Scala:模式匹配

模式匹配

Scala是没有Java中的switch case语法的，相对应的，Scala提供了更加强大的match case语法，即模式匹配，类替代switch case，match case也被称为模式匹配。

match case的语法如下：变量 match { case 值 => 代码 }。此外，match case中，只要一个case分支满足并处理了，就不会继续判断下一个case分支了

def judgeGrade(grade:String){
   grade match{
     case "A" => println("Excellent")
     case "B" =>  println("Good")
     case "C" => println("Just so so")
     case _ => println("you need work harder")
   }
}

如果值为下划线，则代表了不满足以上所有情况下的默认情况如何处理

在模式匹配中使用if守卫

Scala的模式匹配语法，有一个特点在于，可以在case后的条件判断中，不仅仅只是提供一个值，而是可以在值后面再加一个if守卫，进行双重过滤

def judgeGrade(name: String, grade: String) {
  grade match {
    case "A" => println(name + ", you are excellent")
    case "B" => println(name + ", you are good")
    case "C" if name ==  "Leo" => println(name + ",you are smart,come on")
    case "C" if name ==  "Jack" => println(name + ",your parents are very hard,please come on")
    case "C" => println(name + ",your grade is just so so")
    case _ => println(name + ",you need to work harder")
  }
}

在模式匹配中进行变量赋值

Scala的模式匹配语法，有一个特点在于，可以将模式匹配的默认情况，下划线，替换为一个变量名，此时模式匹配语法就会将要匹配的值赋值给这个变量，从而可以在后面的处理语句中使用要匹配的值

def judgeGrade(name: String, grade: String) {
  grade match {
     case "A" => println(name + ", you are excellent")
     case "B" => println(name + ", you are good")
     case "C" => println(name + ", you are just so so")
     case grade if name == "Leo" => println(name + ", you are a good boy, come on, your grade is " + grade)
     case _grade => println("you need to work harder, your grade is " + _grade)
  }
}

对类型进行模式匹配

语法与匹配值其实是一样的，但是匹配类型的话，就是要用“case 变量: 类型 => 代码”这种语法

import java.io._

def processException(e:Exception){
   e match{
      case e1:IllegalArgumentException => println("you have illegal arguments! exception is: " + e1)
      case e2:FileNotFoundException => println("cannot find the file you need read or write!, exception is: " + e2)
      case e3:IOException => println("you got an error while you were doing IO operation! exception is: " + e3)
      case _:Exception => println("cannot know which exception you have!" )
   }
}

对Array和List进行模式匹配

对Array进行模式匹配，分别可以匹配带有指定元素的数组、带有指定个数元素的数组、以某元素打头的数组

def greeting(arr:Array[String]){
    arr match{
       case Array("Leo") => println("Hi,Leo")
       case Array(girl1, girl2, girl3) => println("Hi, girls, nice to meet you. " + girl1 + " and " + girl2 + " and " + girl3)
       case Array("Leo",_*) => println("Hi, Leo, please introduce your friends to me.")
       case _ => println("hey, who are you?")
    }
}

对List进行模式匹配，与Array类似，但是需要使用List特有的::操作符（可以用于将head和tail合并成一个List）

def greeting(list: List[String]) {
  list match {
    case "Leo" :: Nil => println("Hi, Leo!")
    case girl1 :: girl2 :: girl3 :: Nil => println("Hi, girls, nice to meet you. " + girl1 + " and " + girl2 + " and " + girl3)
    case "Leo" :: tail => println("Hi, Leo, please introduce your friends to me.")
    case _ => println("hey, who are you?")
  }
}

case class与模式匹配

Scala中提供了一种特殊的类，用case class进行声明，中文也可以称作样例类。case class其实有点类似于Java中的JavaBean的概念。即只定义field，并且由Scala编译时自动提供getter和setter方法，但是没有method。

case class的主构造函数接收的参数通常不需要使用var或val修饰，Scala自动就会使用val修饰（但是如果你自己使用var修饰，那么还是会按照var来）

class Person
case class Teacher(name: String, subject: String) extends Person
case class Student(name: String, classroom: String) extends Person

Scala自动为case class定义了伴生对象，也就是object，并且定义了apply()方法，该方法接收主构造函数中相同的参数，并返回case class对象

def judgeIdentify(p: Person) {
  p match {
    case Teacher(name, subject) => println("Teacher, name is " + name + ", subject is " + subject)
    case Student(name, classroom) => println("Student, name is " + name + ", classroom is " + classroom)
    case _ => println("Illegal access, please go out of the school!")
  }  
}

Option与模式匹配

Scala有一种特殊的类型，叫做Option。Option有两种值，一种是Some，表示有值，一种是None，表示没有值。

val grades = Map("Leo" -> "A","Jack" -> "B","Jen" -> "C")

def getGreate(name:String){
    var grade:Option[String] = grades.get(name)
    grade match {
      case Some(grade) => println("your grade is " + grade)
      case None => println("Sorry, your grade information is not in the system")
    }
}

Option通常会用于模式匹配中，用于判断某个变量是有值还是没有值，这比null来的更加简洁明了