Match Expression

Match 可以幫我們省略寫很多 if else，而且可以比對不同型態．
scala 的 match 不用寫 break 只會執行符合的那一段，然後 default 是用 _ :

scala> def getNumber(num:Int) = { num match {
     |  case 1 => "one"
     |  case 2 => "two"
     |  case 3 => "three"
     |  case _ => ""
     | }
     |}
getNumber: (num: Int)String

scala> getNumber(2)
res83: String = two

要判斷符合多個條件時使用 | :

scala> def getNumber(num:Int) = { num match {
     |  case 1 | 3 | 5 => "odd"
     |  case 2 | 4 | 6 => "evens"
     |  case _ => "null"
     |  }
     | }
getNumber: (num: Int)String

scala> getNumber(6)
res86: String = evens

scala> getNumber(3)
res87: String = odd

將 match 的結果指定給變數 :

scala> var i = 6
i: Int = 6

scala> val month = i match {
     | case 1 => "January"
     | case 2 => "February"
     | case 3 => "March"
     | case 4 => "April"
     | case 5 => "May"
     | case 6 => "June"
     | case 7 => "July"
     | case 8 => "August"
     | case 9 => "September"
     | case 10 => "October"
     | case 11 => "November"
     | case 12 => "December"
     | case _ => "Invalid month"
     | }
month: String = June

scala> println(month)
June

match 還可以比對不同的型態非常強大 :

scala> def getMultiType(x: Any) = x match {
     |  case true => "truth"
     |  case 'a' | 'A' => "this is a"
     |  case x :: xs => "List"
     |  case Nil => "empty List"
     |  case _ => "null"
     | }
getMultiType: (x: Any)String

scala> getMultiType(List())
res88: String = empty List

scala> getMultiType(List(1,2,3))
res89: String = List

scala> getMultiType(true)
res90: String = truth

• Any 是 scala 所有型態的父類別，底下又分為兩個子類別 AnyVal 和 AnyRef．

再來看更多例子，可以將 Any 對應到的型態，並取得裡面的值 :

scala> def getMultiMatchType(x: Any): String = x match {
     |   case List(0, _, _) => "List has three element first is 0 "
     |   case List(1, _*) => "List element size is dynamic "
     |   case (a, b) => s"Tuple2 $a and $b"
     |   case (a, b, c) => s"Tuple2 $a, $b, and $c"
     |   case s: String => s"this is string: $s"
     |   case i: Int => s"this is int: $i"
     |   case f: Float => s"this is float: $f"
     |   case a: Array[Int] => s"an array of int: ${a.mkString(",")}"
     |   case as: Array[String] => s"an array of strings: ${as.mkString(",")}"
     |   case list: List[_] => s"this the List: $list" case m: Map[_, _] => m.toString
     |   case _ => ""
     | }
getMultiMatchType: (x: Any)String


scala> getMultiMatchType(List(0,2,3))
res91: String = "List has three element first is 0 "

scala> getMultiMatchType(List(1,2,3))
res92: String = "List element size is dynamic "

scala> getMultiMatchType(List(1,2,3,4))
res93: String = "List element size is dynamic "

scala> getMultiMatchType(List(0,1,2,3))
res94: String = this the List: List(0, 1, 2, 3)

scala> getMultiMatchType((1,2))
res1: String = Tuple2 1 and 2

scala> getMultiMatchType((0,1,2))
res2: String = Tuple2 0, 1, and 2

scala> getMultiMatchType(1)
res3: String = this is int: 1

scala> getMultiMatchType(Array(1,2,3))
res4: String = an array of int: 1,2,3

scala> getMultiMatchType(Array("1","2","3"))
res5: String = an array of strings: 1,2,3

總結

• scala 的 Match Expression 可以比對值及型態非常強大．
• scala 的 Any 是 scala 所有型態的父類別，底下又分為兩個子類別 AnyVal 和 AnyRef．