This C++ program takes an integer e and a string input, then applies a simple encryption algorithm to each digit in the input string by adding e to it and taking the result modulo 10. Here's the program rewritten with comments to explain each part:

include

using namespace std;

int main() { int e; // Declare an integer variable e to store the encryption key string input; // Declare a string variable input to store the input string

// Prompt the user to enter the input string followed by the encryption key
cout << "Enter the input string: ";
cin >> input;
cout << "Enter the encryption key: ";
cin >> e;

// Loop through each character of the input string
for (int i = 0; i < input.length(); ++i) {
    // Encrypt the current digit by adding e and taking modulo 10
    // Note: The expression 'input[i] - '0'' converts the character digit to its integer equivalent
    // Then add the encryption key e and take modulo 10 to ensure the result is a single digit
    // Finally, convert the result back to string and output it
    cout << to_string((input[i] - '0' + e) % 10);
}

// Output a newline character for better formatting
cout << endl;

return 0;

}