On Mon, 26 Feb 2001, Martin Gustafsson wrote:
> Hello
>
> I'm a haskell newbie that tries to create a tree with arbitary numbers of childs.
> I create the data structure but i can't do anything on it can someone please help
> me with a small function that sums the values of the leafs, so i don´t loose my hair
> so fast.
>
> The datastructure looks like this and a binary tree built with it would look like
>this:
>
>
> data GeneralTree = Nil | Node (Integer,[GeneralTree])
>
As you said you were a newbie I will ask a few questions about your
datastructure.
Do you know that there is no need to tuple the elements in the Node if you
do not want to. You can write:
data GeneralTree = Nil | Node Integer [GeneralTree]
What is the intended difference between (Node 5 []) and (Node 5 [Nil]) ?
>
> tree =
> (20,
> [
> (-20,[(30,[Nil]),(20,[Nil])]),
> (40,[(65,[Nil]),(-40,[Nil])])
> ]
> )
This is not of type GeneralTree! (And its layout is messed up)
Hint: write the type of every expression you write, and debugging will be
much easier.
tree :: GeneralTree
ERROR tree.hs:8 - Type error in explicitly typed binding
*** Term : tree
*** Type : (a,[(b,[(c,[GeneralTree])])])
*** Does not match : GeneralTree
This is an expression with type GeneralTree:
tree :: GeneralTree
tree = Node 20 [Node (-20) [Node 30 [Nil], Node 20 [Nil]],
Node 40 [Node 65 [Nil], Node (-40) [Nil]]]
Now it should be very easy to write a function to sum the nodes in a tree
sumTree :: GeneralTree -> Integer
sumTree Nil = 0
sumTree (Node n ts) = ... write this yourself
hint - sum and map are very useful functions (defined in the prelude) as
are recursion.
Good luck!
/Lars L
_______________________________________________
Haskell-Cafe mailing list
[EMAIL PROTECTED]
http://www.haskell.org/mailman/listinfo/haskell-cafe
