qsort() of one array with respect to another array

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); }

In which language you are looking for the code? OR You are just looking for quick sort algorithm? — Ravindra Gullapalli, Jan 25 '13 at 14:40
How exactly is the result calculated? Can you show us how that resulting array is constructed from the two inputs? — Dominik Sandjaja, Jan 25 '13 at 14:41
Does this answer your question? [using qsort to sort two arrays simultaneously?](https://stackoverflow.com/questions/27008646/using-qsort-to-sort-two-arrays-simultaneously) — qwr, Jun 16 '21 at 05:12

score 4 · Accepted Answer · answered Jan 25 '13 at 17:19

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;
}

that comparison function may overflow, see https://stackoverflow.com/a/27284248/3163618 — qwr, Jun 16 '21 at 05:13

score 0 · Answer 2 · answered Apr 12 '21 at 17:19

You would need a mechanism to inform the compare function what to compare to rather than the vanilla implementation of comparing with values stored in the addresses passed to it. This can be achieved with a static (global) storage:

#include<stdlib.h>

int *Array= NULL;
void SetArray(int *const array)
{
  Array= array;
}

int basecompare(const void *a, const void *b)
{
  return Array[*((int *) a)]- Array[*((int *) b)];
}
int main(int argc, char *argv[])
{
  int a[]= { 0, 1, 2, 3, 4 };
  int b[]= { 3, 4, 5, 1, 2 };
  size_t len= sizeof(a)/ sizeof(a[0]);

  SetArray(b);

  qsort(a, len, sizeof(int), basecompare);

  return 0;
}

qsort() of one array with respect to another array

2 Answers2