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SequenceMatcher in Python for Longest Common Substring

  • Difficulty Level : Medium
  • Last Updated : 11 Dec, 2017

Given two strings ‘X’ and ‘Y’, print the longest common sub-string.


Input :  X = "GeeksforGeeks", 
         Y = "GeeksQuiz"
Output : Geeks

Input : X = "zxabcdezy", 
        Y = "yzabcdezx"
Output : abcdez

We have existing solution for this problem please refer Print the longest common substring link. We will solve problem in python using SequenceMatcher.find_longest_match() method.

How SequenceMatcher.find_longest_match(aLow,aHigh,bLow,bHigh) method works ?

First we initialize SequenceMatcher object with two input string str1 and str2, find_longest_match(aLow,aHigh,bLow,bHigh) takes 4 parameters aLow, bLow are start index of first and second string respectively and aHigh, bHigh are length of first and second string respectively. find_longest_match() returns named tuple (i, j, k) such that a[i:i+k] is equal to b[j:j+k], if no blocks match, this returns (aLow, bLow, 0).

# Function to find Longest Common Sub-string
from difflib import SequenceMatcher
def longestSubstring(str1,str2):
     # initialize SequenceMatcher object with 
     # input string
     seqMatch = SequenceMatcher(None,str1,str2)
     # find match of longest sub-string
     # output will be like Match(a=0, b=0, size=5)
     match = seqMatch.find_longest_match(0, len(str1), 0, len(str2))
     # print longest substring
     if (match.size!=0):
          print (str1[match.a: match.a + match.size]) 
          print ('No longest common sub-string found')
# Driver program
if __name__ == "__main__":
    str1 = 'GeeksforGeeks'
    str2 = 'GeeksQuiz'



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