Using fold to sum List of object attributes

Question

In this code I'm attempting to double each value and then sum v :

case class s(v : Int)
  val l = List(s(2) , s(3))                      
  l.fold(0) ((_.v * 2) + (_.v * 2))

But I receive error :

  value v is not a member of Any

How can I perform an operation on each attribute of the object within the List?

Answer 1

As the other authors also noted, fold expects the accumulator of the supertype for your class. The class itself is also a supertype for itself, so we can write

case class s(v : Int)
val l = List(s(2) , s(3))                      
l.fold(s(0)){case (acc, elem) => s((acc.v * 2) + (elem.v * 2))}.v

but that indeed feels too heavy. in this case the simple:

l.map(_.v*2).sum

would do better.

Answer 2

The first element in the fold is the starting element and you are calling 0.v, also note the fold signature is def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1, that means that it returns a variable of a type which is a super type of the passed type parameter, use foldLeft instead which allows you to build a value which is a subtype:

l.foldLeft(0) ((acc, curr) => acc + (curr.v * 2))

I used acc and curr to better visualize what was happening inside the fold, this is the abbreviated form:

l.foldLeft(0) (_ + _.v * 2)