Thread: Problem when qsorting an array of floats

Hi,

I am trying to qsort an array of floats and for some reason I am not getting the proper results. I am posting my code here. Could any of you take a look and tell me what I am missing.

Code:

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


int compare(const void*a, const void*b)
{

 int a1 =  ( *(float*) a);
 int a2 =  ( *(float*) b);
 
  return a1 -a2;
}


int main()
{

  float array[] = { 1605748992, 2242276608, 6265311232, 8249718272, 1181892352, 2202173184, 411911232, 6999099904, 11551354880, 5034336768,2997344512,636791104,1221603328,2306947328,7531588096,8709864448,7865203712,1004122048,2171712000,4130571776,4451974656 };

  int i,j, lib_no =21;

  for(i=0; i<lib_no; i++)
  {

    printf("%f\n",array[i]);

  }

  printf("\n\n\n\n");

  qsort(&array[0], lib_no, sizeof(float), compare);


  for(i=0; i<lib_no; i++)
  {

    printf("%f\n",array[i] );

  }

 return 0;
}

When I run this, I am getting an array which is ordered differently from the original array. But not properly sorted.

I think that the problem is something in the the way I am typecasting the void pointers inside the compare function. But not sure what..I was earlier doing it like this..

Code:

int compare(const void*a, const void*b)
{

 return (int) ( *(float*)a - *(float*)b) );
}

and that gave me an output with a different order. But again not properly sorted. Does anyone have any suggestions on what I should try? What am I doing wrong?

Thanks,

Avinash

@Matsp: Sorry bout that.. My bad. But fixing that typecast to

Code:

 int a1 = (int) (* (float *) a)

doesn't mean to make much difference. I still get the same output. Btw, if I assign a float to an int, isn't it usually automatically converted?

@Laserlight: You maybe right. Do u mean i should change my code to something like

Code:

int compare(const void*a, const void*b)
{

 float a1 =  ( *(float*) a);
 float a2 =  ( *(float*) b);
 
  return (int) a1 -a2;
}

.

I tried that and i still get a bad o/p..

Oh.. Tat works!. Thanks a ton for the help.. Not being able to figure that out was driving me nuts.

So.. the problem was that when I just return the difference between 2 numbers, sometimes the return value might be too large that it might overflow int or something. So an very large difference might actually become a negative value. Thus messing up the order. Is that correct?

Yes.. Got it now. Thanks for the explanation. Cheers..