Use of qsort() : C

[Adene]

Hello, I am the student of the student of the Bsc. I had just started to learn the C language. I do not have the knowledge of the C language. I have to create the project on the qsort() function in C language. So, I want to know about the qsort() function. I also want to know how can I Use the qsort() function in C language. Thus, if anyone has the answer for me then reply me.

[Praetor]

Hi, The qsort() function can be used for an Array to sort them. The qsort() function can use the Sedgewick's Quicksort algorithm to sort an array. Every element in the array can be the width bytes in size. the following can be the syntax of the qsort() function :

#include <stdlib.h>
void qsort( void *base, size_t num, 
size_t width, int (*compar) ( const void *, const void *) );

[Katty]

The Following program can demonstrates you the working of the qsort() function :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char *ChrVt[] = { "last", "middle", "first" };
int compare( const void *op1, const void *op2 )
  {
    const char **p3 = (const char **) op3;
    const char **p4 = (const char **) op4;
    return( strcmp( *p3, *p4 ) );
  }
int main( void )
  {
    qsort( ChrVt, sizeof(ChrVt)/sizeof(char *),
      sizeof(char *), compare );
    printf( "%s %s %s\n",
        ChrVt[0], ChrVt[1], ChrVt[2] );
    return( EXIT_SUCCESS );
  }

The Output of the above :

first last middle

[kelfro]

Program :

#include <stdlib.h>
#include <stdio.h>
int compare_doubles (const void *Y, const void *Z)
{
       double y = *((double *)Y);
       double z = *((double *)Z);
       if (y > z)
       {
               return 1;
       }
       else
       {
               if (y < z)
               {
                       return -1;
               }
               else
               {
                       return 0;
               }
       }
}
int main ()
{
       int j = 0, nRW;
       printf("Enter #ROWs: ");
       scanf("%d", &nROW);
       FILE *iPr = fopen("in.txt", "r");
       double *b;
       b = malloc(sizeof(double) * nRW);
       while (!feof(iPr))
       {
               fscanf(iPr, "%lf,", &b[j]);
               printf("%lf\n", b[j]);
               j++;
       }
       qsort((void *)b, nRW, sizeof(double), compare_doubles);
       printf("after sorting:\n");
       for(j = 0; j < nRW; j++)
       {
               printf("%f\n", b[j]);
       }
       return 0;
}

[Zecho]

Hi, The qsort() function can be used for the to sort the buf num items, each of using Quicksort. To compare the buffer items compare function can be use.
The compare function can returns the following :
1. zero if they are equal
2. positive if the first argument is greater than the second.
3. compare should return negative if the first argument is less than the second
The qsort() function can sorts the given buffer in ascending order.