INFO: Sorting Structures with the C qsort() Function (73853)

MORE INFORMATION

In the sample code, the animal structure contains both an integer, which is the key to be sorted on, and an array of char that contains the animal's name. The compare() function receives two pointers to type struct animal and returns -1, 1, or 0 (zero) if the first element is less than, greater than, or equal to the first element, respectively.

Sample Code

/* Compile options needed: none
 *
 * This example program uses the C run-time library function qsort()
 * to sort an array of structures.
 */ 

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

typedef int (*compfn)(const void*, const void*);

struct animal { int  number;
                char name[15];
              };

struct animal array[10]  = { {  1, "Anaconda"    },
                             {  5, "Elephant"    },
                             {  8, "Hummingbird" },
                             {  4, "Dalmatian"   },
                             {  3, "Canary"      },
                             {  9, "Llama"       },
                             {  2, "Buffalo"     },
                             {  6, "Flatfish"    },
                             { 10, "Zebra"       },
                             {  7, "Giraffe"     }  };

void printarray(void);
int  compare(struct animal *, struct animal *);

void main(void)
{
   printf("List before sorting:\n");
   printarray();

   qsort((void *) &array,              // Beginning address of array
   10,                                 // Number of elements in array
   sizeof(struct animal),              // Size of each element
   (compfn)compare );                  // Pointer to compare function

   printf("\nList after sorting:\n");
   printarray();
}

int compare(struct animal *elem1, struct animal *elem2)
{
   if ( elem1->number < elem2->number)
      return -1;

   else if (elem1->number > elem2->number)
      return 1;

   else
      return 0;
}

void printarray(void)
{
   int i;

   for (i = 0; i < 10; i++)
      printf("%d:  Number %d is a %s\n",
               i+1, array[i].number, array[i].name);
}