Velocity Reviews > AVL Balancing

# AVL Balancing

scbauer@gmail.com
Guest
Posts: n/a

 11-22-2006
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
with python. I have seen a few topics on the web about this, and i
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

John Machin
Guest
Posts: n/a

 11-22-2006

(E-Mail Removed) wrote:
> Im working on an AVL tree

Is this homework?

> that consists of balancing the tree everytime
> you add an object.

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?

> I have seen a few topics on the web about this, and i
> 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

scbauer
Guest
Posts: n/a

 11-23-2006

John Machin wrote:
> (E-Mail Removed) wrote:
> > Im working on an AVL tree

>
> Is this homework?

Yes, this is homework.

>
> > that consists of balancing the tree everytime
> > you add an object.

>
> 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 have seen a few topics on the web about this, and i
> > 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

""" 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

> 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

scbauer
Guest
Posts: n/a

 11-23-2006
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

bearophileHUGS@lycos.com
Guest
Posts: n/a

 11-23-2006
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

John Machin
Guest
Posts: n/a

 11-23-2006
scbauer wrote:
> John Machin wrote:
> > (E-Mail Removed) 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
> > > you add an object.

> >
> > 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.

>
> >
> > > I have seen a few topics on the web about this, and i
> > > 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
>
> 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
>

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
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