NAME

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.

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

EXAMPLE

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);
}

SEE ALSO

bsearch(3C), hsearch(3C), lsearch(3C).

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 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.

NOTES

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, pos-
torder 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.