Concatenate

#include <iostream>
#include <string.h>
using namespace std;

void concatenate(char first_string[], char second_string[])
{
    int end_of_first_string = strlen(first_string);

    for (int i = 0; i < strlen(second_string); i++)
    {
        int j = end_of_first_string + i;
        first_string[j] = second_string[i];
    }

    first_string[strlen(first_string)] += 0;
}

int main()
{
    char firstname[20] = "YoonA";
    char lastname[20] = "Lim";

    char desiredname[20];
    strcpy_s(desiredname, firstname);

    cout << "Hi " << desiredname << endl;
    cout << "YoonA has " << strlen(desiredname) << " letters in it" << endl;

    concatenate(firstname, lastname);
    cout << firstname;
}