Question

我正在为Martin Odersky博士mentioned over at Skillmatters进行电话簿助记符的代码审查。

以下是他所拥有的片段：

class Coder(words: List[String]) {

  private val mnemonics = Map(
    '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
    '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

  /** Invert the mnemonics map to give a map from chars 'A' ... 'Z' to '2' ... '9'*/
  private val charCode: Map[Char, Char] =
    for ((digit, str) <- mnemonics; letter <- str) yield (letter -> digit)


  /** Maps a word to the digit string it can represent */
  private def wordCode(word: String): String = word.toUpperCase map charCode

  /** A map from digit strings to the words that represent them, 
    * e,g. 5282 -> Set(Java, Kata, Lava, ...) */

  private val wordsForNum: Map[String, List[String]] = 
    (words groupBy wordCode) withDefaultValue List()

我试图将所有内容声明为函数和变量，以了解withDefaultValue的行为方式，这就是我所得到的：

scala> val words3 = List("moo", "1111")
words3: List[java.lang.String] = List(moo, 1111)

scala> (words3 groupBy wordCode) withDefaultValue List()
java.util.NoSuchElementException: key not found: $
    at scala.collection.MapLike$class.default(MapLike.scala:224)

在视频中，他正在谈论如果这个词没有被映射，那么我们应该得到一个项目列表（这从视频中的17分钟开始）。我收到错误了？我正在使用Scala的REPL 2.9.0.1。

感谢您的时间。

编辑：

我很确定我已经在REPL中正确生成了val和函数def。

scala> :paste
// Entering paste mode (ctrl-D to finish)

val mnemonics = Map(
    '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
    '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

// Exiting paste mode, now interpreting.

mnemonics: scala.collection.immutable.Map[Char,java.lang.String] = Map(8 -> TUV, 4 -> GHI, 9 -> WXYZ, 5 -> JKL, 6 -> MNO, 2 -> ABC, 7 -> PQRS, 3 -> DEF)

scala> :paste
// Entering paste mode (ctrl-D to finish)

val charCode: Map[Char, Char] =
    for ((digit, str) <- mnemonics; letter <- str) yield (letter -> digit)

// Exiting paste mode, now interpreting.

charCode: Map[Char,Char] = Map(E -> 3, X -> 9, N -> 6, T -> 8, Y -> 9, J -> 5, U -> 8, F -> 3, A -> 2, M -> 6, I -> 4, G -> 4, V -> 8, Q -> 7, L -> 5, B -> 2, P -> 7, C -> 2, H -> 4, W -> 9, K -> 5, R -> 7, O -> 6, D -> 3, Z -> 9, S -> 7)

def wordCode(word: String): String = word.toUpperCase map charCode

wordCode: (word: String)String

我也尝试在REPL中定义整个类。

defined class Coder

scala> val words4 = List("hi", "Hello world", "$t@r")
words4: List[java.lang.String] = List(hi, Hello world, $t@r)

scala> var listPhoneNumber = new Coder(words4)
java.util.NoSuchElementException: key not found:  
    at scala.collection.MapLike$class.default(MapLike.scala:224)

scala> val words5 = List("hi","hello","ciao")
words5: List[java.lang.String] = List(hi, hello, ciao)

scala> var listPhoneNumber = new Coder(words5)
listPhoneNumber: Coder = Coder@119c2af

这是我正在使用的整个代码：

class Coder(words: List[String]) {



  private val mnemonics = Map(

    '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",

    '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")



  /** Invert the mnemonics map to give a map from chars 'A' ... 'Z' to '2' ... '9'*/

  private val charCode: Map[Char, Char] =

    for ((digit, str) <- mnemonics; letter <- str) yield (letter -> digit)



  /** Maps a word to the digit string it can represent */

  private def wordCode(word: String): String = word.toUpperCase map charCode



  /** A map from digit strings to the words that represent them, 

    * e,g. 5282 -> Set(Java, Kata, Lava, ...) */

  private val wordsForNum: Map[String, List[String]] = 

    (words groupBy wordCode) withDefaultValue List()



  /** Return all ways to encode a number as a list of words */

  def encode(number: String): Set[List[String]] =

    if (number.isEmpty)

      Set(List())

    else {

      for {

        splitPoint <- 1 to number.length

        word <- wordsForNum(number take splitPoint)

        rest <- encode(number drop splitPoint)

      } yield word :: rest

    }.toSet



  /** Maps a number to a list of all word phrases that can represent it */

  def translate(number: String): Set[String] = encode(number) map (_ mkString " ")

}

Edit2 - 较小的代码段

  val mnemonics = Map(
    '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
    '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")

  val charCode: Map[Char, Char] =
    for ((digit, str) <- mnemonics; letter <- str) yield (letter -> digit)

  def wordCode(word: String): String = word.toUpperCase map charCode

  val words = List("Hello", "1111") // doesn't work

  (words groupBy wordCode) withDefaultValue List()

  val words2 = List("Hello", "Odersky")

  (words2 groupBy wordCode) withDefaultValue List() //works

我刚刚注意到，似乎方法/函数wordCode不会接受任何不在Map中的字符。如果是这种情况，那意味着withDefaultValue是没用的吗？如果是这样，那么我猜所呈现的代码有一点缺陷？

Answer 1

您的最后通知是正确的：只要某个字符不在地图中，函数map charCode的{{1}}部分就会失败，从而引发异常。因此，您无法访问wordCode。

如果存在缺陷，则取决于角色不在地图中时的行为。

似乎无法使用DefaultValue工作

1 个答案: