[Python] AVL Balancing

That's a peculiar kind of AVL tree. Normally it is *not* necessary to
balance the tree every time a node is added -- it's done only if a
constraint is breached.
Perhaps if you could show us what you have done already -- I'd be
expecting a class to represent a node, and maybe another to represent
the root of the tree -- and give us an idea of your Python proficency
level (so that we can tailor the advice accordingly), we could help
you.

Regards,
John

Yes, this is homework.
You're right the AVL Tree shouldnt be updated everytime.
No, this is an Advanced Algorithms class have to use python
class AVLTree:
"""Holds operations of tree"""
def __init__(self):

self.__root = None
self.__stack=[]
stack = self.__stack

def addNode(self, data):
""" Add node for tree"""
return Node(data)

def insert(self, data):
if self.__root == None:
self.__root = Node(data)
else:
current = self.__root
done = False
while not done:
if data < current.get_data():
if current.left == None:
current.left = Node(data)
stack.append(current)

done = True
else:
current = current.left
else:
if current.right == None:
current.right = Node(data)
stack.append(current)

done = True
else:
current = current.right
Basically what im trying to do is use a stack to keep track of the
items in the AVL tree so that rotations will go smoothly. Having
problems with the stack getting initialized and actually storing the
data there. So that i can reference the elements stack[-3].left =
stack[-2].right and so on for balancing.

thanks
scbauer

This is one of the errors that im getting

Traceback (most recent call last):
File "<pyshell#49>", line 1, in <module>
t.insert(5)
File "/Users/stevenbauer/Desktop/AVL.py", line 68, in insert
stack.append(current)
NameError: global name 'stack' is not defined

def __init__(self):
self.__root = None
self.__stack=[]
stack = self.__stack

def addNode(self, data):
.....
stack.append(current)


Some partial suggestions:
- Why do you use __? Use them only when necessary. My suggestion is to
use:
self._stack = []
- What's the purpose of stack = self.__stack ? It it does nothing
there.
- stack.append(current) can't find the stack name, because it's not
present in this scope, it's present into the __init__ only. My
suggestion is to use the instance attribute name.

You can probably fix that problem now.

Bye,
bearophile

It was a rhetorical question :-)
I meant: do you have the ability ("can") (not the permission ("may"))
to do it in another language.
and where's the definition of your Node class?
... and avoiding trouble when there are fewer than 3 elements in the
stack.

I presume that you a righteous dude, and are avoiding the temptation to
google for "AVL tree Python".

So, I'll give you just one hint:

Being righteous, you may wish to examine the ancient scriptures: find
volume 3 of the gospel according to Saint Donald Knuth; therein lies a
description (in arcane notation) of how to insert into a "balanced
tree" without using a stack. If a dumb cluck like me could translate
that into C (with no gotos) and get it to work, a smart lad should have
no trouble translating it into Python.

HTH,
John