How to find the longest common substring from more than two strings in Python?

Common dynamic programming implementations for the Longest Common Substring algorithm runs in O(nm) time. The following is an implementation of the longest common substring algorithm:

Example

def longest_common_substring(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]
print(longest_common_substring('wellbeing', 'welcome'))

Output

wel

This is how it works

• Initially, we initialized the counter array(m) all 0.

• Starting from the 1st row, we will compare the first character of a string s1 with all characters in a string s2.

• While we traverse the characters in s2, if it matches with the character in s1, we increment the counter. It will be saved m[i][j] which is at diagonally one lower position.

At the end we return the longest sub-string using indices we calculated in the loops.

Rajendra Dharmkar

Published on 14-Dec-2017 10:24:29

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