Comparing two strings and returning the difference. Python 3

Question

My goal is to write a program which compares two strings and displays the difference between the first two non-matching characters.

example:

str1 = 'dog'
str2 = 'doc'

should return 'gc'

I know that the code which I have tried to use is bad but I am hoping to receive some tips. Here is my poor attempt to solve the exercise which leads me to nowhere:

# firstly I had tried to split the strings into separate letters
str1 = input("Enter first string:").split()
str2 = input("Enter second string:").split()

# then creating a new variable to store the result after  comparing the strings
result = ''

# after that trying to compare the strings using a for loop
for letter in str1:
    for letter in str2:
        if letter(str1) != letter(str2):
            result = result + letter
            print (result)

`str1` and `str2` are lists; you split the input strings on whitespace. From your example that's not what you intended to do. — Martijn Pieters, Jun 06 '15 at 13:36
Strings are just sequences, so you can l loop over them directly to get characters. To pair up two strings, use `zip()` (see the dupe). — Martijn Pieters, Jun 06 '15 at 13:36
@MartijnPieters, But the `zip()` method won't work for strings of unequal lengths — ZdaR, Jun 06 '15 at 13:39
@ZdaR: it is easy to test for different lengths up front. What should happen in that case? Alternatively, you can use `itertools.izip_longest()` to provide a default value to substitute missing characters once you get past the shortest string, etc. — Martijn Pieters, Jun 06 '15 at 13:42
@ZdaR: my point was that you can *pair up characters*. The details can still be worked out by the OP, we don't have to provide a complete ready-to-run solution here. — Martijn Pieters, Jun 06 '15 at 13:42
@lord63.j: how does that provide the *first differing characters*? — Martijn Pieters, Jun 06 '15 at 13:43
@lord63.j that seems wrong: `set('fail').symmetric_difference(set('file')) == {'e', 'a'}` while it should be `{'a', 'i'}` — fferri, Jun 06 '15 at 13:45
@mescalinum Ah, make sense, seems that I misunderstood the question :( — lord63. j, Jun 06 '15 at 13:53

score 6 · Answer 1 · answered Jun 06 '15 at 13:37

def first_difference(str1, str2):
    for a, b in zip(str1, str2):
        if a != b:
            return a+b

Usage:

>>> first_difference('dog','doc')
'gc'

But as @ZdaR pointed out in a comment, result is undefined (in this case None) if one string is a prefix of the other and has different length.

Youn Elan · Accepted Answer · 2015-06-06T14:08:28.167

I changed the solution by using a single loop.

How about this:

# First, I removed the split... it is already an array
str1 = input("Enter first string:")
str2 = input("Enter second string:")

#then creating a new variable to store the result after  
#comparing the strings. You note that I added result2 because 
#if string 2 is longer than string 1 then you have extra characters 
#in result 2, if string 1 is  longer then the result you want to take 
#a look at is result 2

result1 = ''
result2 = ''

#handle the case where one string is longer than the other
maxlen=len(str2) if len(str1)<len(str2) else len(str1)

#loop through the characters
for i in range(maxlen):
  #use a slice rather than index in case one string longer than other
  letter1=str1[i:i+1]
  letter2=str2[i:i+1]
  #create string with differences
  if letter1 != letter2:
    result1+=letter1
    result2+=letter2

#print out result
print ("Letters different in string 1:",result1)
print ("Letters different in string 2:",result2)

Comparing two strings and returning the difference. Python 3

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