C++ program to Count Common Subsequence in Two Strings

October 14, 2022

Counting Common Subsequence in Two Strings

In this article, we will learn how to write a C++ program to count common subsequence in two strings. We will take two strings as an input, then we will 2-D ”cnt[][]” array that will store the count of common subsequence found. Now we will iterate each character of first string and each character of second string from of the string to its end , then if the characters matches we will check if the previous character of both string are same or not if they are same we will assign ”1 +cnt[i][j – 1] +cnt[i – 1][j]” to our 2D else we will assign ”cnt[i][j – 1] + cnt[i – 1][j] – cnt[i – 1][j – 1]” to our 2D array. As the iteration ends we will get our count.

Algorithm:

Initialize the variables.
Accept the inputs.
Create a function to check common subsequence.
Iterate each character from first string .
Iterate each character from second string.
Match these characters one by one.
If they matches store the count.
Print count.

C++ Code:-

Run

//Count Common Subsequence in two Strings
#include <iostream>
#include <string.h>
using namespace std;

//Function to count the number of subsequences in the string.
  int countsequences(char str[], char str1[])
  {
    int l1 = strlen(str);
    int l2 = strlen(str1);
    int cnt[l1+1][l2+1];
    for (int i = 0; i <= l1; i++)
    {
      for (int j = 0; j <= l2; j++)
       {
       cnt[i][j] = 0; 
       }
    }
//Taking each character from first string.
    for (int i = 1; i <= l1; i++)
   {
//Taking each character from second string.
    for (int j = 1; j <= l2; j++)
    {
    //If characters are same in both the string.

    if(str[i-1] == str1[j-1])
    {
      cnt[i][j] = 1 + cnt[i][j-1] + cnt[i-1][j];
    }
    else
    {
      cnt[i][j] = cnt[i][j-1] + cnt[i-1][j] - cnt[i-1][j-1];
    }
  }
}
return cnt[l1][l2];
}
//Main function for printing the result.
  int main()
 {
    char str[100]="Prep" ,str1[100] = "Prepinsta";

  cout<<"Number of common subsequence is: "<<countsequences(str, str1);
  return 0;
}

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