C/C++ Lessons [Lesson 01]

You are given a file named "rot.c". This file performs ROT-13 encoding on a command line argument. You will need to research ROT encoding (very basic encoding) and then modify the program as specified in the questions. Before you begin coding take some time to look at what the existing code does.

Good Luck.
-Jason

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

#define ROT 13

/* Post: rotate c returns the character ROT positions further along the
   alphabetic character sequence from c, or c if c is not lower-case
*/
static char rotate(char c)
{
	/* Check if c is lower-case or not */
	if (islower(c))
	{
		/* The ciphered character is ROT positions beyond c,
		   allowing for wrap-around
		*/
		return ('a' + (c - 'a' + ROT) % 26);
	}
	else
	{
		return c;
	}
}

int main(int argc, char *argv[])
{
	/* Exit with an error if the number of arguments (including
	   the name of the executable) is not precisely 2
	*/
	if(argc != 2)
	{
		fprintf(stderr, "%s: program expected 1 argument, but instead received %d\n", argv[0], argc-1);
		exit(EXIT_FAILURE);
	}
	else
	{
		/* Calculate the length of the first argument */
		int length = strlen(argv[1]);
		/* Loop for every character in the text */
		for (int i = 0; i < length; i++)
		{
			/* Determine and print the ciphered character */
			printf("%c", rotate(argv[1][i]));
		}
		/* Print one final new-line character */
		printf("\n");
		/* Exit indicating success */
		exit(EXIT_SUCCESS);
	}
	return 0;
}