Caesar - ASCII to char conversion problem

Question

Going through the CS50 exercise Caesar, and I'm still very new to C. My code to validate the key is all working, but as I try to convert from plaintext to the cipher text I keep getting caught up with the conversion form the ASCII number to the char. For example if I run the plaintext with the argv[1] as 27 I get /023/024. I recognize the math is off and if anyone has any pointers about that that would be great, but the main thing I'm curious about is why when I try to assign f into input[i] I get this "/000" type format.

string input = get_string("plaintext: ");
int len = strlen(input);
int i = 0;
while (input[i] != '\0')
{
    int mod = atoi(argv[1]);
    int t = input[i] + mod;
    int f = t % 26;
    input[i] = f;
    i++;
}
printf("ciphertext: %s\n", input);

Answer 1

The issue is that the process is missing a couple of steps. The method that uses modulo requires that the letters, which are represented as ASCII values, must first be converted to numbers from 0 to 25 inclusive, and lastly, after the shift, must be reconverted from a number between 0 and 25 inclusive, back to an ASCII value.

You would likely benefit from reviewing the video that covers this.