CS50 Caesar - Check50 won't validate my code

Question

I know that this question have been asked before, but I still can't find the clue to the problem in my code. My program works apparently fine, but I'm not able to pass the check50 test. From what I understand, the issue may be related to the fact that the null \0 is printed. But I don't know how to modify that. Could you please help me?

This is my code:

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

bool only_digits(string s);
char rotate(char c, int n);
int main(int argc, string argv[])
{
string h = argv[1];
if (argc != 2 || !only_digits(h) || h <= 0)
    {printf("Usage: ./caesar key\n");
    return 1;
    }
else
{
    int key = atoi(argv[1]);

string plaintext = get_string("plaintext:  ");

    int f = strlen(plaintext);
    printf("ciphertext: ");
    for(int q = 0; q < f; q++)
    {
        printf("%c", rotate(plaintext[q], key));
    }
    printf("\n");
}
    return 0;
}


bool only_digits(string s )
{
    for (int i = 0, n = strlen(s); i < n; i++)
{
    char digit = s[i];
    if (!isdigit(digit))
    return false;
}
return true;
}

char rotate(char c, int n)
{
    if(isupper(c) && (c != '\0'))
{
    printf("%c", (((c - 65) + n) % 26) + 65);
}
    else
    if(islower(c) && (c != '\0'))
    {
        printf("%c", (((c - 97) + n) % 26) + 97);
    }
    else
    printf("%c", c);
return 0;
}

This is a caption of check50's check: check50

Answer 1

rotate always returns 0, so printf("%c", rotate(plaintext[q], key)); is causing the letters you output to be interspaced with NUL characters.

I would keep that printf, but change rotate to return the character instead of printing it.