方法迭代器和视图之间有什么区别? [英] What is the difference between the methods iterator and view?

问题描述

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)

除了使用next，hasNext，我什么时候应该选择iterator over view或者view over iterator?

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

推荐答案

迭代器和视图之间存在巨大差异.迭代器只使用一次，按需计算，而视图使用多次，每次重新计算，但只计算需要的元素.例如:

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)

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