Partitioning an Iterator in scala 2.7.5

Question

It looks as though there is no partition method on an Iterator in scala 2.7.5 (there is in 2.8). I'd like to have a partition without losing the laziness of the Iterator, so the following is not an option:

itr.toList.partition( someTest(_) )

Can anyone recommend a way of doing this without implementing my own partition method? For example, is there some way of converting an Iterator into a lazily-evaluated Stream?

Answer 1

Have you tried the Stream.fromIterator method? It produces a stream containing the elements of the given iterator :).

An example would be:

val iter = List(1,2,3,4).elements // just a simple iterator

val str = Stream.fromIterator(iter)

str.partition(_ >= 3)

Hope it helps (and it is the thing you had in mind).

EDIT: Just an example to show that this is lazy (and memoised - as all Streams are).

scala> val iter = new Iterator[Int] {
     | var lst = List(1,2,3,4)
     | def hasNext() = !lst.isEmpty
     | def next() = { val x = lst.head; println(x); lst = lst.tail; x }
     | }

scala> val stream = Stream.fromIterator(iter)
1
stream: Stream[Int] = Stream(1, ?)

scala> stream.partition(_ >= 2)
2
3
4
res1: (Iterable[Int], Iterable[Int]) = (ArrayBuffer(2, 3, 4),ArrayBuffer(1))

scala> stream.partition(_ >= 3)
res2: (Iterable[Int], Iterable[Int]) = (ArrayBuffer(3, 4),ArrayBuffer(1, 2))

NB: left some output out as it was quite verbose.

-- Flaviu Cipcigan