rot_13 — ROT13 Cipher

Allowed functions: write

Write a program that takes a string and displays it, replacing each of its letters by the letter 13 spaces ahead in alphabetical order.

z becomes m and Z becomes M. Case remains unaffected.

The output will be followed by a newline. If the number of arguments is not 1, the program displays a newline.

Examples

$>./rot_13 "abc"
nop
$>./rot_13 "My horse is Amazing." | cat -e
Zl ubefr vf Nznmvat.$
$>./rot_13 "AkjhZ zLKIJz , 23y " | cat -e
NxwuM mYXVWm , 23l $
$>./rot_13 | cat -e
$

Solution

Download rot_13.c

#include <unistd.h>

int main(int ca, char **v) {
	if (ca != 2) {
		write(1, "\n", 1);
		return 0;
	}
	char *str = v[1];
	char c;

	while (*str) {
		c = *str;
		if ((*str >= 'a' && *str <= 'z') || (*str >= 'A' && *str <= 'Z'))
			c = *str + 13 - 26 * (*str % 32 > 13);

		write(1, &c, 1);

		str++;
	}

	write(1, "\n", 1);
	return 0;
}

How It Works

Goal: Encode a string by replacing each letter with the letter 13 positions ahead in the alphabet.

Approach: Add 13 to each letter's ASCII value, and subtract 26 if it would wrap past 'z' or 'Z', using a modulo trick.

Step by step:

1. If argument count is not 2, print a newline and exit.
2. For each character, check if it is a letter (uppercase or lowercase).
3. If it is a letter, compute the shifted character: add 13 and subtract 26 if the letter's position in the alphabet (*str % 32) is greater than 13 (meaning it would overflow past 'z' or 'Z').
4. Write the resulting character (shifted or original) to stdout.

Key concept: ASCII arithmetic and modulo for wrapping — using % 32 to get a letter's 1-based position in the alphabet regardless of case, enabling a single expression to handle wrap-around.