Compare 2 Strings in Python

Question

So, I need to figure out a program that when you input 2 different strings of the same length it will return NOT print the number of differences between the two strings. The order of the characters matters as well.

For example if you input ("abcdef", "aabccf") it should return 4.

("abcdef", "accddf") should return 2.

All I have so far is:

def differencecount ( A, B): counter = 0 str1 = list (A) str2 = list (B) for letter in str1: if letter == str2: counter = counter + 1 return counter

All this does is return 0 though so I feel like I'm missing something.

Answer 1

I would use

def difference(word_one, word_two):
    return sum(l1 != l2 for l1, l2 in zip(word_one, word_two))

Which works like

>>> difference('abcdef', 'abcdef')
0
>>> difference('abcdef', 'abcabc')
3

Answer 2

You can zip the strings together and then count how many different pairs there are:

def chardifferencecounter(x, y):
    return len([1 for c1, c2 in zip(x, y) if c1 != c2])

>>> chardifferencecounter('abcdef', 'aabccf')
4
>>> chardifferencecounter('abcdef', 'accddf')
2

Explanation:

Zipping the strings together produces this:

>>> s1 = 'abcdef'
>>> s2 = 'aabccf'
>>> zip(s1, s2)
[('a', 'a'), ('b', 'a'), ('c', 'b'), ('d', 'c'), ('e', 'c'), ('f', 'f')]

so it takes a character from the same position in each string and pairs them together. So you just need to count how many pairs are different. That can be done using a list comprehension to create a list with those pairs that are the same filtered out, and then get the length of that list.

Answer 3

Just for a different look here is a solution that doesn't use zip or enumerate:

def chardifferencecounter(x,y):
    if len(x) != len(y):
        raise Exception('Please enter strings of equal length')
    return sum(x[i] != y[i] for i in range(len(x)))

Note that this solution also raises an exception when x and y are of different lengths, which is what you wanted in your comment.