PRODUCT  :  TURBO C                                NUMBER  :  398
  VERSION  :  1.0
       OS  :  PC-DOS
     DATE  :  January 6, 1988                          PAGE  :  1/3

    TITLE  :  QSORT




  qsort  is  an  implementation  of  the  Quicksort  algorithm  and
  requires four parameters:   the  array to sort, the length of the
  array, the width of the array, and a comparison function.

  qsort 's length and width parameters are  fairly straightforward.
  If you are sorting an array declared as:

              int array[100];

  the  length  of the array is 100 and the width of  the  array  is
  "sizeof(int)".   Generally, the length and width of any array can
  be  expressed as:  "sizeof(array)/sizeof(array[0])" (the size  of
  array divided by  the  size  of  the first element of array), and
  "sizeof(array[0])" (the  size  of  the  first  element  of array)
  respectively.

  The fourth parameter (the comparison routine) usually  causes the
  most  trouble.    The  reference  guide  states  that the  fourth
  parameter is a pointer to a  function.   This means that you must
  give  the name of a routine as the  fourth  parameter;  you  must
  supply this routine yourself.

  qsort will call your  routine  to  do  all comparisons.  Pointers
  will be passed to the array elements which are  to  be  compared.
  You do not  have  to  be  concerned  how this is done, qsort will
  handle this for you.  Your routine must return  an  integer,  the
  value of which will be:

    positive   -  if the first element is greater than the second.

            0  -  if the elements are equal.

    negative   -  if the second element is greater than the first.

  To sort the above array in ascending order, the routine is:

  int fcmp(int *num1, int *num2)
  {
      return (*num1 - *num2);
  }

  qsort can  also  sort  arrays  of  structures.   If the array was
  defined:













  PRODUCT  :  TURBO C                                NUMBER  :  398
  VERSION  :  1.0
       OS  :  PC-DOS
     DATE  :  January 6, 1988                          PAGE  :  2/3

    TITLE  :  QSORT




  struct address
  {
      char LastName[16];
      char FirstName[11];
      char Street[30];
      char City[21];
      char State[3];
      char Zip[6];
  } AddressBook[100];

  To sort "AddressBook" on "LastName" the fcmp routine would be:

  int fcmp(struct address *first, struct address *second)
  {
      return (strcmp(first->LastName,second->LastName));
  }

  To  sort the same array on "Zip" in  the  above  routine,  simply
  change "LastName" to "Zip".  The call to qsort for  both examples
  would look like:

    qsort(AddressBook, sizeof(AddressBook)/sizeof(AddressBook[0]),
           sizeof(AddressBook[0]), fcmp);

  The best way to put this all together is a sample program.

  /* The program will generate 100 random numbers.  Store them
   * in an array.  Print them out.  Call qsort to sort them
   * and print them out again.
   */

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

  #define ARRAYLEN 100

  int array[ARRAYLEN];       /* Array to sort */


















  PRODUCT  :  TURBO C                                NUMBER  :  398
  VERSION  :  1.0
       OS  :  PC-DOS
     DATE  :  January 6, 1988                          PAGE  :  3/3

    TITLE  :  QSORT




  /* Comparison routine passed to and used by qsort() */
  int fcmp(int *num1, int *num2)
  {
      return (*num1 - *num2);
  }

  main()
  {
     long along;
     int i;

     /* Seed the random number generator */
     srand((unsigned) time(&along));
     for (i=0; i<ARRAYLEN; i++)
         array[i] = rand() % ARRAYLEN;

     /* Print the random array */
     printf("\nOriginal array\n");
     for (i=0; i<ARRAYLEN; i++)
     {
         printf("%4d ",array[i]);
         if ( !((i+1) % 15) )
             putchar('\n');
     }

     /* Call qsort() */
    qsort(array,sizeof(array)/sizeof(*array),sizeof(*array),fcmp);

     /* Print the sorted array */
     printf("\n\nSorted array\n");
     for (i=0; i<ARRAYLEN; i++)
     {
         printf("%4d ",array[i]);
         if ( !((i+1) % 15) )
             putchar('\n');
     }
  }

  DISCLAIMER: You  have the right to use this technical information
  subject to the terms  of  the  No-Nonsense License Statement that
  you received with  the  Borland product to which this information
  pertains.