How to return all positives and the first negative number in a list using functional programming?

Question

Imagine I have an unsorted list of positive & negative ints. I want to return a list containing all the positive ints, and the first negative number, but then ignore all subsequent negative numbers from the list, while preserving the ordering.

Imperatively I could do:

l = [1, 2, -4, 5, -6, -1, 3]
out = []
first = true
for n in l:
    if n >= 0:
        out.push(n)
    else if first:
        out.push(n)
        first = false

// out = [1, 2, -4, 5, 3]

How could I do this with FP in Scala? I was thinking (probably won't compile...):

val l = List(1, 2, -4, 5, -6, -1, 3)
val posl = l.map(_ >= 0)
val negl = l.zipWithIndex.map((n, i) => if (n < 0) (i, n) else (None, None)).head
// now split posl at negl._1, and create a new list of leftSlice :: negl._2 :: rightSlice?

Is this the right approach, or is there a more elegant, succinct way?

Answer 1

It wouldn't be a proper functional programming question without a slightly-too-clever recursion+pattern matching answer.

def firstNegAllPos(l:List[Int]):List[Int] = {
   l match{
      case x::xs if x>=0 => x::firstNegAllPos(xs)
      case x::xs if x<0 => x::xs.filter(_>=0)
      case Nil => Nil
   }
}