scala turning an Iterator[Option[T]] into an Iterator[T]

Question

I have an Iterator[Option[T]] and I want to get an Iterator[T] for those Options where T isDefined. There must be a better way than this:

it filter { _ isDefined} map { _ get }

I would have thought that it was possible in one construct... Anybody any ideas?

Answer 1

In the case where it is an Iterable

val it:Iterable[Option[T]] = ...
it.flatMap( x => x )                //returns an Iterable[T]

In the case where it is an Iterator

val it:Iterator[Option[T]] = ...
it.flatMap( x => x elements )       //returns an Iterator[T]
it.flatMap( _ elements)             //equivalent

Answer 2

In newer versions this is now possible:

val it: Iterator[Option[T]] = ...
val flatIt = it.flatten

Answer 3

This works for me (Scala 2.8):

it.collect {case Some(s) => s}

Answer 4

To me, this is a classic use case for the monadic UI.

for {
  opt <- iterable
  t   <- opt
} yield t

It's just sugar for the flatMap solution described above, and it produces identical bytecode. However, syntax matters, and I think one of the best times to use Scala's monadic for syntax is when you're working with Option, especially in conjunction with collections.

I think this formulation is considerably more readable, especially for those not very familiar with functional programming. I often try both the monadic and functional expressions of a loop and see which seems more straightforward. I think flatMap is hard name for most people to grok (and actually, calling it >>= makes more intuitive sense to me).