In Scala, how do I fold a List and return the intermediate results?

Question

I've got a List of days in the month:

val days = List(31, 28, 31, ...)

I need to return a List with the cumulative sum of days:

val cumDays = List(31, 59, 90)

I've thought of using the fold operator:

(0 /: days)(_ + _)

but this will only return the final result (365), whereas I need the list of intermediate results.

Anyway I can do that elegantly?

Answer 1

Scala 2.8 has the methods scanLeft and scanRight which do exactly that.

For 2.7 you can define your own scanLeft like this:

def scanLeft[a,b](xs:Iterable[a])(s:b)(f : (b,a) => b) =
  xs.foldLeft(List(s))( (acc,x) => f(acc(0), x) :: acc).reverse

And then use it like this:

scala> scanLeft(List(1,2,3))(0)(_+_)
res1: List[Int] = List(0, 1, 3, 6)

Answer 2

I'm not sure why everybody seems to insist on using some kind of folding, while you basically want to map the values to the cumulated values...

val daysInMonths = List(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)

val cumulated = daysInMonths.map{var s = 0; d => {s += d; s}}

//--> List[Int] = List(31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365)

Answer 3

You can simply perform it:

daysInMonths.foldLeft((0, List[Int]()))
                     {(acu,i)=>(i+acu._1, i+acu._1 :: acu._2)}._2.reverse