rotone — Shift Letters by One

Allowed functions: write

Write a program that takes a string and displays it, replacing each of its letters by the next one in alphabetical order.

z becomes a and Z becomes A. 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

$>./rotone "abc"
bcd
$>./rotone "Les stagiaires du staff ne sentent pas toujours tres bon." | cat -e
Mft tubhjbjsft ev tubgg of tfoufou qbt upvkpvst usft cpo.$
$>./rotone "AkjhZ zLKIJz , 23y " | cat -e
BlkiA aMLJKa , 23z $
$>./rotone | cat -e
$

Solution

Download rotone.c

#include <unistd.h>

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

		if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))
			c = *str + 1 - 26 * (*str % 32 > 25);

		write(1, &c, 1);
		str++;
	}
	write(1, "\n", 1);
	return 0;
}

How It Works

Goal: Encode a string by shifting each letter one position forward in the alphabet, wrapping 'z' to 'a' and 'Z' to 'A'.

Approach: Add 1 to each letter's ASCII value, and subtract 26 if it would wrap past the end of the alphabet.

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.
3. If it is a letter, add 1 to its ASCII value. If the letter's position in the alphabet (*str % 32) is greater than 25 (i.e., it is 'z' or 'Z'), subtract 26 to wrap back to 'a' or 'A'.
4. Write the resulting character to stdout.

Key concept: ASCII arithmetic and alphabet wrapping — using % 32 to determine a letter's position and conditionally subtracting 26 to wrap from the end of the alphabet back to the start.