c - Why is pointer-to-pointer being used in this prog?

Question

The following program shows how to build a binary tree in a C program. It uses dynamic memory allocation, pointers and recursion. A binary tree is a very useful data-structure, since it allows efficient insertion, searching and deletion in a sorted list. As such a tree is essentially a recursively defined structure, recursive programming is the natural and efficient way to handle it.

tree
   empty
   node left-branch right-branch

left-branch
   tree

right-branch
   tree

Here's the code:

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

struct tree_el {
   int val;
   struct tree_el * right, * left;
};

typedef struct tree_el node;

void insert(node ** tree, node * item) {
   if(!(*tree)) {
      *tree = item;
      return;
   }
   if(item->val<(*tree)->val)
      insert(&(*tree)->left, item);
   else if(item->val>(*tree)->val)
      insert(&(*tree)->right, item);
}

void printout(node * tree) {
   if(tree->left) printout(tree->left);
   printf("%d
",tree->val);
   if(tree->right) printout(tree->right);
}

void main() {
   node * curr, * root;
   int i;

   root = NULL;

   for(i=1;i<=10;i++) {
      curr = (node *)malloc(sizeof(node));
      curr->left = curr->right = NULL;
      curr->val = rand();
      insert(&root, curr);
   }

   printout(root);
}

Why is pointer-to-pointer used?

Answer 1

Because the insert method needs to modify the root of the tree.