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