CODEWITHSUNDEEP
c++iteratorinfinite-loopavl-tree
Netsuki
Netsuki

Reputation: 53

C++ AVL tree iterator will not increment properly

I have implemented a class iterator inside my AvlTree class. My AvlTree node is as follows:

    struct AvlNode
{
    Comparable element;
    list<int> lines; //line occurrences
    bool flag; //checks validity
    AvlNode   *left;
    AvlNode   *right;
    AvlNode   *parent; //parent pointer 
    int       height;

    AvlNode( const Comparable & theElement, AvlNode *lt, AvlNode *rt, AvlNode *pt,
                                                     int h = 0, bool b = true )
      : element( theElement ), left( lt ), right( rt ), parent( pt ), height( h ), flag( b ) { }
};

My iterator is as follows:

     class iterator
 {
    protected:

        friend class AvlTree<Comparable>;
        AvlNode * node;

        AvlNode * findInOrderSuccessor(AvlNode * & t)
        {
            AvlNode * temp;
            //node has a right child
            // so successor is leftmost node of right subtree
            if(t->right != NULL)
            {
                temp = t->right; //go right
                //go all the way left
                while(temp->left != NULL)
                {
                    temp = temp->left; 
                }
                return temp;
            }

            //node has no right child
            //if we are someone's left child, go up one
            if(t->parent->left == t)
            {
                //return your parent
                temp = t->parent;
                return temp;
            }
            //if we are someone's right child, go up until the current node
            //is someone's left child, then go up one more
            temp = t->parent;
            while(temp->parent->left != temp)
            {
                temp = temp->parent; //go up 
            }
            //return your parent
            temp = t->parent;
            return temp;

        }

    public:
        iterator(AvlNode * p) : node(p)
            { }

        //overload * to make *iterator return the element of its node
        Comparable & operator*() 
            { return node->element; }

        iterator operator++ (int) //postfix operator
        {
            node = findInOrderSuccessor(node);
            return iterator(node);
        }

        // == comparison overload
        bool operator==(iterator  rhs)
            { return node == rhs.node; }
        // != comparison overload
        bool operator!=(iterator  rhs)
            { return !(*this == rhs); }
};

My AvlTree also has a begin and end iterator as public members:

    //begin iterator points to leftmost node
iterator begin()
{ //return pointer to leftmost node
    AvlNode *temp = root;
    while(temp->left != NULL)
        temp = temp->left;
    return iterator(temp);
}

//end iterator points to one after rightmost node
iterator end()
{ //return NULL right pointer of rightmost node
    AvlNode * temp = root;
    while(temp->right != NULL)
        temp = temp->right;
    return iterator( temp->right );
}

My problem is that when I try to run the following in main:

    for(AvlTree<string>::iterator itr = tree.begin(); itr != (tree.end()); itr++)
        cout << *itr << endl;

Instead of outputting all the words in the string tree inorder, I get an infinite loop of the first in order item in the tree. I can't seem to figure out why it would not be moving past the first item.

Upvotes: 2

Views: 1960

Answers (1)

Ambroz Bizjak
Ambroz Bizjak

Reputation: 8095

The following iteration code works (from my AVL tree; substitute left and right for link[0] and link[1]):

BAVLNode * BAVL_GetFirst (const BAVL *o)
{
    if (!o->root) {
        return NULL;
    }

    BAVLNode *n = o->root;
    while (n->link[0]) {
        n = n->link[0];
    }

    return n;
}

BAVLNode * BAVL_GetNext (const BAVL *o, BAVLNode *n)
{
    if (n->link[1]) {
        n = n->link[1];
        while (n->link[0]) {
            n = n->link[0];
        }
    } else {
        while (n->parent && n == n->parent->link[1]) {
            n = n->parent;
        }
        n = n->parent;
    }

    return n;
}

As far as your code is concerned, first, end() does not need to find the rightmost node in order to return iterator(NULL); it could just return that without looking at the tree.

The actual error in your algorithm however seems to be here:

        temp = t->parent;
WRONG:  while(temp->parent->left != temp)
        {
            temp = temp->parent; //go up 
        }
        //return your parent
WRONG:  temp = t->parent;
        return temp;

    }

The first line I flagged may attempt a NULL pointer dereference, and should be changed to:

while(temp->parent && temp->parent->left != temp)

And the second one to:

temp = temp->parent;

Also, you might have noticed from my code that the following is now superfulous; it can be removed, and will be handled exactly the same way by the (fixed) remaining code. It also suffers from the same NULL pointer dereference I pointed out just above.

//if we are someone's left child, go up one
if(t->parent->left == t)
{
    //return your parent
    temp = t->parent;
    return temp;
}

Upvotes: 1

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