Question

Specifically I’m looking at Problem 1 here

http://pavelfatin.com/scala-for-project-euler/

The code as listed is as follows

val r = (1 until 1000).view.filter(n => n % 3 == 0 || n % 5 == 0).sum

I can follow everything except for “view”. In fact if I take out view the code still compiles and produces exactly the same answer.

Answer 1

View produces a lazy collection, so that calls to e.g. filter do not evaluate every element of the collection. Elements are only evaluated once they are explicitly accessed. Now sum does access all elements, but with view the call to filter doesn’t create a full Vector. (See comment by Steve)

A good example of the use of view would be:

scala> (1 to 1000000000).filter(_ % 2 == 0).take(10).toList
java.lang.OutOfMemoryError: GC overhead limit exceeded

Here Scala tries to create a collection with 1000000000 elements to then access the first 10. But with view:

scala> (1 to 1000000000).view.filter(_ % 2 == 0).take(10).toList
res2: List[Int] = List(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)