Question

I want to use qsort() to sort array a with respect to b. Can you give me the function?

a={0,1,2,3,4}
b={3,4,5,1,2}

answer must be {3,4,0,1,2}

Please give me the code of function.

like :
int compare (const void *a,const void *b)
{
return(*(char *)a-*(char *)b);
}

Answer 1

This isn’t possible the way you currently have it because qsort() takes in one array and compares elements of the array against each other. You would need to create a single array of a struct containing both values, like so:

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

typedef struct {
    int a;
    int b;
} c_type;

int
compare(const void *a, 
        const void *b) {
    return ((c_type *)a)->b - ((c_type *)b)->b;
}

int
main(int argc,
     char *argv[])
{
    int i = 0;
    c_type array[] = {{0, 3}, {1, 4}, {2, 5}, {3, 1}, {4, 2}};

    qsort(array, sizeof array / sizeof(*array), sizeof(*array), compare);

    for ( i = 0; i < sizeof array / sizeof(*array); i++ ) {
       printf("%d\t", array[i].a);
    }
    printf("\n");

    return 0;
}