c89 - Problem trying to use the C qsort function

Question

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

float values[] = { 4, 1, 10, 9, 2, 5, -1, -9, -2,10000,-0.05,-3,-1.1 };

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

int main ()
{

    int i;

    qsort (values, 13, sizeof(float), compare);

    for (i = 0; i < 13; i++)
    {
        printf ("%f ",values[ i ]);
    }
    putchar('
');

    return 0;
}

The result is:

-9.000000 -3.000000 -2.000000 -1.000000 -1.100000 -0.050000 1.000000 2.000000 4.000000 5.000000 9.000000 10.000000 10000.000000

It's wrong because the order of -1 and -1.1 is changed. I believe it is happening because my "compare" function.

How can I fix this?

Thanks

Answer 1

Your comparison function is broken. It says, for example, that -1.0 is equal (equivalent) to -1.1, since (int) ((-1.0) - (-1.1)) is zero. In other words, you yourself told qsort that the relative order of -1.0 and -1.1 does not matter. Why are you surprised that in the resultant ordering these values are not sorted?

In general, you should avoid comparing numerical values by subtracting one from another. It just doesn't work. For floating-point types it might produce imprecise results for quite a few different reasons, one of which you just observed yourself. For integer types it might overflow.

The generic idiom for comparing two numerical values a and b for qsort looks as (a > b) - (a < b). Remember it and use it. In your case that would be

int compare (const void * a, const void * b)
{
  float fa = *(const float*) a;
  float fb = *(const float*) b;
  return (fa > fb) - (fa < fb);
}

In C code it might make perfect sense to define a macro

#define COMPARE(a, b) (((a) > (b)) - ((a) < (b)))

and use it instead of spelling out the comparisons explicitly.