Concatenate Two Strings Without Using function

#include <stdio.h>

int main()

{

    char str1[100], s2tr[100], i, j;

    printf("Enter first string: ");

    scanf("%s", str1);

    printf("Enter second string: ");

    scanf("%s", str2);

    // calculate the length of string s1

    // and store it in i  
    for(i = 0; str1[i] != '\0'; ++i);

    for(j = 0; str2[j] != '\0'; ++j, ++i)

    {

        str1[i] = str2[j];

    }

    str1[i] = '\0';

    printf("After concatenation: %s", str1);

    return 0;

}

Output:-