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Using fold to sum List of object attributes

In this code, I am trying 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 am getting the error:

  value v is not a member of Any

How can I perform an operation on each attribute of an object in a List?

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3 answers


As other authors have pointed out, fold

expect a supertype battery 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 it's really too hard. in this case simple:



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

will be better.

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The first element in the fold is the start element and you call 0.v

, also note that the signature is the signature def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1

, this means it returns a type variable that is the supertype of the passed type parameter, use foldLeft

instead that allows you to construct a value that 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 a shorthand form:

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


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You can't do what you want, as your accumulator fold

is of type Int

, not type s

. Execution _.v * 2

for Int

doesn't make sense.

You can either do foldLeft

(or foldRight

):

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

Or even easier, use map

and sum

:

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