What is the difference between the methods iterator and view?

Question

scala> (1 to 10).iterator.map{_ * 2}.toList res1: List[Int] = List(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)  scala> (1 to 10).view.map{_ * 2}.force res2: Seq[Int] = Vector(2, 4, 6, 8, 10, 12, 14, 16, 18, 20) 

Other than using next,hasNext, when should I choose iterator over view or view over iterator?

Answer 1

There's a huge difference between iterators and views. Iterators are use once only, compute on demand, while views are use multiple times, recompute each time, but only the elements needed. For instance:

scala> val list = List(1,2,3).map{x => println(x); x * 2} 1 2 3 list: List[Int] = List(2, 4, 6)  scala> list(2) res14: Int = 6  scala> list(2) res15: Int = 6  scala> val view = List(1,2,3).view.map{x => println(x); x * 2} view: scala.collection.SeqView[Int,Seq[_]] = SeqViewM(...)  scala> view(2) 3 res12: Int = 6  scala> view(2) 3 res13: Int = 6  scala> val iterator = List(1,2,3).iterator.map{x => println(x); x * 2} iterator: Iterator[Int] = non-empty iterator  scala> iterator.drop(2).next 1 2 3 res16: Int = 6  scala> iterator.drop(2).next [Iterator.next] (Iterator.scala:29) (access lastException for the full trace) 

Answer 2

view produces a lazy collection/stream. It's main charm is that it won't try and build the whole collection. This could prevent a OutOfMemoryError or improve performance when you only need the first few items in the collection. iterator makes no such guarantee.

One more thing. At least on Range, view returns a SeqView, which is a sub-type of Seq, so you can go back or start again from the beginning and do all that fun sequency stuff.

I guess the difference between an iterator and a view is a matter of in-front and behind. Iterators are expected to release what has been seen. Once next has been called, the previous is, hopefully, let go. Views are the converse. They promise to not acquire what has not been requested. If you have a view of all prime numbers, an infinite set, it has only acquired those primes you've asked for. It you wanted the 100th, 101 shouldn't be taking up memory yet.