Thread: Minor problem, hope someone can help

Im new to C, and have been teaching myself for a few days now. I'm trying to write some code which will reverse the digits of a number provided as an argument when the code is run from the command line. Here's what I've come up with:

Code:

/*
 *  Test.c
 *  lab2
 *
 *  Created by ** on 14/01/09.
 *  Copyright 2009 UWA. All rights reserved.
 *
 */

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


int reverse(char s[])
{
	int length = strlen(s);
	int y = atoi(s);
	for (int r = 0; r < length; r++)
	{
		s[r] = (y % 10);
		y = (y / 10);
	}
	return atoi(s);
}

int main(int argc, char *argv[])
{
	int y = (reverse(argv[1]));
	printf("%d\n", y);
	return 0;
}

I have tested it as much as I can and from what I can see, the string s[r] is being filled correctly with the input string numbers, but in reverse, though for some reason when i return the integer equivalent of this string using atoi, I always get 0 as the response.
Help would be much appreciated. Thankyou.

Originally Posted by h4rrison.james

Of course.

Thanks for your help.

The other option, of course, is to not modify the s[] input (which is nice to the caller, as the caller may need to make a copy, which is extra work), and just calculate (using multiplication by 10 and addition) to produce the integer - and it's probably no slower than using atoi(). It would then also work to do something like this:

Code:

x = reverse("12345");

which won't work on many modern OS's where a string literal (in this case "12345") is not writable.

--
Mats