NAME
bsearch() — binary search a sorted table
SYNOPSIS
#include <stdlib.h>
void *bsearch(
const void *key,
const void *base,
size_t nel,
size_t 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 indicating where
a datum may be found.
The table must be previously sorted in increasing order
according to a provided comparison function.
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 size of each element in the table.
compar
is the name of the comparison function,
which 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 zero
indicating that the first argument (the key) is to be considered
less than, equal to, or greater than the second argument (the array element).
NOTES
The pointers to the key and the element at the base of
the table should be of type pointer-to-element,
and cast to type pointer-to-void.
The comparison function need not compare every byte,
so arbitrary data can be contained in the elements
in addition to the values being compared.
Although declared as type pointer-to-void,
the value returned should be cast into type pointer-to-element.
RETURN VALUE
A
NULL
pointer is returned if the key cannot be found in the table.
EXAMPLES
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 code fragment 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>
#define TABSIZE 1000
struct node { /* these are stored in the table */
char *string;
int length;
};
struct node table[TABSIZE]; /* table to be searched */
.
.
.
{
struct node *node_ptr, node;
/* routine to compare 2 nodes */
int node_compare(const void *, const void *);
char str_space[20]; /* space to read string into */
.
.
.
node.string = str_space;
while (scanf("%s", node.string) != EOF) {
node_ptr = (struct node *)bsearch((void *)(&node),
(void *)table, TABSIZE,
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: %s\n", node.string);
}
}
}
/* This routine compares two nodes based on an
alphabetical ordering of the string field.
*/
int
node_compare(const void *node1, const void *node2)
struct node *node1, *node2;
{
return strcoll(((const struct node *)node1)->string,
((const struct node *)node2)->string);
}
WARNINGS
If the table being searched contains two or more entries
that match the selection criteria, a random entry is returned by
bsearch()
as determined by the search algorithm.
STANDARDS CONFORMANCE
bsearch(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, ANSI C
Printable version
