BSEARCH(3C-SVR4)    RISC/os Reference Manual     BSEARCH(3C-SVR4)



NAME
     bsearch - binary search a sorted table

SYNOPSIS
     #include <stdlib.h>

     void *bsearch (const void *key, const void *base, sizet
     nel,
         sizet size, int (*compar)(const void *, const void *));

DESCRIPTION
     bsearch is a binary search routine generalized from Knuth
     (6.2.1) Algorithm B.  It returns a pointer into a table (an
     array) indicating where a datum may be found or a null
     pointer if the datum cannot be found.  The table must be
     previously sorted in increasing order according to a com-
     parison function pointed to by compar.  key points to a
     datum instance to be sought in the table.  base points to
     the element at the base of the table.  nel is the number of
     elements in the table.  size is the number of bytes in each
     element.  The function pointed to by compar is called with
     two arguments that point to the elements being compared.
     The function must return an integer less than, equal to, or
     greater than 0 as accordingly the first argument is to be
     considered less than, equal to, or greater than the second.

EXAMPLE
     The example below searches a table containing pointers to
     nodes consisting of a string and its length.  The table is
     ordered alphabetically on the string in the node pointed to
     by each entry.

     This program reads in strings and either finds the
     corresponding node and prints out the string and its length,
     or prints an error message.

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

          struct node {            /* these are stored in the table */
               char *string;
               int length;
          };
          static struct node table[] =  /* table to be searched */
          {
               { "asparagus", 10 },
               { "beans", 6 },
               { "tomato", 7 },
               { "watermelon", 11 },
          };




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BSEARCH(3C-SVR4)    RISC/os Reference Manual     BSEARCH(3C-SVR4)



          main()
          {
               struct node *node_ptr, node;
               /* routine to compare 2 nodes */
               static int node_compare(const void *, const void *);
               char str_space[20];   /* space to read string into */

               node.string = str_space;
               while (scanf("%20s", node.string) != EOF) {
                    node_ptr = bsearch( &node,
                            table, sizeof(table)/sizeof(struct node),
                            sizeof(struct node), node_compare);
                    if (node_ptr != NULL) {
                         (void) printf("string = %20s, length = %d\n",
                              node_ptr->string, node_ptr->length);
                    } else {
                         (void)printf("not found: %20s\n", node.string);
                    }
               }
               return(0);
          }

          /* routine to compare two nodes based on an  */
          /* alphabetical ordering of the string field */
          static int
          node_compare(const void *node1, const void *node2)
          {
               return (strcmp(
                         ((const struct node *)node1)->string,
                         ((const struct node *)node2)->string));
          }

SEE ALSO
     hsearch(3C), lsearch(3C), qsort(3C), tsearch(3C).

DIAGNOSTICS
     A null pointer is returned if the key cannot be found in the
     table.

NOTES
     The pointers to the key and the element at the base of the
     table should be of type pointer-to-element.

     The comparison function need not compare every byte, so
     arbitrary data may be contained in the elements in addition
     to the values being compared.

     If the number of elements in the table is less than the size
     reserved for the table, nel should be the lower number.






 Page 2                 Printed 11/19/92