Recursive Function for Substring Search in C++

Given two strings Str and subStr as input. The goal is to find whether text present in subStr exists in Str as substring or not. The string X is called a substring of Y if whole X is present in Y at least once. We will use a recursive approach to do this.

For Example

Input − Str = “tutorialspoint” subStr=”Point”

Output − Given string does not contain substring!

Explanation − The string Point is not substring of tutorialspoint

Input − Str = “globalization” subStr=”global”

Output − Given string contains substring!

Explanation − The string global is substring of globalization

Approach used in the below program is as follows

In this approach we check if subStr is substring of Str in a recursive manner. The steps for recursion will be:-

• 1. Pass both strings to a recursive function where pointers will point to current character positions of both strings

• If string is ended but pattern has more characters left, then return 0 as pattern not found and we reached at the end of string.

• If the current character is the last character in the pattern then it is found in string, return 1.

• If both current characters are the same, then move both pointers to next positions.

• If both current characters do not match, then move the pointer for string to the next position.

• Take the input strings as character arrays Str and subStr.

• Function match(char *str1, char *substr1) takes two substrings and returns 1 if substr1 and str1 are the same.

• Both pointers point to characters present in strings, initially at starting positions.

• If substr is empty, then return 0.

• If both strings are empty, then also return 0.

• If both current characters are equal then recursively check for next characters using match(str1 + 1, substr1 + 1)

• Function checksubString(char *str2, char *substr2) takes both strings and returns 1 if substr2 is present in str2.

• If current characters pointed by str2 and substr2 are the same then check if consecutive characters also match using match () function. If it returns 1 then return 1.

• If reached the end of str2 then return 0.

• Otherwise recursively check for next character of str2 using checksubString(str2 + 1, substr2);

• If all conditions conditions fail then also recursively check using checksubString(str2 + 1, substr2);

• Print result according to return value.

Example

#include<iostream>
using namespace std;
int match(char *str1, char *substr1){
   if (*substr1 == '\0'){
      return 1;
   }
   if (*str1 == '\0'){
      if(*substr1 != '\0'){
         return 0;
      }
   }
   if (*str1 == *substr1){
      return match(str1 + 1, substr1 + 1);
   }
   return 0;
}
int checksubString(char *str2, char *substr2){
   if (*str2 == *substr2){
      if(match(str2, substr2)){
         return 1;
      }
   }
   if (*str2 == '\0'){
      return 0;
   }
   else{
      return checksubString(str2 + 1, substr2);
   }

   return checksubString(str2 + 1, substr2);
}
int main(){
   char Str[]="tutorialspoint";
   char subStr[]="point";

   if(checksubString(Str,subStr)==1){
      cout << "Given string contains substring!";
   }
   else{
      cout << "Given string does not contain substring!"; }
   return 0;
}

Output

If we run the above code it will generate the following Output

Given string contains substring!