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理解流中的递归

scala

0

Srinivas · 技术社区 · 7 年前

  val bigIntFromStream : Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: bigIntFromStream.zip(bigIntFromStream.tail).map{x =>
    println(s"stream = ${bigIntFromStream} + stream.tail = ${bigIntFromStream.tail} + zipped value = ${bigIntFromStream.zip(bigIntFromStream.tail)}")
    println(s"Calculating value for ${x}, {x._1} = ${x._1}, {x._2} = ${x._2}")
    x._1 + x._2}

代码的输出如下

0 1 stream = Stream(0, 1, ?) + stream.tail = Stream(1, ?) + zipped value = Stream((0,1), ?)

Calculating value for (0,1), {x._1} = 0, {x._2} = 1
1 stream = Stream(0, 1, 1, ?) + stream.tail = Stream(1, 1, ?) + zipped value = Stream((0,1), ?)

Calculating value for (1,1), {x._1} = 1, {x._2} = 1
2 stream = Stream(0, 1, 1, 2, ?) + stream.tail = Stream(1, 1, 2, ?) + zipped value = Stream((0,1), ?)

Calculating value for (1,2), {x._1} = 1, {x._2} = 2
3 stream = Stream(0, 1, 1, 2, 3, ?) + stream.tail = Stream(1, 1, 2, 3, ?) + zipped value = Stream((0,1), ?)

Calculating value for (2,3), {x._1} = 2, {x._2} = 3
5 stream = Stream(0, 1, 1, 2, 3, 5, ?) + stream.tail = Stream(1, 1, 2, 3, 5, ?) + zipped value = Stream((0,1), ?)

Calculating value for (3,5), {x._1} = 3, {x._2} = 5

我的问题是x如何得到实际值和尾部之间的最后一个zip值?如果我遗漏了什么,请告诉我

2 回复 | 直到 7 年前

1

jwvh 7 年前

回想一下 Stream 一开始是作为一个单一的元素,承诺在需要的时候得到更多。即使所有元素都已完全定义,情况也是如此。

val ns = Stream(1,2,3,4,5)
//ns: scala.collection.immutable.Stream[Int] = Stream(1, ?)

val paired = ns zip ns.tail
//paired: scala.collection.immutable.Stream[(Int, Int)] = Stream((1,2), ?)
// The rest are (2,3), (3,4) and (4,5), but they haven't been realized yet.

另外,当你索引一个这是一个线性运算。换句话说,如果你想 paired(3) 代码将逐步通过(“实现”) paired(0) ,然后 paired(1) paired(2) ,最后 成对(3) .

溪流 您发布的内容基本上是:

// This is concept, not code.
Stream(a=0, b=1, c=a+b, d=b+c, e=c+d, ...)

溪流

2

0

Mansoor Baba Shaik 7 年前

object Test extends App {
  var bigIntFromStream : Stream[BigInt] = Stream(0,1)

  def addItems(n: Int)(x: Stream[BigInt]): Unit = {
    if(n > 0){
      val lastIndex: Int = bigIntFromStream.indices.last
      bigIntFromStream ++= Stream(bigIntFromStream(lastIndex - 1) + bigIntFromStream(lastIndex))
      addItems(n-1)(bigIntFromStream)
    }
  }

  addItems(10)(bigIntFromStream)

  println(bigIntFromStream.mkString(","))
}