带有 Map 和 For 的 Scala 迭代器
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Scala Iterator with Map and For
提问by ssanj
Given:
鉴于：
val list = List("one","two","three")
val it = list.toIterator
I can run:
我可以跑：
list map ("_" +) -> List(_one, _two, _three)
for (i <- list) yield("_" + i) -> List(_one, _two, _three)
If I run the same on the iterator I get:
如果我在迭代器上运行相同的，我得到：
it map ("_" + ) -> Iterator[java.lang.String] = empty iterator
for (i <- it) yield("_" + i) -> Iterator[java.lang.String] = empty iterator
Shouldn't I get back another (non-empty) Iterator[String] after I run map/for on it?
在我运行 map/for 之后，我不应该取回另一个（非空）Iterator[String] 吗？
回答by IttayD
scala> def ints(n: Int): Stream[Int] = n #:: ints(n + 1)
ints: (n: Int)Stream[Int]
scala> val list = List("one","two","three")
list: List[java.lang.String] = List(one, two, three)
scala> val it = list.toIterator
it: Iterator[java.lang.String] = non-empty iterator
scala> it map ("_" + )
res24: Iterator[java.lang.String] = non-empty iterator
scala> it map ("_" + )
res25: Iterator[java.lang.String] = non-empty iterator
scala> for (i <- it) yield("_" + i)
res26: Iterator[java.lang.String] = non-empty iterator
Maybe you used your iterator?
也许您使用了迭代器？
scala> res26.foreach{println}
_one
_two
_three
scala> res26
res28: Iterator[java.lang.String] = empty iterator
Since iterators are stateful and not resettable, once you used it, it is empty and can't be used again.
由于迭代器是有状态的且不可重置，一旦你使用它，它就是空的，不能再次使用。
Instead, you can use views:
相反，您可以使用视图：
scala> val v = list.view
v: java.lang.Object with scala.collection.SeqView[java.lang.String,List[java.lang.String]] = SeqView(one, two, three)
scala> v map ("_" + )
res29: scala.collection.SeqView[java.lang.String,Seq[_]] = SeqViewM(...)
scala> for (i <- v) yield("_" + i)
res30: scala.collection.SeqView[java.lang.String,Seq[_]] = SeqViewM(...)
scala> res29.foreach{println}
_one
_two
_three
scala> res29
res32: scala.collection.SeqView[java.lang.String,Seq[_]] = SeqViewM(...)
scala> res29.foreach{println}
_one
_two
_three
回答by Eugene Yokota
See Iterators.
请参阅迭代器。
There's an important difference between the
foreachmethod on iterators and the same method on traversable collections: When called to an iterator,foreachwill leave the iterator at its end when it is done. So callingnextagain on the same iterator will fail with aNoSuchElementException. By contrast, when called on on a collection,foreachleaves the number of elements in the collection unchanged (unless the passed function adds to removes elements, but this is discouraged, because it may lead to surprising results).
foreach迭代器上的方法和可遍历集合上的相同方法之间有一个重要区别：当调用迭代器时，foreach将在完成后将迭代器留在其末尾。因此，next再次调用同一个迭代器将失败并显示NoSuchElementException. 相比之下，当在集合上调用时，集合foreach中元素的数量保持不变（除非传递的函数添加到删除元素，但不鼓励这样做，因为它可能会导致令人惊讶的结果）。
...
...
As you can see, after the call to
it.map, theititerator has advanced to its end.
如您所见，在调用 之后
it.map，it迭代器前进到它的末尾。
