Recursive function to do substring search

Difficulty Level : Easy
Last Updated : 23 May, 2019

Given a text txt[] and a pattern pat[], write a recursive function “contains(char pat[], char txt[])” that returns true if pat[] is present in txt[], otherwise false.

Examples:

1) Input:   txt[] =  "THIS IS A TEST TEXT"
            pat[] = "TEST"
  Output:  true


2) Input:  txt[] =  "geeksforgeeks"
           pat[] = "quiz"
  Output:  false;

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Below is recursive algorithm.

contains(tex[], pat[])
    1) If the current character is the last character of the text, but pat
       has more characters, return false.

    2) Else If the current character is the last character of the pattern,
       then return true

    3) Else If current characters of pat and text match, then
        return contains(text + 1, pat + 1);

    4) Else If current characters of pat and text don't match
        return contains(text + 1, pat);

Below is the implementation of the above algorithm.

C++

// Recursive C++ program to find if a given pattern is 
// present in a text
#include<iostream>
using namespace std;
  
bool exactMatch(char *text, char *pat)
{
    if (*text == '\0' && *pat != '\0')
        return false;
  
    // Else If last character of pattern reaches
    if (*pat == '\0')
        return true;
  
    if (*text == *pat)
        return exactMatch(text + 1, pat + 1);
  
    return false;
}
  
// This function returns true if 'text' contain 'pat'
bool contains(char *text, char *pat)
{
    // If last character of text reaches
    if (*text == '\0')
        return false;
  
    // If current characters of pat and text match
    if (*text == *pat)
        if(exactMatch(text, pat))
            return 1;
        else
          return contains(text + 1, pat);
  
    // If current characters of pat and tex don't match
    return contains(text + 1, pat);
}
  
// Driver program to test above function
int main()
{
    cout << contains("geeksforgeeks", "geeks") << endl;
    cout << contains("geeksforgeeks", "geeksquiz") << endl;
    cout << contains("geeksquizgeeks", "quiz") << endl;
    return 0;
}

Python3

# Recursive Python3 program to find if a given pattern is 
# present in a text
  
def exactMatch(text, pat, text_index, pat_index):
    if text_index == len(text) and pat_index != len(pat):
        return 0
   
    # Else If last character of pattern reaches
    if pat_index == len(pat):
        return 1
   
    if text[text_index] == pat[pat_index]:
        return exactMatch(text, pat, text_index+1, pat_index+1)
   
    return 0
  
   
# This function returns true if 'text' contain 'pat'
def contains(text, pat, text_index, pat_index):
    # If last character of text reaches
    if text_index == len(text):
        return 0
   
    # If current characters of pat and text match
    if text[text_index] == pat[pat_index]:
        if exactMatch(text, pat, text_index, pat_index):
            return 1
        else:
            return contains(text, pat, text_index+1, pat_index)
   
    # If current characters of pat and tex don't match
    return contains(text , pat, text_index+1, pat_index)
   
# Driver program to test the above function
  
print(contains("geeksforgeeks", "geeks", 0, 0))
print(contains("geeksforgeeks", "geeksquiz", 0, 0))
print(contains("geeksquizgeeks", "quiz", 0, 0))
  
# This code is contributed by ankush_953.

Output:

1
0
1

This article is contributed by Bhupinder. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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