greatest common divisor for strings in python

Question

I would like to have a python function that given the list

mystrings = ['abcde', 'abcdf', 'abcef', 'abcnn']

returns the string 'abc', i.e., the longest piece contained by all the elements in the list. I have a solution which just loops through the slices of mystring[0], and compares it to the rest, and breaks out of the loop, whenever the first unmatching substring is found. However, I suspect that there must be a more efficient, elegant, and pythonic way of doing this.

Could someone point out how to do this properly?

see http://en.wikipedia.org/wiki/Longest_common_substring_problem — NPE, Oct 24 '11 at 17:07

JBernardo · Accepted Answer · 2011-10-24T17:21:30.817

6

The way you described, you want the biggest substring at the starting point:

>>> os.path.commonprefix(['abcde', 'abcdf', 'abcef', 'abcnn'])
'abc'

edited Oct 24 '11 at 17:21

answered Oct 24 '11 at 17:11

JBernardo

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No. He searches for the common substring, not the common prefix. – Martin Thurau Oct 24 '11 at 17:16
Sorry, if I wasn't clear in OP: I wanted to find the longest common prefix, so Bernardo's solution is what I was looking for. Many thanks! – v923z Oct 24 '11 at 17:24

score 3 · Answer 2 · answered Oct 24 '11 at 17:11

3

Once you realize that "Longest Common Substring" is the problem you're describing, it's easy to find what you want:

eg, from Wikibooks:

def LongestCommonSubstring(S1, S2):
    M = [[0]*(1+len(S2)) for i in xrange(1+len(S1))]
    longest, x_longest = 0, 0
    for x in xrange(1,1+len(S1)):
        for y in xrange(1,1+len(S2)):
            if S1[x-1] == S2[y-1]:
                M[x][y] = M[x-1][y-1] + 1
                if M[x][y]>longest:
                    longest = M[x][y]
                    x_longest  = x
            else:
                M[x][y] = 0
    return S1[x_longest-longest: x_longest]

answered Oct 24 '11 at 17:11

jwd

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This is the naive way of doing it. Moreover, it only works with two strings. – Sven Marnach Oct 24 '11 at 17:13
Thanks, but this is about as complicated as my original solution:( – v923z Oct 24 '11 at 17:13

greatest common divisor for strings in python

2 Answers2