Qsort

I saw this example on a website:

Code:

/* qsort example */
#include <stdio.h>
#include <stdlib.h>

int values[] = { 40, 10, 100, 90, 20, 25 };

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

int main ()
{
  int * pItem;
  int n;
  qsort (values, 6, sizeof(int), compare);
  for (n=0; n<6; n++)
  {
    printf ("%d ",values[n]);
  }
  return 0;
}

I understand all of it but the compare function. Could someone explain to me in simple terms what the compare function is doing?

Thanks,

Shane

That compare function is subtracting a from b. First it casts each void * as an int *, and then dereferences them. The end result is a positive integer if a > b, 0 if a = b, and a negative integer if a < b.

Except it's safer to write it out in a slightly longer form

Code:

int compare (const void * a, const void * b)
{
  int v1 = *(int*)a;
  int v2 = *(int*)b;
  if ( v1 < v2 ) return -1;
  if ( v1 > v2 ) return +1;
  return 0;
}

A simple subtraction has the possibility of underflow, and thus the possibility of producing the wrong answer.

Thanks to both of you.