Question

通过Sedgwick和Wayne的算法练习继续我的旅行我遇到了一个我必须实现RandomBag的练习。最初RandomBag应该实现Iterable（在java中），它的Iterator必须以随机顺序提供项目。

这是我的ImmutableRandomBag的伴随对象：

object ImmutableRandomBag{
  case class Node[Item](item: Item, next: Option[Node[Item]])
  def apply[Item](maybeNode: Option[Node[Item]], size: Int): ImmutableRandomBag[Item] = new ImmutableRandomBag(maybeNode, size)
}

这是班级本身的开始：

class ImmutableRandomBag[Item](maybeNode: Option[Node[Item]], size: Int) extends Iterable[Item]{

      override def isEmpty: Boolean = size == 0

      def add(item: Item) = {
        ImmutableRandomBag(Some(Node(item, maybeNode)), size +1)
      }
   ...
   }

我的理解是val size应该从Iterable特征中覆盖def大小。在测试add方法时，我得到了IndexOutOfBounException：

class RandomBagSpec extends BaseSpec {

  trait RandomBag{
    val begin = new ImmutableRandomBag[Connection](None, 0)
  }

  ...

  "Adding an item to empty RandomBag" should "return another bag with size 1" in new RandomBag {
    val bag = begin.add(Connection(0,1))
    bag.size should equal(1)
  }
}

虽然在构造函数参数中正确评估了调试大小，但我不确定IndexOutOfBoundException来自哪里，但每当我调用add方法时我都会得到它。也许这个问题会从以下问题中萌发出来。在ImmutableRandomBag中还有Iterator实现：

...
override def iterator: Iterator[Item] = new RandomIterator[Item](maybeNode)

  private class RandomIterator[Item](first: Option[Node[Item]]) extends Iterator[Item]{

    first match {
      case Some(node) => random(node)
      case None =>
    }

    var current: Int = 0
    var container: Vector[Item] = Vector()

    override def hasNext: Boolean = current < ImmutableRandomBag.this.size

    override def next(): Item = {
      val item = container(current)
      current += 1
      item
    }

    def random(first: Node[Item]) = {
      @tailrec
      def randomHelper(next: Option[Node[Item]], acc: List[Item]):List[Item]= next match {
        case None => acc
        case Some(node) => randomHelper(node.next, node.item::acc)
      }

      val items = randomHelper(Some(first), List[Item]())
      container = Random.shuffle(items).toVector
    }
  }
}

我在同一规格中有不同的测试：

...
"Random Bag's iterator" should "contain all items passed to parent iterable" in new RandomBag{
    val connections = List(Connection(0,1), Connection(1,0), Connection(1,1))
    var localRB = begin
    for(c <- connections) localRB = localRB.add(c)
    assert(localRB.iterator.forall(conn=> connections.contains(conn)) == true)
  }
...

我还得到一个带有以下堆栈的IndexOutOfBoundException：

    [info] RandomBagSpec:
[info] Random Bag's iterator
[info] - should contain all items passed to parent iterable *** FAILED ***
[info]   java.lang.IndexOutOfBoundsException: 0
[info]   at scala.collection.immutable.Vector.checkRangeConvert(Vector.scala:123)
[info]   at scala.collection.immutable.Vector.apply(Vector.scala:114)
[info]   at ca.vgorcinschi.algorithms1_3_34.ImmutableRandomBag$RandomIterator.next(ImmutableRandomBag.scala:31)
[info]   at scala.collection.Iterator.forall(Iterator.scala:956)
[info]   at scala.collection.Iterator.forall$(Iterator.scala:954)
[info]   at ca.vgorcinschi.algorithms1_3_34.ImmutableRandomBag$RandomIterator.forall(ImmutableRandomBag.scala:18)
[info]   at ca.vgorcinschi.algorithms1_5_19.RandomBagSpec$$anon$1.<init>(RandomBagSpec.scala:16)
[info]   at ca.vgorcinschi.algorithms1_5_19.RandomBagSpec.$anonfun$new$1(RandomBagSpec.scala:12)
[info]   at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
[info]   at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)

问题似乎来自调用Iterator的下一个方法，而且容器Vector确实不包含任何元素：

但为什么在next之前调用random？

Answer 1

val size应该从Iterable trait
覆盖def大小

val会有，但你没有;你只需要一个非case类的构造函数参数。实际上它是private val并且无法覆盖任何内容。

但为什么下一次被随机调用呢？

它不是;在RandomIterator的构造函数中，random在初始化程序first match ...之前被调用（作为container = Vector()的一部分）。仅在构造函数之后调用next。

Scala中基于节点的可执行实现

1 个答案: