tsearch(S) 6 January 1993 tsearch(S) Name tsearch, tfind, tdelete, twalk - manage binary search trees Syntax cc . . . -lc #include <search.h> void *tsearch (key, rootp, compar) void *key, **rootp; int (*compar)(); void *tfind (key, rootp, compar) void *key, **rootp; int (*compar)(); void *tdelete (key, rootp, compar) void *key, **rootp; int (*compar)(); void twalk (root, action) void *root; void (*action)(); Description The tsearch, tfind, tdelete, and twalk routines manipulate 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. The user-supplied function must return an integer less than, equal to, or greater than 0, according to whether the first argument is to be con- sidered 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. The tsearch function 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 is set to point to the datum that is at the root of the new tree. Like tsearch, tfind searches for a datum in the tree, returning a pointer to it if found. However, if it is not found, tfind returns 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 is 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 argu- ment 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), depend- ing 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-void. Similarly, although declared as type pointer-to-void, 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 <search.h> #include <stdio.h> #define STRSZ 10000 #define NODSZ 500 struct node { /* pointers to these are stored in the tree */ char *string; int length; }; char string_space[STRSZ]; /* space to store strings */ struct node nodes[NODSZ]; /* 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++ < NODSZ) { /* set node */ nodeptr->string = strptr; nodeptr->length = strlen(strptr); /* put node into the tree */ (void) tsearch((void *)nodeptr, (void **) &root, node_compare); /* adjust pointers, so we don't overwrite tree */ strptr += nodeptr->length + 1; nodeptr++; } twalk((void *)root, print_node); } /* This routine compares two nodes, based on an 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 second time twalk encounters it or if it is a leaf. */ void print_node(node, order, level) struct node **node; VISIT order; int level; { if (order == postorder || order == leaf) { (void)printf("string = %*s, length = %d\n", STRSZ/NODSZ, (*node)->string, (*node)->length); } } Diagnostics A NULL pointer is returned by tsearch if there is not enough space avail- able 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. Warnings The root argument to the twalk routine is one level of inderection less than the rootp arguments to the tsearch and tdelete routines. There are two nomenclatures used to refer to the order in which tree nodes are visited. The tsearch function uses preorder, postorder, and endorder to respectively refer to visiting a node before any of its chil- dren, after its left child and before its right, and after both its chil- dren. 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. Note If the calling function alters the pointer to the root, results are unpredictable. See also bsearch(S), hsearch(S), lsearch(S) Standards conformance tdelete, tfind, tsearch and twalk are conformant with: X/Open Portability Guide, Issue 3, 1989.