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scalafunctional-programmingfold

Sum list of Option[Int] Scala using fold

I was trying to sum all elements in a list, they are of type Option[Int], if any of them are None, the whole result is None, if all are Some(value), I want the sum of all of them:

val ls = List(Some(2), Some(5), Some(4), Some(1))

ls.fold(None)((rs,x) => for(n <- x; m <- rs) yield {n+m})

but I always get None, no matter what.

Solution

  • Try this also: 

    val ls = List(Some(2), Some(5), Some(4), Some(1))
val sum = ls.foldLeft(Option(0))((so, io) => so.flatMap(s => io.map(s + _)))

    If any single value in the list is None, then sum is reported as None; otherwise - if all values are defined - you get a Some containing the sum of elements. If the list is empty, then you'd get Some(0) as the result.