Expert: Narendra - 9/24/2005

Question
Hello,
I was doing some search online and I found this link and decided it try it out. Here is the problem I am having. I want to sort a multdimensional data according to the first column. First I tried to sort  an array which contains doubles in th efollowing code by using the built in function qsort. But after run I just don't get the sorted array and I am wondering what it is happening. Input file will be assumed comma separated txt file.

note:I have only a  week of experience with C and if it is a common question you don't even have to consider it. The compiler I ma using is lcc.

here is the code:
#include <stdlib.h>
#include <stdio.h>
//sort an array containing doubles:initial try!


int compare_doubles (const double * x,const double * y)
{
   if (*x > *y)
      return 1;
   else if (*x < *y)
      return -1;
   else
      return 0;
}


int main (){

  int i=0,nROW,nCOL;
  
  printf("Enter #ROWs:   ");
  scanf("%d",&nROW);

  FILE *iPtr=fopen("in.txt","r");


float *a;
  a= malloc(sizeof(double *)*nROW);

  while (!feof(iPtr))   
        {
        fscanf(iPtr,"%f,",&a[i]);
     printf("%f\n",a[i]);
     i++;
     }



qsort(a, nROW, sizeof(double),compare_doubles);

printf("after sorting:\n");

  for(i=0;i<nROW;i++)
  {
      printf("%f\n",a[i]);
     }

  return 0;

}


Answer
Here is your corrected code.
Compare it with your previous code and you will find the difference.
I have one suggestion for you. Whenever you program, follow a uniform indentation. It will help you and others in debugging the program. It will improve readability.

#include <stdlib.h>
#include <stdio.h>
//sort an array containing doubles:initial try!


int compare_doubles (const void *X, const void *Y)
{
       double x = *((double *)X);
       double y = *((double *)Y);

       if (x > y)
       {
               return 1;
       }
       else
       {
               if (x < y)
               {
                       return -1;
               }
               else
               {
                       return 0;
               }
       }
}


int main ()
{

       int i = 0, nROW;

       printf("Enter #ROWs: ");
       scanf("%d", &nROW);

       FILE *iPtr = fopen("in.txt", "r");
       double *a;

       a = malloc(sizeof(double) * nROW);

       while (!feof(iPtr))
       {
               fscanf(iPtr, "%lf,", &a[i]);
               printf("%lf\n", a[i]);
               i++;
       }

       qsort((void *)a, nROW, sizeof(double), compare_doubles);

       printf("after sorting:\n");

       for(i = 0; i < nROW; i++)
       {
               printf("%f\n", a[i]);
       }

       return 0;
}