sorting an array of strings using qsort

Question

So i have an array of strings called nova_str[50][1024] and what i want is to sort it using qsort the problem is that its not sorting anything.

My output:

* fcb
* bvb

Correct output:

* bvb
* fcb

as you can see the array isnt being sorted and i cant dont know why, so any help would be appreciated.

Program:

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


int string_cmp (const void * a, const void * b ) {
    const char * pa = *(const char * const * ) a;
    const char * pb = *(const char * const * ) b;

    return strcmp(pa,pb);
}

void print_array(char array[][1024], int len) 
{ 
    int i;
    for(i=0; i<len; i++)
    {
        printf("* %s\n",array[i]);
    }

}

int main(void)
{
    char nova_str[50][1024];
    strcpy(nova_str[0],"fcb");
    strcpy(nova_str[1],"bvb");
    qsort(nova_str,1, sizeof(char *)*1024, string_cmp);
    print_array(nova_str,2);
}

There are 2 elements in the array, but you are sorting only 1, it should be something like qsort(nova_str, 2, sizeof *nova_str, string_cmp); . — isrnick
– isrnick, Commented May 14, 2020 at 19:12
You sort 1 element only, and you pass the wrong element size as sizeof(char *)*1024. That is 1024 time the size of a pointer. qsort(nova_str, 2, sizeof nova_str[0], string_cmp) would work. — Weather Vane
– Weather Vane, Commented May 14, 2020 at 19:14
Also qsort(nova_str, 2, 1024, strcmp); will work without making your own comparator function. — Weather Vane
– Weather Vane, Commented May 14, 2020 at 19:19
your compare function is wrong, qsort (if called correctly as suggested) will pass just the pointer your strings, not a pointer to pointer. — MarcoLucidi
– MarcoLucidi, Commented May 14, 2020 at 19:23
@WeatherVane using strcmp directly will emit a warning of "incompatible pointer type" (at least on gcc) — MarcoLucidi
– MarcoLucidi, Commented May 14, 2020 at 19:41

bhristov · Accepted Answer · 2020-05-14 19:40:03Z

4

This will work.

// You are getting a pointer from qsort, not a pointer to a pointer.
int string_cmp (const void * a, const void * b ) {
    const char * pa = (const char *) a;
    const char * pb = (const char *) b;

    return strcmp(pa,pb);
}

void print_array(char array[][1024], int len) 
{ 
    int i;
    for(i=0; i<len; i++)
    {
        printf("* %s\n",array[i]);
    }

}

int main(void)
{
    char nova_str[50][1024];
    strcpy(nova_str[0],"fcb");
    strcpy(nova_str[1],"bvb");
    // the size is 2, not 1
    // you also want the correct size of the elements
    // getting the size of the first element will ensure this
    qsort(nova_str,2, sizeof(nova_str[0]), string_cmp);
    print_array(nova_str,2);
}

I hope that this helps.

answered May 14, 2020 at 19:40

bhristov

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