diff tree5/tree.c tree6/tree.c
3c3
<  *  (version 5: keyed by a string instead of int)
---
>  *  (version 6: introduce function pointer)
24c24
<   struct treenode *root;
---
>   struct treenode *root;                     // root of the tree
36a37,39
> static void tree_print_helper(tree_t *tree, treenode_t *node, int depth, 
> 			      FILE *fp, 
> 			      void (*itemprint)(FILE *fp, const char *key, void *data) );
43c46
<   
---
> 
61c64
<     
---
> 
146a150,184
> /**************** tree_print() ****************/
> void
> tree_print(tree_t *tree, FILE *fp, 
> 	   void (*itemprint)(FILE *fp, const char *key, void *data) )
> {
>   if (tree != NULL) {
>     tree_print_helper(tree, tree->root, 0, fp, itemprint);
>   }
> }
> 
> /**************** tree_print_helper() ****************/
> /* Recursive function to print nodes of the tree, 
>  * return data for the found node, or NULL if not found.
>  */
> static void  // not visible outside this file
> tree_print_helper(tree_t *tree, treenode_t *node, int depth, FILE *fp, 
> 		  void (*itemprint)(FILE *fp, const char *key, void *data) )
> {
>   if (node != NULL) {
>     // print the left subtree
>     tree_print_helper(tree, node->left, depth+1, fp, itemprint);
>     
>     // print the current node
>     fprintf(fp, "%*s", depth+1, " ");	// indent one space per depth
>     if (itemprint != NULL) {		// print the node
>       (*itemprint)(fp, node->key, node->data); 
>     }
>     fputc('\n', fp);			// print a newline
> 
>     // print the right subtree
>     tree_print_helper(tree, node->right, depth+1, fp, itemprint);
>   }
>   return;
> }
> 
diff tree5/tree.h tree6/tree.h
3c3
<  *  (version 5: keyed by a string instead of int)
---
>  *  (version 6: introduce function pointer)
16,18c16
< /* Create a new (empty) tree;
<  * return NULL if error.
<  */
---
> /* Create a new (empty) tree; return NULL if error. */
31a30,35
> /* Print the whole tree, recursively.
>  * Provide a function that will print an item.
>  */
> void tree_print(tree_t *tree, FILE *fp, 
> 		void (*itemprint)(FILE *fp, const char *key, void *data) );
> 
diff tree5/treetest.c tree6/treetest.c
11a12,13
> void myprint(FILE *fp, const char *key, void *string);
> 
24d25
<   tree_insert(tree, "alice", datastring);
27a29
>   tree_insert(tree, "alice", datastring);
35,36c37,38
<   printf("find %s returns %s\n", "irina", tree_find(tree, "irina"));
<   printf("find %s returns %s\n", "eve", tree_find(tree, "eve"));
---
>   printf("find %s returns %s\n", "irina", (char*) tree_find(tree, "irina"));
>   printf("find %s returns %s\n", "eve", (char*) tree_find(tree, "eve"));
40,41c42,46
<   printf("find %s returns %s\n", "eve", tree_find(tree, "eve"));
<   printf("find null returns %s\n", tree_find(NULL, "eve"));
---
>   printf("find %s returns %s\n", "eve", (char*) tree_find(tree, "eve"));
>   printf("find null returns %s\n", (char*) tree_find(NULL, "eve"));
>   
>   printf("\nThe tree:\n");
>   tree_print(tree, stdout, myprint);
42a48,54
> 
> /* print the given string to the given file */
> void myprint(FILE *fp, const char *key, void *string)
> {
>   fprintf(fp, "[%s]: '%s'", key, (char*)string);
> }
>