String sorting example in Objective-C

sort-string.m

#import <Foundation/Foundation.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

static int compare_char(const char *a, const char *b)
{
    if (*a > *b) {
        return 1;
    } else if (*a < *b) {
        return -1;
    } else {
        return 0;
    }
}

@interface NSString (Sorting)
- (NSString *)stringBySortingCharacters;
@end

@implementation NSString (Sorting)
- (NSString *)stringBySortingCharacters
{
    const char *s = [self UTF8String];
    char *s2 = (char *) calloc([self length]+1, 1);
    if (!s2) return nil;
    strncpy(s2, s, [self length]);
    qsort(s2, [self length], 1, compare_char);
    NSString *ret = [NSString stringWithUTF8String:s2];
    free(s2);
    return ret;
}
@end

int main(int argc, char **argv)
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSString *s1 = @"string";
    NSString *s2 = @"the quick brown fox jumps over the lazy dog";
    printf("Sorted: %s\n", [[s1 stringBySortingCharacters] UTF8String]);
    printf("Sorted: %s\n", [[s2 stringBySortingCharacters] UTF8String]);

    [pool release];
    return 0;
}