Question

I tried to implement a binary search tree for the purpose of (re-)learning C. The problem is that the this current = new; does not work as desired because the tree.root is still a null pointer after adding two nodes. What’s wrong with that?

#include <stdlib.h>
#include <stdio.h>


typedef struct BinaryNode {
    int key;
    double value;
    struct BinaryNode *left;
    struct BinaryNode *right;
    } BinaryNode;

typedef struct BinaryTree {
    struct BinaryNode *root;
    } BinaryTree;


static void binary_tree_insert_recursive(BinaryNode *current, BinaryNode *new) {
    if (current == NULL || current->key == new->key) {
        current = new;
    } else if (current->key > new->key) {
        binary_tree_insert_recursive(current->left, new);
    } else if (current->key < new->key) {
        binary_tree_insert_recursive(current->right, new);
    }
}

void binary_tree_insert(BinaryTree *tree, int key, double value) {
    BinaryNode *new = (BinaryNode *) malloc(sizeof(BinaryNode));
    new->key = key;
    new->value = value;
    binary_tree_insert_recursive(tree->root, new);
}

int main(void) {
    BinaryTree tree;
    binary_tree_insert(&tree, 5, 123);
    binary_tree_insert(&tree, 10, 123);
    printf("%p\n", tree.root);
    return 0;
}

Thank you!

Answer 1

I believe the problem with current = new; is that you are changing your local copy of current. After the function is done, this modification is not visible.

I suspect you want something like:

static void binary_tree_insert_recursive(BinaryNode **current, BinaryNode **new)
{
    if (*current == NULL || (*current)->key == (*new)->key) {
    *current = *new;
    /* ... */

Well explained in the C FAQ.