如何创建一个内容是表达式结果的有限迭代器？

Question

我想创建一个Iterator通过(重复)计算表达式来获取其下一个元素,并且我希望表达式能够返回某个值来终止它.

我发现的唯一一件事就是Iterator.continually(),这似乎是无限的.重要的是,在next()调用表达式之前不应该对表达式进行求值Iterator.

有没有办法得到这种行为？

例如:

def getNext = {
  // some complicated code
  val next = ... // either a STOP value or a real value to be returned by the iterator
} 

val myIter = Iterator.continually(getNext) // want this to stop at some point

Answer 1

Iterator.continually通常与takeWhile:

var count = 0
def complexCompute(): Int = { count +=1; println("eval " + count); count }

val iter = Iterator.continually { complexCompute() }
iter.takeWhile(_ < 3).foreach(println)

哪个印刷品:

eval 1
1
eval 2
2
eval 3

因此,如果确定计算是否应该继续的条件可以在计算之外进行评估,那么这非常有效.

基本上我想我Iterator.continually(getNext()).takeWhile(_ != certainValue)会说会实现你想要做的事情.这是懒惰的评价.

Answer 2

你看过scala.collection.immutable.Stream吗？它的目的是创建一个像对象的序列,其中下一个元素被懒惰地评估.它可以是有限的或无限的.

例如:

Welcome to Scala version 2.9.0.final (Java HotSpot(TM) Client VM, Java 1.6.0_24).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import collection.immutable._
import collection.immutable._

scala> def next(i: Int): Stream[Int] = Stream.cons(i*i, next(i*i))
next: (i: Int)scala.collection.immutable.Stream[Int]

scala> val stream = next(2)
stream: scala.collection.immutable.Stream[Int] = Stream(4, ?)

scala> stream.find(_ > 1000)
res0: Option[Int] = Some(65536)