Thread: problem in creating a tree

  1. #1
    Registered User
    Join Date
    Mar 2005

    problem in creating a tree

    Hi, I encountered some problems in creating a tree and using preorder algorithm to travel it.

    // Create a binary search tree
    struct TreeNode;
    typedef struct TreeNode *PtrToNode;
    typedef struct TreeNode *Tree;
    using namespace std;
    struct TreeNode {
                    int data;
    	PtrToNode left;
    	PtrToNode right;
    // insert a new node into a binary search tree
    // using a recursive algorithm
    void insert(Tree tree, int item) {
    	if( tree == 0 ) {
    		tree = (Tree)malloc(sizeof(struct TreeNode));
    		if( tree == 0 ) {
    			cout << "Fatal error!" << endl;
    		else {
    			tree->data = item;
    			tree->left = tree->right = 0;
    	else if( item < tree->data )
    		insert( tree->left, item);
    	else if( item > tree->data )
    		insert( tree->right, item );
    // create a new binary search tree
    void createTree( Tree tree, int a[], int n ) {
    	for( int i = 0; i < n; i++ )
    		insert( tree, a[i] );
    void preorder( Tree tree ) {
    	if( tree != 0 ) {
    		cout << tree ->data << ' ';
    		preorder( tree->left );
    		preorder( tree->right );
    int main(void) {
    	const int maxsize = 5;
    	int a[maxsize] = {5,3,9,1,10};
    	Tree T = 0;
    	preorder( T );
    	return 0;
    so why there is no output to the screen??
    thanks for your help~~~

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    > tree = (Tree)malloc(sizeof(struct TreeNode));
    Use new in C++, not malloc.

    The main problem is you don't return the tree to main - remember, it's pass by value.
    To fix this, make insert pass the tree by reference
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Registered User
    Join Date
    Jan 2005
    You are passing the tree pointer by value to your createTree and insert functions. You should pass the pointers by reference so that the new pointer value created by the call to malloc will be saved in the original pointer variable passed to the function.

    Note that you have to make createTree take a reference as well.

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