Scala：不可变的 Map 正在改变它的值

Question

我正在编写一个代码来确定一个可着色的图表。我正在使用不可变地图来确定回溯时的顶点颜色。

在 flatMap 循环的第二次交互中，我的 Map 丢失了一个元素。我无法编写一个更简洁的示例来呈现相同的错误，那么这是我的整个代码：

import scala.collection.immutable
import scala.collection.mutable

class GraphUndirected[Vertex](vertices: Set[Vertex], edges: Set[(Vertex, Vertex)]) {
  private val adjacencyList: Map[Vertex, Set[Vertex]] =
    (
      edges.flatMap{ case (a, b) => {Seq( (a, Set(b)), (b, Set(a)))}}
        ++ vertices.map((_, Set()))
      )
      .groupBy(_._1)
      .mapValues(
        _.map(_._2)
          .reduce(_ ++ _)
          .toSet
      )

  override def toString(): String = adjacencyList.toString

  def kColorable[C](colors: Set[C]): Option[Map[Vertex, C]] = {
    vertices.toSeq.view.flatMap( v => {
      val colorsMap = immutable.Map[Vertex,C]()
      kColorable[C](colors, v, colorsMap)
    }).headOption
  }

  def kColorable[C](colors: Set[C], v: Vertex, colorMap: Map[Vertex, C]): Option[Map[Vertex, C]] = {
    println(v, colorMap)

    colors.toSeq.view.flatMap( c => {
      if (kColorableIsSafe[C](c, v, colorMap)) {
        val newColorMap = colorMap + (v -> c)
        println("\tnewColorMap", newColorMap)

        if (newColorMap.size == vertices.size) {
          Some(newColorMap)
        } else {
          adjacencyList(v).toSeq.view.flatMap( u => {
            println("\t\tnewColorMap", newColorMap)

            if (newColorMap.contains(u)) {
              None
            } else {
              kColorable[C](colors, u, newColorMap)
            }
          }).headOption
        }
      } else {
        None
      }
    }).headOption
  }

  def kColorableIsSafe[C](c: C, v: Vertex, colorMap: Map[Vertex, C]): Boolean = {
    adjacencyList(v).forall( u => {
      (!colorMap.contains(u)) ||
        (colorMap(u) != c)
    })
  }
}

val myGraph1 = new GraphUndirected[Int](
  Set(1,2,3,4,5,6,7,8,9),
  Set(
    (1,2),
    (3,2), (3,4),
    (5,7), (5,9),
    (6,4),
    (8,2), (8,9)
  )
)
println(myGraph1.kColorable[String](Set("BLACK", "WHITE")))

运行这段代码会产生：

(5,Map())
(       newColorMap,Map(5 -> BLACK))
(               newColorMap,Map(5 -> BLACK))
(7,Map(5 -> BLACK))
(       newColorMap,Map(5 -> BLACK, 7 -> WHITE))
(               newColorMap,Map(5 -> BLACK, 7 -> WHITE))
(               newColorMap,Map(5 -> BLACK))
(9,Map(5 -> BLACK))

这对我来说没有意义。在第一次交互中，它打印newColorMap,Map(5 -> BLACK, 7 -> WHITE)，但在第二次交互中，“7->WHITE”消失了newColorMap,Map(5 -> BLACK)。

newColorMap 不应该是不可变的，不改变它的值？

---

我的 Scala 版本：

$ scala -version
Scala code runner version 2.12.2 -- Copyright 2002-2017, LAMP/EPFL and Lightbend, Inc.

Answer 1

您的代码对我来说似乎没有错误。它返回预期的输出。具有欺骗性的是您选择在逐步执行程序时输出程序内部状态的方式。

要了解发生了什么，请尝试运行同一程序的此版本。只需将代码复制并粘贴到 Kcolor.scala 中，将其放入其文件夹并输入

scalac Kcolor.scala
scala Kcolor

这是 Kcolor.scala 的内容

import scala.collection.immutable

object Kcolor extends App {

  val myGraph1 = new GraphUndirected[Int](
    Set(5,7,9),
    Set((5,7), (5,9))
  )
  println(myGraph1.kColorable[String](Set("BLACK", "WHITE")))

}

class GraphUndirected[Vertex](vertices: Set[Vertex], edges: Set[(Vertex, Vertex)]) {
  val adjacencyList: Map[Vertex, Set[Vertex]] =
    (
      edges.flatMap{ case (a, b) => {Seq( (a, Set(b)), (b, Set(a)))}}
        ++ vertices.map((_, Set()))
      )
      .groupBy(_._1)
      .mapValues(
        _.map(_._2)
          .reduce(_ ++ _)
          .toSet
      )

  override def toString(): String = adjacencyList.toString

  def kColorable[C](colors: Set[C]): Option[Map[Vertex, C]] = {
    vertices.flatMap( v => {
      val colorsMap = immutable.Map[Vertex,C]()
      kColorable[C](colors, v, colorsMap)
    }).headOption
  }

  def kColorable[C](colors: Set[C], v: Vertex, colorMap: Map[Vertex, C]): Option[Map[Vertex, C]] = {
    println(v, colorMap)

    val res = colors.flatMap( c => {
      if (kColorableIsSafe[C](c, v, colorMap)) {
        val newColorMap = colorMap + (v -> c)
        println("\tnewColorMap", v, c, newColorMap)

        if (newColorMap.size == vertices.size) {
          Some(newColorMap)
        } else {
          val res1 = adjacencyList(v).flatMap( u => {
            println("\t\tnewColorMap", u, newColorMap)

            if (newColorMap.contains(u)) {
              None
            } else {
              kColorable[C](colors, u, newColorMap)
            }
          })
          println(res1)
          res1.headOption
        }
      } else {
        None
      }
    }).headOption
    println("end")
    res
  }

  def kColorableIsSafe[C](c: C, v: Vertex, colorMap: Map[Vertex, C]): Boolean = {
    adjacencyList(v).forall( u => {
      (!colorMap.contains(u)) ||
        (colorMap(u) != c)
    })
  }
}

我已经包含了输出的前几行：

(5,Map())
(       newColorMap,5,BLACK,Map(5 -> BLACK))
(               newColorMap,7,Map(5 -> BLACK))
(7,Map(5 -> BLACK))
(       newColorMap,7,WHITE,Map(5 -> BLACK, 7 -> WHITE))
(               newColorMap,5,Map(5 -> BLACK, 7 -> WHITE))
Set()
end
(               newColorMap,9,Map(5 -> BLACK))
(9,Map(5 -> BLACK))
(       newColorMap,9,WHITE,Map(5 -> BLACK, 9 -> WHITE))
(               newColorMap,5,Map(5 -> BLACK, 9 -> WHITE))
Set()
end

程序正在做的是它到达 flatMap 的末尾而无法创建彩色地图，并且它正在输出 None。所以行间

(               newColorMap,5,Map(5 -> BLACK, 7 -> WHITE))

和

(               newColorMap,9,Map(5 -> BLACK))

它正在到达当前 flatMap 的末尾并跳回到外循环的下一次迭代。

你的程序很有趣。我想起来很开心。感谢分享。