BSEARCH(S) XENIX System V BSEARCH(S)
Name
bsearch - Performs a binary search.
Syntax
#include <search.h>
char *bsearch (key, base, nel, width, compar)
char *key;
char *base;
unsigned nel, width;
int (*compar)();
Description
bsearch is a binary search routine generalized from Knuth
(6.2.1) Algorithm B. It returns a pointer into a table
indicating the location at which a datum may be found. The
table must be previously sorted in increasing order
according to a provided comparison function, compar. key is
a pointer to the datum to be located in the table. base is
a pointer to the elements at the base of the table. nel is
the number of elements in the table. width is the size of
an element in bytes. compar is the name of the comparison
routine. It is called with two arguments which are pointers
to the elements being compared. The routine must return an
integer less than, equal to, or greater than zero, depending
on whether 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. The nodes consist of a string and its length. The
table is ordered alphabetically on the string in the node
pointed to by each entry.
The following code fragment reads in strings and either
finds the corresponding node and prints out the string and
its length, or prints an error message, (as shown on the
next page).
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BSEARCH(S) XENIX System V BSEARCH(S)
#include <stdio.h>
#include <search.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;
int node_compare( ); /* routine to compare 2 nodes */
char str_space[20]; /* space to read string into */
.
.
.
node.string = str_space;
while (scanf(``%s'', node.string) !=EOF) {
node_ptr = (struct node *)bsearch((char *)(&node),
(char *)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(node1,node2)
struct node *nodel, *node2;
{
return strcmp(nodel->string, node2->string);
}
See Also
hsearch(S), lsearch(S), qsort(S), tsearch(S)
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BSEARCH(S) XENIX System V BSEARCH(S)
Diagnostics
If the key cannot be found in the table, a NULL (0) pointer
is returned.
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-character. The comparison function need not
compare every byte, so arbitrary data may be contained in
the elements in addition to the values being compared.
Although declared as type pointer-to-character, the value
returned should be cast into pointer-to-element.
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