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

For any:

val s:Seq[Int] = ...

You can use one of those:

s.tail.scanLeft(s.head)(_ + _)
s.scanLeft(0)(_ + _).tail

or folds proposed in other answers but... be aware that Landei's solution is tricky and you should avoid it.

BE AWARE

s.map { var s = 0; d => {s += d; s}} 
//works as long `s` is strict collection

val s2:Seq[Int] = s.view //still seen as Seq[Int]
s2.map { var s = 0; d => {s += d; s}} 
//makes really weird things! 
//Each value'll be different whenever you'll access it!

I should warn about this as a comment below Landei's answer but I couldn't :(.

Answer 2

Fold your list into a new list. On each iteration, append a value which is the sum of the head + the next input. Then reverse the entire thing.

scala> val daysInMonths = List(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
daysInMonths: List[Int] = List(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)

scala> daysInMonths.foldLeft(Nil: List[Int]) { (acc,next) => 
     | acc.firstOption.map(_+next).getOrElse(next) :: acc    
     | }.reverse                                             
res1: 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

Answer 4

Fold into a list instead of an integer. Use pair (partial list with the accumulated values, accumulator with the last sum) as state in the fold.

Answer 5

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 6

You can also create a monoid class that concatenates two lists while adding to the second one the last value from the first. No mutables and no folds involved:

case class CumSum(v: List[Int]) { def +(o: CumSum) = CumSum(v ::: (o.v map (_ + v.last))) }
defined class CumSum

scala> List(1,2,3,4,5,6) map {v => CumSum(List(v))} reduce (_ + _)
res27: CumSum = CumSum(List(1, 3, 6, 10, 15, 21))