How do I correctly sort strings in C?

Question

Here is my code:

#include <stdio.h>
#include <string.h>
#include <errno.h>

int cmp(const void *a, const void *b) {
    const char **ia = (const char **)a;
    const char **ib = (const char **)b;
    return strcmp(*ia, *ib);
}

void print_array(char **array, size_t len) {
    size_t i;
    for(i=0; i<len; i++) {
        printf("%s, ", array[i]);
    }
    putchar('\n');
}

int main(int argc, char *argv[]) {

        char *strings[] = { "z1.doc", "z100.doc",  "z2.doc", "z3.doc", "z20.doc"};
        size_t strings_len = sizeof(strings) / sizeof(char *);
        print_array(strings, strings_len);
        qsort(strings, strings_len, sizeof(char *), cmp);
        print_array(strings, strings_len);

        system("PAUSE");
        return 1;
}

the actual output is

z1.doc, z100.doc, z2.doc, z20.doc, z3.doc

and I want it to be

z1.doc, z2.doc, z3.doc, z20.doc, z100.doc

What am doing wrong?

Answer 1

The actual output is correct, the string "z100.doc" is less than "z2.doc". The strcmp compares character by character and when it gets to the '1' that is less than '2' and it stops there, so z100 < z2.

If you name the files z001.doc, z002.doc, z003.doc, z020.doc, z100.doc it will sort the way you want.

Answer 2

Change your comparator to say:

return (atoi(*ib + 1) - atoi(*ia + 1));

Answer 3

You want a natural sorting algorithm as opposed to the ASCIIbetical sorting algorithm provided by default. See Jeff's blog entry for a long rant about this, and some nice links to implementations of this natural algorithm. Just as a warning, the correct implementation (as opposed to the hacky answer you've accepted) is quite complicated, so don't try to implement it yourself, take someone else's (hopefully public domain or liberally licensed) implementation and use that instead.