Scala list folding

Question

I'm trying to learn scala. It's hard.

I currently have a tree in the form

class Node(children:List[Node], value:Int){
}

I want to calculate a total cost defined by value + the sum of the totalcost of the children. My java background made me do this:

def totalCost() {
 var total = value
 for (child <- children){
 total = total+child.totalCost
 }
 return total
}

now I know I should be folding or reducing, but it's not coming out. Could some of you frendly helpers give me a hand here?

• Martijn: I don't quite understand how this question is not within scope. @Michael K: Could you review your closure, and if it is in fact out of scope, explain how, so I won't be asking out of scope questions on this site again?
• Landei: I would second that. The question was to review the given totalCost method, which works fine, but isn't very idiomatic.
• Cem Kalyoncu: Probably it should be in stack overflow, it should be moved instead of getting closed.
• Michael K: I apologize; this was flagged as OT and I didn't read the post closely enough. This does have working code and is on topic here.

Answer 1

There is already a sum function:

def totalCost = value + children.map(_.totalCost).sum

Answer 2

A left-fold is an elegant solution too:

class Node (children:List[Node], value:Int) {
 def totalCost : Int = (value /: children) (_ + _.totalCost)
}

• Suma: Just a note for recent readers: /: is not used any more (it might even be deprecated, though I am not sure - see github.com/scala/bug/issues/9607). People write foldLeft instead.