# Count common characters in two strings in C++

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We are given with the two strings let’s say str1 and str2 and the task is to find the count of common characters in two strings i.e. if str1[i] = str[j], then they will be considered as a pair and count will increased to 1 and if str1[i]!=str2[j] then they willn’t be considered as a pair and count willn’t increase to 1.

## For Example

Input − str1 = “hello”
str2 = “heoo”
Output − count is: 3

Explanation − str1 = str2 i.e. h ; str1 = str2 i.e. e ; str1!=str2 i.e. l and o; str1=str2 i.e. o. So, the pairs with similar letters are 3 and 1 pair containing different letters.

Input − str1 = “point”
str2 = “print”
Output − count is: 4

Explanation − str1 = str2 i.e. p ; str1 != str2 i.e. o and r ; str1 =str2 i.e. i; str1=str2 i.e. n; str1=str2 i.e. t. So, the pairs with similar letters are 4 and 1 pair containing different letters.

## Approach used in the below program is as follows

• Input the two strings str1 and str2

• Calculate the size of both the strings using length() function that will return an integer value as per the number of letters in the string including the spaces.

• Initially, Initialize the frequency of characters in both the string with 0.

• Now, for updating the frequency of str1 apply “f1[str1[i] - 'a']++” that will increase the frequency with every iteration and apply the same process with str2

• For calculating the count of pairs apply min() function for f1 and f2.

• Display the result

## Example

Live Demo

#include <iostream>
using namespace std;
// Function to count the valid indices pairs
int pairs(string str1, int size1, string str2, int size2){
// f1 and f2 for frequencies of characters
// of string str1 and str2
int f1 = { 0 };
int f2 = { 0 };
// 'c' To count the valid pairs
int i, c = 0;
//updating the frequencies of str1 and st2
for (i = 0; i < size1; i++){
f1[str1[i] - 'a']++;
}
for (i = 0; i < size2; i++){
f2[str2[i] - 'a']++;
}
// Find the count of valid pairs
for (i = 0; i < 26; i++){
c += (min(f1[i], f2[i]));
}
return c;
}
// main function
int main(){
string str1 = "tutorialspoint", str2 = "codingground";
int size1 = str1.length(), size2 = str2.length();
cout<<”Total pairs with str1[i]=str2[j] are: ”;
cout << pairs(str1, size1, str2, size2);
return 0;
}

## Output

If we run the above code it will generate the following output −

Total pairs with str1[i]=str2[j] are − 6