INFO: Sorting Structures with the C qsort() Function

• The C Run-time (CRT) included with: - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 5.1, 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

The Microsoft C and QuickC run-time library function qsort() is useful for sorting data; however, it is necessary to provide qsort() a compare function for the type of data being sorted. This sometimes causes confusion when that type is a structure. The sample program below illustrates how to use qsort() to sort an array of structures.

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);
   }

Keywords          : CRTIss kbcode kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0 WINDOWS:1.0,1.5,1.51,1.52;  WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo