TSEARCH(3C) TSEARCH(3C)
tsearch, tfind, tdelete, twalk - manage binary search trees
#include <search.h>
void *tsearch (const void *key, void **rootp,
int (*compar)(const void *, const void *));
void *tfind (const void *key, void * const *rootp,
int (*compar)(const void *, const void *));
void *tdelete (const void *key, void **rootp,
int (*compar)(const void *, const void *));
void twalk ((const void *root, void const (*action)(void *, VISIT, int));
tsearch, tfind, tdelete, and twalk are routines for manipulating binary
search trees. They are generalized from Knuth (6.2.2) Algorithms T and
D. All comparisons are done with a user-supplied routine. This routine
is called with two arguments, the pointers to the elements being
compared. It returns an integer less than, equal to, or greater than 0,
according to whether the first argument is to be considered less than,
equal to or greater than the second argument. The comparison function
need not compare every byte, so arbitrary data may be contained in the
elements in addition to the values being compared.
tsearch is used to build and access the tree. Key is a pointer to a
datum to be accessed or stored. If there is a datum in the tree equal to
*key (the value pointed to by key), a pointer to this found datum is
returned. Otherwise, *key is inserted, and a pointer to it returned.
Only pointers are copied, so the calling routine must store the data.
Rootp points to a variable that points to the root of the tree. A NULL
value for the variable pointed to by rootp denotes an empty tree; in this
case, the variable will be set to point to the datum which will be at the
root of the new tree.
Like tsearch, tfind will search for a datum in the tree, returning a
pointer to it if found. However, if it is not found, tfind will return a
NULL pointer. The arguments for tfind are the same as for tsearch.
Tdelete deletes a node from a binary search tree. The arguments are the
same as for tsearch. The variable pointed to by rootp will be changed if
the deleted node was the root of the tree. Tdelete returns a pointer to
the parent of the deleted node, or a NULL pointer if the node is not
found.
Twalk traverses a binary search tree. Root is the root of the tree to be
traversed. (Any node in a tree may be used as the root for a walk below
that node.) Action is the name of a routine to be invoked at each node.
This routine is, in turn, called with three arguments. The first
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argument is the address of the node being visited. The second argument
is a value from an enumeration data type typedef enum { preorder,
postorder, endorder, leaf } VISIT; (defined in the <search.h> header
file), depending on whether this is the first, second or third time that
the node has been visited (during a depth-first, left-to-right traversal
of the tree), or whether the node is a leaf. The third argument is the
level of the node in the tree, with the root being level zero.
The pointers to the key and the root of the tree should be of type
pointer-to-element, and cast to type pointer-to-character. Similarly,
although declared as type pointer-to-character, the value returned should
be cast into type pointer-to-element.
The following code reads in strings and stores structures containing a
pointer to each string and a count of its length. It then walks the
tree, printing out the stored strings and their lengths in alphabetical
order.
#include <search.h>
#include <stdio.h>
struct node { /* pointers to these are stored in the tree */
char *string;
int length;
};
char string_space[10000]; /* space to store strings */
struct node nodes[500]; /* nodes to store */
struct node *root = NULL; /* this points to the root */
main( )
{
char *strptr = string_space;
struct node *nodeptr = nodes;
void print_node( ), twalk( );
int i = 0, node_compare( );
while (gets(strptr) != NULL && i++ < 500) {
/* set node */
nodeptr->string = strptr;
nodeptr->length = strlen(strptr);
/* put node into the tree */
(void) tsearch((char *)nodeptr, (char **) &root,
node_compare);
/* adjust pointers, so we don't overwrite tree */
strptr += nodeptr->length + 1;
nodeptr++;
}
twalk((char *)root, print_node);
}
/*
This routine compares two nodes, based on an
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alphabetical ordering of the string field.
*/
int
node_compare(node1, node2)
char *node1, *node2;
{
return strcmp(((struct node *)node1)->string,
((struct node *) node2)->string);
}
/*
This routine prints out a node, the first time
twalk encounters it.
*/
void
print_node(node, order, level)
char **node;
VISIT order;
int level;
{
if (order == postorder || order == leaf) {
(void)printf("string = %20s, length = %d\n",
(*((struct node **)node))->string,
(*((struct node **)node))->length);
}
}
bsearch(3C), hsearch(3C), lsearch(3C).
A NULL pointer is returned by tsearch if there is not enough space
available to create a new node.
A NULL pointer is returned by tfind and tdelete if rootp is NULL on
entry.
If the datum is found, both tsearch and tfind return a pointer to it. If
not, tfind returns NULL, and tsearch returns a pointer to the inserted
item.
The root argument to twalk is one level of indirection less than the
rootp arguments to tsearch and tdelete.
There are two nomenclatures used to refer to the order in which tree
nodes are visited. tsearch uses preorder, postorder and endorder to
respectively refer to visiting a node before any of its children, after
its left child and before its right, and after both its children. The
alternate nomenclature uses preorder, inorder and postorder to refer to
the same visits, which could result in some confusion over the meaning of
postorder.
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CAVEAT
If the calling function alters the pointer to the root, results are
unpredictable.
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