scala笔记第九天（模式匹配）

引言

模式匹配是Scala中非常有特色，非常强大的一种功能。模式匹配，其实类似于Java中的swich case语法，即对一个值进行条件判断，然后针对不同的条件，进行不同的处理。

但是Scala的模式匹配的功能比Java的swich case语法的功能要强大地多，Java的swich case语法只能对值进行匹配。但是Scala的模式匹配除了可以对值进行匹配之外，还可以对类型进行匹配、对Array和List的元素情况进行匹配、对case class进行匹配、甚至对有值或没值（Option）进行匹配。

而且对于Spark来说，Scala的模式匹配功能也是极其重要的，在spark源码中大量地使用了模式匹配功能。因此为了更好地编写Scala程序，并且更加通畅地看懂Spark的源码，学好模式匹配都是非常重要的。

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。Scala的match case与Java的switch case最大的不同点在于，Java的switch case仅能匹配变量的值，比1、2、3等；而Scala的match case可以匹配各种情况，比如变量的类型、集合的元素、有值或无值。match case的语法如下：变量 match { case 值 => 代码 }。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。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。（与Java不同，java的switch case需要用break阻止）

class Moshi {
  def judgGrade(grade :String): Unit ={
    grade match {
      case "A" => println("very good")
      case "B" => println("good")
      case "C" => println("just so so")
      case _ => println("you need work harder")
    }
  }
}


object TextDemo {
  def main(args: Array[String]): Unit = {
    //************16********1******模式匹配
    val a = new Moshi
    a.judgGrade("r") //you need work harder
  }

}

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。

class Demo_16_2 {
  def judgeGrad(name :String, grad:String): Unit ={
    grad match {
      case "A" => println(name + "you are excellent")
      case "B" => println(name + "you are good")
      case  "C" => println(name + "yu are just so so")
      case _ if name == "leo" => println(name + "you are a good boy")
      case _ => println("you need to work harder")
    }
  }
}


object TextDemo {
  def main(args: Array[String]): Unit = {
    //************16********2****** 在模式匹配中使用if守卫
    val a = new Demo_16_2
    a.judgeGrad("leo","E") //leoyou are a good boy
    a.judgeGrad("tom","B") //tomyou are good
    a.judgeGrad("tom","E") //you need to work harder
  }
}

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将下划线，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。
但是对于下划线_这种情况，所有不满足前面的case的值，都会进入_这种默认情况进行处理，此时如果我们在处理语句中需要拿到具体的值进行处理呢？那就需要使用这种在模式匹配中进行变量赋值的语法！！

class Demo_16_3 {
  def judgeGrad(name :String, grad:String): Unit ={
    grad match {
      case "A" => println(name + "you are excellent")
      case "B" => println(name + "you are good")
      case  "C" => println(name + "yu are just so so")
      case baa => println("you grad is " + baa + " work harder")
    }
  }
}


object TextDemo {
  def main(args: Array[String]): Unit = {
    //************16********3****** 在模式匹配中使用if守卫
    val a = new Demo_16_3
	a.judgeGrad("tom","E")  //you grad is E work harder

  }
}

对类型的匹配

object TextDemo {
  def main(args: Array[String]): Unit = {

  def processException(e : Exception): Unit ={
    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!" )
  }
}
    processException(new IllegalArgumentException)  //you have illegal arguments! exception is: java.lang.IllegalArgumentException
    processException(new NullPointerException) //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。 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。

Array

object TextDemo {
  def main(args: Array[String]): Unit = {
    //************16********5******对Array和List进行模式匹配
      def greeding_16_5(arr :Array[String]): Unit ={
      arr match {
        case Array("leo") => println("Hi , leo")
        case Array(boy1,boy2,boy3) => println("Hi ,boy,nice to meet you. " + "  and "+boy1+" and "+boy2+" and "+boy3)
        case Array("leo",_*) => println("Hi, Leo, please introduce your friends to me." + arr.foreach(println(_)))
        case _a => println("hey, who are you?" + _a.foreach(println(_)))

      }
    }
//      val a = Array("leo")
//      greeding_16_5(a)   //Hi , leo
//      val b = Array("tom")
//      greeding_16_5(b) //tom   hey, who are you?()
//        val c = Array("wangzhaojun","xishi","diaochan")
//        //greeding_16_5(c)  //Hi ,boy,nice to meet you.   and wangzhaojun and xishi and diaochan
//        val d = Array("wangzhaojun","xishi","diaochan","yangyuhuan")
//        greeding_16_5(d)
    //wangzhaojun
    //xishi
    //diaochan
    //yangyuhuan
    //hey, who are you?()

List

object TextDemo {
  def main(args: Array[String]): Unit = {
    //************16********5******对Array和List进行模式匹配
    def greeting_15_5_2(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." + list.foreach(println(_)))
        case _ => println("hey, who are you?")
      }
    }


//        val l = List("Leo")
//        greeting_15_5_2(l)  //Hi, Leo!
          val f = List("Leo","zhangfei","kangkang")
          //greeting_15_5_2(f)   //Hi, girls, nice to meet you. Leo and zhangfei and kangkang
          val f2 = List("Leo","ton","tom","joker","tyler","join")
          greeting_15_5_2(f2)
    //Leo
    //ton
    //tom
    //joker
    //tyler
    //join
    //Hi, Leo, please introduce your friends to me.()

  }
}

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。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。

object Demo2 {
  val grades = Map("Leo" -> "A", "Jack" -> "B", "Jen" -> "C")

  def getGrade_16_7(name: String) {
    val grade = 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")
    }


    }
  def main(args: Array[String]): Unit = {
    //-------16----------7--Option与模式匹配
    getGrade_16_7("Jack") //your grade is B
    //getGrade_16_7("Tom")  //Sorry, your grade information is not in the system
  }
}