Need help using qsort with an array of structs

Question

Now, I have seen various examples, but I don't get what they mean.

Here's my structure

typedef struct profile{
    char gender[1];
    double soc;
       . . .
} PROFILE;

where soc is social security number that I'm going to be sorting by.

I know you need a compare function, but I don't know how to come up with the exact thing I need.

Answer 1

Here is an example of using qsort for an array of structs in C

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

typedef struct {
    int price;
    int id;
} order;

int compare(const void *a, const void *b) {
  
    order *orderA = (order *)a;
    order *orderB = (order *)b;
  
    return (orderB->price - orderA->price);
}

int main() {
    order list[6];

    srand(time(NULL));

    printf("Before sorting\n");
    for (int i = 0; i < 6; i++) {   
        list[i].price = rand() % 10;
        list[i].id = i; 
        printf("Order id = %d Price = %d\n", list[i].id, list[i].price);            
    }

    qsort(list, 6, sizeof(order), compare);

    printf("AFTER sorting\n");
    for (int n = 0; n < 6; n++) {
        printf("Order id = %d Price = %d\n", list[n].id, list[n].price);    
    }       
    return 0;
}

Answer 2

Your Soc should almost certainly not be of type double, but anyway here's an example of what a compare function needs to return:

int compare(const void *p1, const void *p2)
{
    const struct profile *elem1 = p1;    
    const struct profile *elem2 = p2;

   if (elem1->soc < elem2->soc)
      return -1;
   else if (elem1->soc > elem2->soc)
      return 1;
   else
      return 0;
}

Thanks for pointing out the const void *.

Here is a complete example (archived): Sorting Structures with the C qsort() Function

Answer 3

The strict version of a comparator takes two constant void pointers:

int compare(const void *v1, const void *v2)
{
    const struct profile *p1 = v1;
    const struct profile *p2 = v2;
    if (p1->gender > p2->gender)
        return(+1);
    else if (p1->gender < p2->gender)
        return(-1);
    else if (p1->soc > p2->soc)
        return(+1);
    else if (p1->soc < p2->soc)
        return(-1);
    else
        return(0);
}

This compares the gender field first, then the soc field. This is how you handle any multipart comparison.

Answer 4

To sort the array, use qsort() and pass a comparison function.

Here is one that produces the correct result for all possible values of the price member:

typedef struct profile {
    char gender[1];
    double soc;
    int price;
    ...
} PROFILE;

int compare_price(const void *a, const void *b) {
    const PROFILE *oa = a;
    const PROFILE *ob = b;

    return (oa->price > ob->price) - (oa->price < ob->price);
}

int compare_soc(const void *a, const void *b) {
    const PROFILE *oa = a;
    const PROFILE *ob = b;

    return (oa->soc > ob->soc) - (oa->soc < ob->soc);
}

Notes:

• the simple subtraction of values produces incorrect results if the difference does not fit in the int type. For example -2 and INT_MAX cannot be correctly compared with the subtraction method. It would not work for floating point values either.

• the above method can be used for all comparable types, including double except for NaN.

If you wish to handle NaN, here is how to group them at the end:

#include <math.h>

int compare_soc_nan_at_the_end(const void *a, const void *b) {
    const PROFILE *oa = a;
    const PROFILE *ob = b;

    if (isnan(oa->soc)) {
        return isnan(ob->soc) ? 0 : 1;
    } else
    if (isnan(ob->soc)) {
        return -1;
    } else {
        return (oa->soc > ob->soc) - (oa->soc < ob->soc);
    }
}