Count occurrences of a substring recursively

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

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 

Output:

2
2

My Personal Notes

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

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