FAQ
Im working on an AVL tree that consists of balancing the tree everytime
you add an object. Im not quite grasping the concept on how to do this
clearly understand how when the balance of an AVL tree get to 2 or -2
you have to do a R, L, LR, or a RL rotation. Could anyone possibly help
me/point me in the right direction as far as checking the balance and
actually balancing the tree? Thanks in advance

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• at Nov 22, 2006 at 1:16 am ⇧ scba... at gmail.com wrote:
Im working on an AVL tree
Is this homework?
that consists of balancing the tree everytime
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.
Im not quite grasping the concept on how to do this
with python.
Could you do it in any other language?
clearly understand how when the balance of an AVL tree get to 2 or -2
you have to do a R, L, LR, or a RL rotation. Could anyone possibly help
me/point me in the right direction as far as checking the balance and
actually balancing the tree? Thanks in advance
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
• at Nov 23, 2006 at 12:04 am ⇧ John Machin wrote:
scba... at gmail.com wrote:
Im working on an AVL tree
Is this homework?
Yes, this is homework.
that consists of balancing the tree everytime
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.
You're right the AVL Tree shouldnt be updated everytime.
Im not quite grasping the concept on how to do this
with python.
Could you do it in any other language?
No, this is an Advanced Algorithms class have to use python
clearly understand how when the balance of an AVL tree get to 2 or -2
you have to do a R, L, LR, or a RL rotation. Could anyone possibly help
me/point me in the right direction as far as checking the balance and
actually balancing the tree? Thanks in advance
class AVLTree:
"""Holds operations of tree"""
def __init__(self):

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

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
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.
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
• at Nov 23, 2006 at 12:16 am ⇧ 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
• at Nov 23, 2006 at 12:26 am ⇧ scbauer wrote:
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

.....
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
• at Nov 23, 2006 at 12:29 am ⇧ scbauer wrote:
John Machin wrote:
scba... at gmail.com wrote:
Im working on an AVL tree
Is this homework?
Yes, this is homework.
It was a rhetorical question :-)
that consists of balancing the tree everytime
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.
You're right the AVL Tree shouldnt be updated everytime.
Im not quite grasping the concept on how to do this
with python.
Could you do it in any other language?
No, this is an Advanced Algorithms class have to use python
I meant: do you have the ability ("can") (not the permission ("may"))
to do it in another language.
clearly understand how when the balance of an AVL tree get to 2 or -2
you have to do a R, L, LR, or a RL rotation. Could anyone possibly help
me/point me in the right direction as far as checking the balance and
actually balancing the tree? Thanks in advance
class AVLTree:
"""Holds operations of tree"""
def __init__(self):

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

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
and where's the definition of your Node class?
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.
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.
... 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

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

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