Cumulative sum without var scala [duplicate] - scala

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

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

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 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))

Answer 7

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 8

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)