[Haskell-beginners] Re: How to delete a range from AVL tree.

17 Sep 2010

      Excuse me for the repost.
I'm sending it again because I have not find a reply from
beginners-bounces@haskell.org and I want to exclude (possible) problems
related to the mailing list.
I would be very much appritiated for any help or advice.

Alexander.

2010/9/16 Alexander.Vladislav.Popov 
...
Hi.
Help me to solve the problem I faced with.
I'm trying to delete items from the AVL tree 'itree' that are between 0 and
2.
But as you can see the result tree contains value of 1.
...
import Data.Tree.AVL
import Data.COrdering
...
mydelete' :: (Ord a) => a -> a -> AVL a -> AVL a
mydelete' p1 p2 tree = let res = delete cmp tree in
                       case contains res cmp of
                         True      -> mydelete' p1 p2 res
                         otherwise -> res
                       where
                         cmp  = between p1 p2
...
between a b x = if x < a
                then LT
                else if x > b
                     then GT
                     else EQ
...
itree = push ((sndByCC compare) 3) 3 (pair 1 2)
idel  = mydelete' 0 2
*Main> idel itree
P (Z E 1 E) 3 E
*Main> idel $ idel itree
P (Z E 1 E) 3 E
*Main>
Explain me why I got following results and where I made error.
*Main> (mydelete' 0 3) itree
E
*Main> (mydelete' 0 2) itree
P (Z E 1 E) 3 E
*Main> (mydelete' 1 2) itree
P (Z E 1 E) 3 E
*Main> (mydelete' 1 1) itree
Z (Z E 1 E) 2 (Z E 3 E)
*Main> (mydelete' 2 1) itree
Z (Z E 1 E) 2 (Z E 3 E)
*Main> (mydelete' 0 1) itree
Z (Z E 1 E) 2 (Z E 3 E)
*Main> (mydelete' 0 2) itree
P (Z E 1 E) 3 E
*Main> (mydelete' (-1) 2) itree
P (Z E 1 E) 3 E
Alexander