Coursera Scala 4-6：模型匹配

Coursera Scala 4-6：模型匹配

匹配值

val times = 1

times match {
  case 1 => "one"
  case 2 => "two"
  case _ => "some other number"
}

 List(('a',1), ('b',2), ('a',1)) match {
        case Nil => println("null")
        case (C, n) :: ps => (C, n+1) :: ps   得到List(('a',2), ('b',2), ('a',1))
        case p :: ps => p            得到('a',1)
        case p :: ps => ps            得到List(('b',2), ('a',1))

      }
  }

匹配類型

def bigger(o: Any): Any = {
  o match {
    case i: Int if i < 0 => i - 1
    case i: Int => i + 1
    case d: Double if d < 0.0 => d - 0.1
    case d: Double => d + 0.1
    case text: String => text + "s"
  }
}

匹配類成員

def calcType(calc: Calculator) = calc match {
  case _ if calc.brand == "hp" && calc.model == "20B" => "financial"
  case _ if calc.brand == "hp" && calc.model == "48G" => "scientific"
  case _ if calc.brand == "hp" && calc.model == "30B" => "business"
  case _ => "unknown"
}

樣本類Case Class

使用樣本類可以方便得存儲和匹配類的內容。你不用new關鍵字就可以創建它們

scala> case class Calculator(brand: String, model: String)
defined class Calculator

scala> val hp20b = Calculator("hp", "20b")
hp20b: Calculator = Calculator(hp,20b)

使用樣本類進行模式匹配:

e match{
   case pattern => expr 
}

在沒有匹配項時拋出 MatchError

Pattern可以是:

• 小寫字母開頭表示變量(保留字除外) 用於綁定值到這個變量
• 大寫開頭表示常量 兩者是否相等來進行匹配(==)

構造器pattern C(p1,....,pn) 匹配所有C類型(或子類型)通過p1...p構造的
(注意類的定義要用case class)
同樣的變量隻能在Pattern出現一次

順序匹配下去

 trait Expr
 case class Number(n:Int) extends Expr
 case class Sum(e1:Expr,e2:Expr) extends Expr

 object exprs {
     def show(e:Expr): String = e match {
         case Number(x) => x.toString
         case Sum(l.r) => show(l)+"+"+show(r)
     }
 }
 show(Sum(Number(1),Number(44)))               > res0:String=1+44

---

例子:

  trait Expr {  
  def isNumber: Boolean  
  def isSum: Boolean  
  def numValue: Int  
  def leftOp: Expr  
  def rightOp: Expr  
  def show():String = this match{  
      case Number(n) => n+""  
      case Sum(e1,e2) => e1+"+"+e2  
  }  
}

另一個例子:

Scala代碼  
def show(e: Expr): String = e match {  
   case Sum(Prod(x1, x2), Prod(x3, x4)) =>  
     if (x1 == x3) show(x1) + "*" + "(" + show(x2) + "+" + show(x4) + ")"  
     else if (x1 == x4) show(x1) + "*" + "(" + show(x2) + "+" + show(x3) + ")"  
     else if (x2 == x3) show(x2) + "*" + "(" + show(x1) + "+" + show(x4) + ")"  
     else if (x2 == x4) show(x2) + "*" + "(" + show(x1) + "+" + show(x3) + ")"  
     else show(_Prod(x1, x2)) + "+" + show(_Prod(x3, x4))  
   case Sum(n1, n2) => show(n1) + "+" + show(n2)  
   case Prod(Sum(n1, n2), n3) => "(" + show(_Sum(n1, n2)) + ")" + "*" + show(n3)  
   case Prod(n3, Sum(n1, n2)) => show(n3) + "*(" + show(_Sum(n1, n2)) + ")"  
   case Prod(n1, n2) => show(n1) + "*" + show(n2)  
   case _ => e.toString()  
 }

Reference

https://twitter.github.io/scala_school/zh_cn/basics2.html
https://hongjiang.info/scala-pattern-matching-1/