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我正在做一个代码审查电话簿助记符马丁Odersky博士mentioned over at Skillmatters。似乎无法使用默认值工作
这里是什么，他有一个片段：
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()
我试图声明一切，函数和变量怎么看withDefaultValue的行为，这是我得到了什么：
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$class.default(MapLike.scala:224)
在他正在谈论的视频如果单词未被映射，那么我们应该得到一个项目列表（这从视频中的17分钟开始）。我收到一个错误？我正在使用Scala的REPL 2.9.0.1。
谢谢你的时间。
编辑：
我敢肯定，我已经产生的VAL和REPL DEF正常工作。
scala> :paste
// Entering paste mode (ctrl-D to finish)
val mnemonics = Map(
'2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
'6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")
// Exiting paste mode, now interpreting.
mnemonics: scala.collection.immutable.Map[Char,java.lang.String] = Map(8 -> TUV, 4 -> GHI, 9 -> WXYZ, 5 -> JKL, 6 -> MNO, 2 -> ABC, 7 -> PQRS, 3 -> DEF)
scala> :paste
// Entering paste mode (ctrl-D to finish)
val charCode: Map[Char, Char] =
for ((digit, str) <- mnemonics; letter <- str) yield (letter -> digit)
// Exiting paste mode, now interpreting.
charCode: Map[Char,Char] = Map(E -> 3, X -> 9, N -> 6, T -> 8, Y -> 9, J -> 5, U -> 8, F -> 3, A -> 2, M -> 6, I -> 4, G -> 4, V -> 8, Q -> 7, L -> 5, B -> 2, P -> 7, C -> 2, H -> 4, W -> 9, K -> 5, R -> 7, O -> 6, D -> 3, Z -> 9, S -> 7)
def wordCode(word: String): String = word.toUpperCase map charCode
wordCode: (word: String)String
我也尝试在REPL中定义整个类。
defined class Coder
scala> val words4 = List("hi", "Hello world", "[email protected]")
words4: List[java.lang.String] = List(hi, Hello world, [email protected])
scala> var listPhoneNumber = new Coder(words4)
java.util.NoSuchElementException: key not found:
at scala.collection.MapLike$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 = [email protected]
这里是整个代码我使用：
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 - 小片段：
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
我刚刚注意到它好像方法/函数wordCode不会采用任何不在地图中的字符。如果是这种情况，这是否意味着withDefaultValue是无用的？如果是这样，那么我认为提出的代码有一点缺陷？
当我用'：paste'命令将你的“整个代码”粘贴到REPL中时，它编译时没有错误。我正在使用2.9.1.RC3。你可以试着想出一个行为意外的小例子吗？ –
添加了一个较小的片段，谢谢。 – mythicalprogrammer
该代码有错误。他在OSCON 2011上展示的代码已更正。 –