Errors in ROT13 code (C) - c

Question

My program should take in a char, and encode it using ROT13 if it is a letter, and otherwise leave it the same, and then print the result.
My code below works for all lowercase letters, and uppercase letters A-M, but fails on uppercase letters N-Z, and other symbols/numbers. Any help appreciated :)
#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#define TRUE 1
#define FALSE 0
#define UPPER_START 65
#define UPPER_END 90
#define LOWER_START 97
#define LOWER_END 122
#define UPPER_MID 77
#define LOWER_MID 109
void testEncode (void); int isValid (char cipherChar); char encode (char letter);
int main (int argc, char* argv[]) {
char cipherChar;
scanf("%c", &cipherChar);
if (isValid(cipherChar) == TRUE) {
printf("%c", encode (cipherChar));
} else if (isValid(cipherChar) == FALSE) {
printf("%c", cipherChar);
}
return EXIT_SUCCESS;
}
int isValid (char cipherChar) {
int valid;
if ((cipherChar >= UPPER_START) &&
(cipherChar <= UPPER_END)) {
valid = TRUE;
} else if ((cipherChar >= LOWER_START) &&
(cipherChar <= LOWER_END)) {
valid = TRUE;
} else {
valid = FALSE;
}
return valid;
}
char encode (char letter) {
if ((letter <= UPPER_MID) || (letter <= LOWER_MID)) {
letter = letter + 13;
} else {
letter = (letter - 13);
}
return letter;
}

Answer 1

ROT13 generally doesn't affect characters other than [A-Za-z], so I'd say just ignore those. unless you have some reason to rotate them as well. For example, sometimes 0-9 are treated with ROT5.
In any case, your if statement with the clause (letter <= UPPER_MID) || (letter <= LOWER_MID) basically says, "any character before 'a' gets 13 added to it", so that's why [N-Z] aren't working correctly. Instead, try breaking out the upper- and lower-case logic and handling the wrap-around per-case, like this:
if (letter >= UPPER_START && letter <= UPPER_END) {
letter = letter + 13;
if (letter > UPPER_END) {
letter -= 13;
}
} else if (letter >= LOWER_START && letter <= LOWER_END) {
letter = (letter + 13);
if (letter > LOWER_END) {
letter -= 13;
}
}