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

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)