如何在 Scala 中返回迭代器？

Question

我必须做什么才能从方法/类返回迭代器？如何将这一特征添加到类中？

Answer 1

您可以扩展 Iterator，这需要您实现next 和 hasNext 方法：

  class MyAnswer extends Iterator[Int] {
    def hasNext = true
    def next = 42
  }

但是，如果扩展 Iterable，它要求您实现 elements （或 2.8 中的 iterator）：

  class MyAnswer extends Iterable[Int] {
    def iterator = new Iterator[Int] {
      def hasNext = true
      def next = 42
    }
  }

常见的习惯用法似乎是将迭代器公开给某些私有集合，如下所示：

  class MyStooges extends Iterable[String] {
    private val stooges = List("Moe", "Larry", "Curly")
    def iterator = stooges.iterator
  }

Answer 2

对于方法，只需 yield：

def odd(from: Int, to: Int): List[Int] = 
  for (i <- List.range(from, to) if i % 2 == 1) yield i

Answer 3

这两个答案从下面的帖子中得到了帮助，并感谢@Dima。

• 如何为现有单链表实现迭代器？

• 为什么这个可迭代实现会产生 stackoverflow？

假设您有一个类链表。而要求是打印列表中的所有元素。

trait LinkedList {
  def nodeValue: Int
  def tailList: LinkedList
}

class Node(val nodeValue: Int, val tailList: LinkedList) extends LinkedList

object Nil extends LinkedList {
  def nodeValue = throw new IllegalAccessException("head of Nil")
  def tailList = throw new IllegalAccessException("tail of Nil")
}

val singleLinkedList = new Node(1,Nil)
val chainedLinkedList = new Node(2,singleLinkedList)
print(chainedLinkedList)
A$A44$A$A44$Node@7b7a2c78res0: Unit = ()

现在让我们为这个类实现迭代器。

trait LinkedList extends Iterator[Int]{
  def nodeValue: Int
  def tailList: LinkedList
}

class Node(val nodeValue: Int, val tailList: LinkedList) extends LinkedList {
  var ptr: LinkedList = this

  //The following two are mandatory for extending Iterator
  override def hasNext: Boolean = ptr match { case Nil => false; case _=> true}

  override def next(): Int = {
    val result = ptr.nodeValue
    ptr = ptr.tailList
    result
  }
}

object Nil extends LinkedList {
  def nodeValue = throw new IllegalAccessException("head of Nil")
  def tailList = throw new IllegalAccessException("tail of Nil")

  //The following two are mandatory for extending Iterator
  override def hasNext: Boolean = false
  override def next(): Int = throw new IllegalAccessException("next of Nil")
}

val singleLinkedList = new Node(1,Nil)
val chainedLinkedList = new Node(2,singleLinkedList)

//Printing this first Time
chainedLinkedList.foreach(println)
//Prints 2 1

//Printing second Time
chainedLinkedList.foreach(println)
//No output

在迭代器实现中，一旦 ptr 到达末尾，它就无法前进。可迭代的实现解决了这个问题。

trait LinkedList extends Iterable[Int]{
  val nodeValue: Int
  val tailList: LinkedList
  override def toString(): String = this.mkString(" -> ")
}

class Node(val nodeValue: Int, val tailList: LinkedList) extends LinkedList {

  override def iterator: Iterator[Int] = Iterator
    .iterate(this: LinkedList)(_.tailList)
    .takeWhile(_ != Nil)
    .map(_.nodeValue)
}

object Nil extends LinkedList {
  lazy val nodeValue= throw new IllegalAccessException("head of Nil")
  lazy val tailList = throw new IllegalAccessException("tail of Nil")

  override def iterator: Iterator[Int] = Iterator.empty
}

val singleLinkedList = new Node(1,Nil)
val chainedLinkedList = new Node(2,singleLinkedList)

//Printing this first Time
chainedLinkedList.foreach(println)
Output 2 -> 1
chainedLinkedList.foreach(println)
Output 2 -> 1