标签：style c class blog code java
Scala中的match, 比起以往使用的switch-case有著更強大的功能,
1. 傳統方法
def toYesOrNo(choice: Int): String = choice match { case 1 => "yes" case 0 => "no" case _ => "error" } // toYesOrNo(1)=>"yes" // toYesOrNo(0)=>"no" // toYesOrNo(33)=>"error"
佔位符“_”表示預設的情形, 若不想使用"_"也可以寫成以下的形式,
def toYesOrNo(choice: Int): String = choice match { case 1 => "yes" case 0 => "no" case whaterver => "whaterver " }
2. 類型模式
可以使用保留字match來判斷類型
def f(x: Any): String = x match { case i:Int => "integer: " + i case _:Double => "a double" case s:String => "I want to say " + s } // f(1) → “integer: 1″Typ // f(1.0) → “a double” // f(“hello”) → “I want to say hello”
3. Functional approach to pattern matching
以下是一個Factorial的傳統遞迴方法
def fact(n: Int): Int =
if (n == 0) 1
else n * fact(n - 1)改以pattern matching來實現, 又會如何呢??
def fact(n: Int): Int = n match { case 0 => 1 case n => n * fact(n - 1) }
4. 模式匹配與集合
來試試一個集合加總的遞迴實現, 我們可能會寫出以下的代碼
def length[A](list : List[A]) : Int = { if (list.isEmpty) 0 else 1 + length(list.tail) }
def length[A](list : List[A]) : Int = list match { case _ :: tail => 1 + length(tail) case Nil => 0 }
"Nil"代表集合為空時,
"_::tailX" 應該被理解成, “a list with whatever head followed by a tail.”我的解釋是能將一個list拆分為list.head與list.tail
接著我們可以來看看多個參數的pattern matching要怎麼做呢??
def parseArgument(arg : String, value: Any) = (arg, value) match { case ("-l", lang) => setLanguageTo(lang) case ("-o" | "--optim", n : Int) if ((0 < n) && (n <= 5)) => setOptimizationLevelTo(n) case ("-o" | "--optim", badLevel) => badOptimizationLevel(badLevel) case ("-h" | "--help", null) => displayHelp() case bad => badArgument(bad) }
在pattern中還有最重要的case class, 下一篇繼續介紹....
參考資料:
Playing with Scala’s pattern matching
[Scala] Pattern Matching(模式匹配),布布扣,bubuko.com
[Scala] Pattern Matching(模式匹配)
标签：style c class blog code java
原文地址：http://www.cnblogs.com/YangMark/p/3746108.html
