Scala - increasing prefix of a sequence

Question

I was wondering what is the most elegant way of getting the increasing prefix of a given sequence. My idea is as follows, but it is not purely functional or any elegant:

Answer 1

If by elegant you mean concise and self-explanatory, it's probably something like the following:

sequence.inits.dropWhile(xs => xs != xs.sorted).next

inits gives us an iterator that returns the prefixes longest-first. We drop all the ones that aren't sorted and take the next one.

If you don't want to do all that sorting, you can write something like this:

sequence.scanLeft(Some(Int.MinValue): Option[Int]) {
  case (Some(last), i) if i > last => Some(i)
  case _ => None
}.tail.flatten

If the performance of this operation is really important, though (it probably isn't), you'll want to use something more imperative, since this solution still traverses the entire collection (twice).