tsearch(S)

tsearch, tfind, tdelete, twalk -- manage binary search trees

Synopsis

   #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 (void root, void(action) (void , VISIT, int));

Description

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.

In the following, ``nodes'' are internal data structures, the first element of which is a key, that is, a pointer to data stored at the node. Pointers to nodes can be used as pointers to pointers to stored data.

tsearch is used to build and access the tree. key is a pointer to the data to be accessed or stored. If there is data in the tree equal to key (the value pointed to by key), a pointer to the node storing the pointer to the data is returned. Otherwise, a new node is allocated, key is inserted, and a pointer to the new node 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 a newly allocated node, which will store the key at the root of the new tree.

Like tsearch, tfind will search for data equal to key in the tree, returning a pointer to the data 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 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.

Return values

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 data equal to key is found, both tsearch and tfind return a pointer to the node containing a pointer to the data. If not, tfind returns NULL, and tsearch returns a pointer to the node containing the newly inserted key.

Usage

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 <string.h>
   #include <stdio.h>
   #include <search.h>
   
   struct node {
   	char *string;
   	int length;
   };
   char string_space[10000];
   struct node nodes[500];
   void *root = NULL;
   

   int node_compare(const void *node1, const void *node2) {
   	return strcmp(((const struct node *) node1)->string,
   		      ((const struct node *) node2)->string);
   }
   

   void print_node(void **node, VISIT order, int level) {
   	if (order == preorder || order == leaf) {
   		printf("length=%d, string=%20s\n",
   		(*(struct node **)node)->length,
   		(*(struct node **)node)->string);
   	}
   }
   

   main() {
   	char *strptr = string_space;
   	struct node *nodeptr = nodes;
   	int i = 0;
   

   	while (gets(strptr) != NULL && i++ < 500) {
   		nodeptr->string = strptr;
   		nodeptr->length = strlen(strptr);
   		(void) tsearch((void *)nodeptr,
   				&root, node_compare);
   		strptr += nodeptr->length + 1;
   		nodeptr++;
   	}
   	twalk(root, print_node);
   }

References

bsearch(S), hsearch(S), lsearch(S)

Notices

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 refer respectively 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.

If the calling function alters the pointer to the root, results are unpredictable.