Count occurrences of a substring recursively

Difficulty Level : Easy
Last Updated : 19 May, 2021

Given two strings str1 and str2, the task is to count the number of times “str2” occurs in “str1” using recursion.
Examples:

Input : str1 = "geeksforgeeks", str2 = "geek"
Output : 2

Input: kanekihiishishi
Output: 3

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Suppose the problem has n parts, divide the problem in such a way that considers n-1 parts already done after which the operation to be performed is limited to only one part. Thereby, dividing the recursion approach into two cases i.e. base case and the recursive case.
In this particular problem, the base case involves the fact that if the length of str1 is less than that of str2.
Now, talking about the recursive case, compare first substring of str1 with str2 and recur for remaining str1.

C++

// Recursive C++ program for counting number of substrings
#include <iostream>
#include <string>
using namespace std;
 
// Recursive function to count
// the number of occurrences of "hi" in str.
int countSubstrig(string str1, string str2)
{
    int n1 = str1.length();
    int n2 = str2.length();
 
    // Base Case
    if (n1 == 0 || n1 < n2)
        return 0;
 
    // Recursive Case
    // Checking if the first substring matches
    if (str1.substr(0, n2).compare(str2) == 0)
        return countSubstrig(str1.substr(n2-1), str2) + 1;
 
    // Otherwise, return the count from
    // the remaining index
    return countSubstrig(str1.substr(n2-1), str2);
}
 
// Driver function
int main()
{
    string str1 = "geeksforgeeks", str2 = "geeks";
    cout << countSubstrig(str1, str2) << endl;
 
    str1 = "hikakashi", str2 = "hi";
    cout << countSubstrig(str1, str2) << endl;
    return 0;
}

Java

// Recursive Java program for
// counting number of substrings
class GFG
{
 
// Recursive function to
// count the number of
// occurrences of "hi" in str.
static int countSubstrig(String str1,
                         String str2)
{
    int n1 = str1.length();
    int n2 = str2.length();
 
    // Base Case
    if (n1 == 0 || n1 < n2)
        return 0;
 
    // Recursive Case
    // Checking if the first
    // substring matches
    if (str1.substring(0, n2).equals(str2))
        return countSubstrig(str1.substring(n2 - 1),
                                            str2) + 1;
 
    // Otherwise, return the count
    // from the remaining index
    return countSubstrig(str1.substring(n2 - 1),
                                        str2);
}
 
// Driver Code
public static void main(String args[])
{
    String str1 = "geeksforgeeks",
           str2 = "geeks";
    System.out.println(countSubstrig(str1,
                                     str2));
 
    str1 = "hikakashi";
    str2 = "hi";
    System.out.println(countSubstrig(str1,
                                     str2));
 
}
}
 
// This code is contributed
// by Arnab Kundu

Python3

# Recursive Python3 program for
# counting number of substrings
 
# Recursive function to
# count the number of
# occurrences of "hi" in str.
def countSubstrig(str1, str2):
     
    n1 = len(str1);
    n2 = len(str2);
     
    # Base Case
    if (n1 == 0 or n1 < n2):
        return 0;
 
    # Recursive Case
    # Checking if the first
    # substring matches
    if (str1[0 : n2] == str2):
        return countSubstrig(str1[n2 - 1:],
                             str2) + 1;
 
    # Otherwise, return the count
    # from the remaining index
    return countSubstrig(str1[n2 - 1:],
                         str2);
 
# Driver Code
if __name__ == '__main__':
     
    str1 = "geeksforgeeks";
    str2 = "geeks";
    print(countSubstrig(str1, str2));
 
    str1 = "hikakashi";
    str2 = "hi";
    print(countSubstrig(str1, str2));
 
# This code is contributed by Princi Singh

C#

// Recursive C# program for
// counting number of substrings
using System;
class GFG
{
 
// Recursive function to
// count the number of
// occurrences of "hi" in str.
static int countSubstrig(String str1,
                         String str2)
{
    int n1 = str1.Length;
    int n2 = str2.Length;
 
    // Base Case
    if (n1 == 0 || n1 < n2)
        return 0;
 
    // Recursive Case
    // Checking if the first
    // substring matches
    if (str1.Substring(0, n2).Equals(str2))
        return countSubstrig(str1.Substring(n2 - 1),
                                            str2) + 1;
 
    // Otherwise, return the
    // count from the remaining
    // index
    return countSubstrig(str1.Substring(n2 - 1),
                                        str2);
}
 
// Driver Code
public static void Main()
{
    string str1 = "geeksforgeeks",
           str2 = "geeks";
    Console.Write(countSubstrig(str1,
                                str2));
    Console.Write("\n");
     
    str1 = "hikakashi";
    str2 = "hi";
    Console.Write(countSubstrig(str1,
                                str2));
 
}
}
 
// This code is contributed
// by Smita

Javascript

<script>
 
// Recursive js program for counting number of substrings
 
// Recursive function to count
// the number of occurrences of "hi" in str.
function countSubstrig( str1, str2){
    let n1 = str1.length;
    let n2 = str2.length;
 
    // Base Case
    if (n1 == 0 || n1 < n2)
        return 0;
 
    // Recursive Case
    // Checking if the first substring matches
    if (str1.substr(0, n2) == (str2))
        return countSubstrig(str1.substr(n2-1), str2) + 1;
 
    // Otherwise, return the count from
    // the remaining index
    return countSubstrig(str1.substr(n2-1), str2);
}
 
// Driver function
let str1 = "geeksforgeeks", str2 = "geeks";
document.write( countSubstrig(str1, str2),'<br>');
str1 = "hikakashi", str2 = "hi";
document.write( countSubstrig(str1, str2),'<br>');
 
 
</script>

Output:

2
2

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Count occurrences of a substring recursively

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