Problems implementing a binary tree

I am not understanding why the following code is not working. I am assuming the problem is in the InsertNode function.

Code:

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

typedef struct Node {
        int value;
        struct Node* left;
        struct Node* right;
} Node;

Node* InitNode(int value);
void InsertNode(Node* node, Node* newnode);
void PrintTree(Node* node);

Node* InitNode(int value)
{
        Node* node = (Node*)malloc(sizeof(Node));
        node->value = value;
        node->left = NULL;
        node->right = NULL;
        return(node);
}

void InsertNode(Node* node, Node* newnode)
{
        if (node == NULL)
        {
                node = newnode;
                return;
        }
        if (newnode->value < node->value)
                InsertNode(node->left, newnode);
        else
                InsertNode(node->right, newnode);
}

void PrintTree(Node* node)
{
        if (node == NULL)
                return;
        PrintTree(node->left);
        printf(" %3d", node->value);
        PrintTree(node->right);
}

int main()
{
        Node* root = InitNode(5);
        Node* left = InitNode(4);
        Node* right = InitNode(6);

        InsertNode(root, left);
        InsertNode(root, right);

        PrintTree(root);

        return(0);
}

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When you do node=newnode, you are changing the values passed in to the function -- which means that those changes are not reflected in the main program (hence no node is actually inserted). (We've gone around this bit a couple times in the last few days, so read some recent threads.)

My problem stems from assuming by passing a pointer that I can change the location of where the pointer points to. I carried this over from my experience with integers where I would pass in a pointer to the int, modify that, and when the function was done, the changes persisted.

Apparently, for pointers, I needed to be passing in a pointer to the pointer. It actually does make sense the more I think about it. When passing a value to a function that you want the change to persist you need to pass a pointer to that value (even if it is already a pointer).

Here is my modified insert function which now works:

Code:

---

void InsertNode(Node** node, Node* newnode)
{
        if ((*node) == NULL)
        {
                *node = newnode;
                return;
        }
        if (newnode->value < (*node)->value)
                InsertNode(&(*node)->left, newnode);
        else
                InsertNode(&(*node)->right, newnode);
}

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Quote:

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Originally Posted by noops

When passing a value to a function that you want the change to persist you need to pass a pointer to that value (even if it is already a pointer).

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That's exactly it!
Answering your own questions like that suggests that you have great potential as a software developer.