[SOLVED] Find common substring between two strings

Issue

I’d like to compare 2 strings and keep the matched, splitting off where the comparison fails.

So if I have 2 strings –

string1 = apples
string2 = appleses

answer = apples

Another example, as the string could have more than one word.

string1 = apple pie available
string2 = apple pies

answer = apple pie

I’m sure there is a simple Python way of doing this but I can’t work it out, any help and explanation appreciated.

Solution

Its called Longest Common Substring problem. Here I present a simple, easy to understand but inefficient solution. It will take a long time to produce correct output for large strings, as the complexity of this algorithm is O(N^2).

def longestSubstringFinder(string1, string2):
    answer = ""
    len1, len2 = len(string1), len(string2)
    for i in range(len1):
        match = ""
        for j in range(len2):
            if (i + j < len1 and string1[i + j] == string2[j]):
                match += string2[j]
            else:
                if (len(match) > len(answer)): answer = match
                match = ""
    return answer

print longestSubstringFinder("apple pie available", "apple pies")
print longestSubstringFinder("apples", "appleses")
print longestSubstringFinder("bapples", "cappleses")

Output

apple pie
apples
apples

Answered By – thefourtheye

Answer Checked By – Candace Johnson (BugsFixing Volunteer)