Question

When we want to change the value of an ordinary variable by in a function we pass it using call by reference. But I am not able to understand the intricacies when we have to pass a pointer variable(like a node of a binary tree) using call by refence. I understand that if we want to modify the poiter variable to point to another node we have to use call by reference. But what if we have to modify the data element of the root. I thought that to change it also we would need a call by reference. But the following code snippet is giving an output of 10, 10, 10 even though I have passed the root node of the tree using call by value in the function modifyTree. Am I missing something over here?

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


struct node
{
int data;
struct node* left;
struct node* right;
};

/* Helper function that allocates a new node with the
given data and NULL left and right pointers. */
struct node* newNode(int data)
{
struct node* node = (struct node*)malloc(sizeof(struct node));

node->data = data;
node->left = NULL;
node->right = NULL;
return(node);
}

/*  This function sets the data fields of some of the nodes of tree to 10*/
void modifyTree(struct node* node)
{

node->data = 10;
node->left->data = 10;
node->right->data = 10;
}

int main()
{
struct node *root = newNode(1);
root->left            = newNode(2);
root->right          = newNode(3);
root->left->left     = newNode(4);
root->left->right   = newNode(5);

modifyTree(root);

printf("%d\n", root->data);
printf("%d\n", root->left->data);
printf("%d\n", root->right->data);

getchar();
return 0;
}

Answer 1

You are passing the pointer by value, but the pointer still points at the same thing. I’ll use some hypothetical values to demonstrate.

In main you allocate a new struct node. Let’s say it gets created at memory location 0x12345. So now your struct node *root contains 0x12345.

You now call modifyTree(root);. root gets passed by value to the root argument of modifyTree.

That root now contains 0x12345. It’s pointing at the same memory location.

So when you access that location with node->data = 10, you are accessing the same memory you created in main.