any unnecessary intermediate data structures and it will work on Python 2.3 - 2.7. For Python 3.x replace xrange for range.

Answer 2

Python really comes with batteries included. Have a look at difflib

>>> import difflib
>>> a='HELPMEPLZ'
>>> b='HELPNEPLX'
>>> s = difflib.SequenceMatcher(None, a, b)
>>> for block in s.get_matching_blocks():
...     print block
Match(a=0, b=0, size=4)
Match(a=5, b=5, size=3)
Match(a=9, b=9, size=0)

difflib is very powerful and a some study of the documentation is really recommended.

Answer 3

>>> from itertools import izip
>>> s1 = 'HELPMEPLZ'
>>> s2 = 'HELPNEPLX'
>>> [i for i,(a1,a2)  in enumerate(izip(s1,s2)) if a1!=a2]
[4, 8]