TSEARCH(3)

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

SYNOPSIS
#include <search.h>

char *tsearch((char *) key, (char **) rootp, compar)
int (*compar)( );

char *tfind((char *) key, (char **) rootp, compar)
int (*compar)( );

char *tdelete((char *) key, (char **) rootp, compar)
int (*compar)( );

void twalk((char *) root, action)
void (*action)( );

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.

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 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-pointer-to-character. Similarly, although declared as type pointer-to-character, the value returned should be cast into type pointer-to-element.

EXAMPLES

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>

    void twalk();
    char *tsearch();

    struct node {		/* pointers to these are stored in the tree */
    	char *string;
    	int count;
    };

    #define MAXNODES	12
    #define MAXSTRING	100
    #define MINSTRING	3		/* char, newline, eos */

    char string_space[MAXSTRING];		/* space to store strings */
    struct node node_space[MAXNODES];	/* nodes to store */
    struct node *root = NULL;		/* this points to the root */

    main()
    {
    	char *strptr = string_space;
    	int maxstrlen = MAXSTRING;
    	struct node *nodeptr = node_space;
    	int node_compare();
    	void print_node();
    	struct node **found;
    	int length;

    	while (fgets(strptr, maxstrlen, stdin) != NULL) {
    		/* remove the trailing newline */
    		length = strlen(strptr);
    		strptr[length-1] = 0;
    		/* set node */
    		nodeptr->string = strptr;
    		/* locate node into the tree */
    		found = (struct node **)
    		    tsearch((char *) nodeptr, (char **) &root, node_compare);
    		/* bump the count */
    		(*found)->count++;

    		if (*found == nodeptr) {
    			/* node was inserted, so get a new one */
    			strptr += length;
    			maxstrlen -= length;
    			if (maxstrlen < MINSTRING)
    				break;
    			if (++nodeptr >= &node_space[MAXNODES])
    				break;
    		}
    	}
    	twalk((char *)root, print_node);
    }

    /*
         This routine compares two nodes, based on an
         alphabetical ordering of the string field.
    */

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

    /* Print out nodes in alphabetical order */
    /*ARGSUSED2*/
    void
    print_node(node, order, level)
    	struct node **node;
    	VISIT order;
    	int level;
    {
    	if (order == postorder || order == leaf) {
    		(void) printf("string = %20s,  count = %d,
    		    (*node)->string, (*node)->count);
    	}
    }

SEE ALSO
bsearch(3), hsearch(3), lsearch(3)

DIAGNOSTICS

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 tsearch(), 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.

WARNINGS
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 visting 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.

BUGS

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