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:

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

Works on 2.7.7:

def stepSum (sums: List [Int], steps: List [Int]) : List [Int] = steps match { 
     case Nil => sums.reverse.tail                                                  
     case x :: xs => stepSum (sums.head + x :: sums, steps.tail) }

days
res10: List[Int] = List(31, 28, 31, 30, 31)

stepSum (List (0), days) 
res11: List[Int] = List(31, 59, 90, 120, 151)