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?

  • \$\begingroup\$ 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? \$\endgroup\$
    – Martijn
    Commented Jul 23, 2011 at 19:27
  • \$\begingroup\$ I would second that. The question was to review the given totalCost method, which works fine, but isn't very idiomatic. \$\endgroup\$
    – Landei
    Commented Jul 23, 2011 at 19:33
  • \$\begingroup\$ Probably it should be in stack overflow, it should be moved instead of getting closed. \$\endgroup\$ Commented Jul 24, 2011 at 9:50
  • \$\begingroup\$ 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. \$\endgroup\$
    – Michael K
    Commented Jul 25, 2011 at 3:07

2 Answers 2


There is already a sum function:

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

A left-fold is an elegant solution too:

class Node (children:List[Node], value:Int) {
  def totalCost : Int = (value /: children) (_ + _.totalCost)
  • \$\begingroup\$ 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. \$\endgroup\$
    – Suma
    Commented Nov 6, 2020 at 12:43

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