I can't print character in C using printf - c

Question

I am trying to print the output of the function "char rotate(char c, int n)" but it will print only the numbers and not the characters. any help is appreciate it.
#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
bool only_digits(string arg);
char rotate(char c, int n);
int main(int argc, string argv[])
{
// Make sure program was run with just one command-line argument
if (argc !=2)
{
printf("Usage: ./caesar key\n");
return 1;
};
// Make sure every character in argv[1] is a digit
if ( only_digits(argv[1])==0)
{
printf("Usage: ./caesar key\n");
return 1;
};
// Convert argv[1] from a `string` to an `int`
int key = atoi(argv[1]);
// Prompt user for plaintext//
string text = get_string("plaintext: \n");
// For each character in the plaintext:
printf("ciphertext: ");
for (int i = 0; i < strlen(text); i++)
{
char ch = (rotate(text[i], key));
printf("%c", ch);
};
}
bool only_digits(string arg)
{
// Rotate the character if it's a letter
for (int i = 0; i < strlen(arg); i++)
{
if (!isdigit(arg[i]))
{
return false;
}
}
return true;
}
char rotate(char c, int n){
if (isupper(c))
{
c -=65;
c = (c + n) % 26;
c = c + '0';
return c;
}
else if (islower(c))
{
c -=97;
c = (c + n) % 26;
c = c + '0';
return c;
}
else
return c;
}
I tried different ways of casting the integers into characters and experimenting with a debugger and using printf statement to figure out what's causing this to no avail.
Edit1:
in my original post I deleted bunch of my code thinking it's not relevant to my issue but some of the questions asked about those parts, so hopefully now it's more clear.
when I type abc it print no thing at all, just "plaintext: "

Answer 1

OP eventually determined a major issue.
Code was substring a '0' when an 'A' or 'a' should have been subtracted.
To handle other issues:
is...(ch) has trouble when ch < 0
Best to use unsigned char.
Avoid magic numbers like 65
Avoid negatives and overflow errors
c + n may overflow
The sum may be negative. So let us work with certain positive numbers.
Unneeded else
// In main(), bring `key` into the range [0...25];
int key = atoi(argv[1]);
key &= 26; // key now in the [-25 ... 25] range
if (key < 0) key += 26;
// n is in the 0...25 range.
static char rotate(char c, int n) {
unsigned char uch = (unsigned char) c;
if (isupper(uch)) {
uch = (uch - 'A' + n) % 26;
return uch + 'a';
}
if (islower(uch)) {
uch = (uch - 'a' + n) % 26;
return uch + 'a';
}
return c;
}