Only evaluate position when reaching cumulative sum threshold in LazyList-CodePudding

I would like to find a position of reaching a fixed threshold (e.g. 10) in a lazy list of 2's e.g.:

LazyList.
    continually(2).
    zipWithIndex.
    scanLeft((0,0)){case (acc, el) => (acc._1   el._1, el._2)}.
    takeWhile(_._1 <= 10)

res72: LazyList[(Int, Int)] = LazyList(
  (0, 0),
  (2, 0),
  (4, 1),
  (6, 2),
  (8, 3),
  (10, 4)
)

Is it possible to do the same without collecting all the intermediate results, in functional way, only returning final position when the cumulative sum is reached, with foldLeft e.g. or similar?

CodePudding user response：

Yes, you can use collectFirst, also I would move the zipWithIndex after the scanLeft

LazyList
  .continually(2)
  .scanLeft(0)((acc, x) => acc   x)
  .zipWithIndex
  .collectFirst {
    case (x, idx) if (x >= 10) => idx
  }

That returns an Option[Int] because the collect may never match, but you know it will in this case so you can just .get or .getOrElse(new IllegalStateException("This should have never happened"))