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scala--match模式匹配

程序员文章站 2022-06-14 17:14:31
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scala中模式匹配,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。

 

1、基础语法--对变量的匹配:(案例:成绩评价)

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,在每个case语句最后,不需要break,break是隐含的

案例:

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

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

此案例是如果不满足"A","B",再进行if判断,因为模式匹配是从上往下进行的

 

案例:

def judgeGrade(name: String, grade: String) {
  grade match {
    case "A" => println(name + ", you are excellent")
    case "B" => println(name + ", you are good")
    case _  if name == "leo" => println(name + ", you are a good boy, come on")
    case _ => println("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

案例:

def judgeGrade(name: String, grade: String) {
  grade match {
    case "A" => println(name + ", you are excellent")
    case "B" => println(name + ", you are good")
    case grade if name == "leo" => println(name + ", you are a good boy, come on, your grade is " + grade)
    case grade  => println(name + ", you should study, your grade is " + grade)
  }
}

2、对类型进行模式匹配(案例:异常处理)

match可以直接匹配类型,这点是java的switch case绝对做不到的。

案例:

import java.io._
def processException(e: Exception) {
  e match {
    case e1: IllegalArgumentException => println("you have illegal arguments! exception is: " + e1)
    case e2: IOException => println("you got an IOError! exception is: " + e3)
    case _: Exception => println("cannot know which exception you have!" )
  }
}

3、对Array和List的元素进行模式匹配(案例:对朋友打招呼)

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

四种情况匹配下面四种结果:greeting("Leo") , greeting("marry","lucy","rise"), greeting("Leo","tom"), greeting("jack")

案例:

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

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,这个操作符的含义就是连接list的元素,连接Nil就是空,说明后面没有值。tail表示以"Leo"

开头,后面的都是"尾"

案例:

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?")
  }
}

4、case class与模式匹配(案例:学校门禁)

Scala中提供了一种特殊的类,用case class进行声明,也可以称作样例类。它类似于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默认val修饰。自己也可以用var定义。同时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

案例:

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

当传入老师可学生的对象的时候是可以匹配到相应情况的,但假如你定义了Worker类继承Person,就是最后一种情况了

 

只要在声明类时 在class关键字前加上case关键字 这个类就成为了样例类

样例类和普通的区别在于:

(1)默认实现序列化接口

(2)默认自动覆盖 toString equals hashCode方法

(3)不需要new可以直接生成对象

(4)支持模式匹配

object TermTest extends scala.App {
  def printTerm(term: Term) {
    term match {
      case Var(n) =>
        print(n)
      case Fun(x, b) =>
        print("^" + x + ".")
        printTerm(b)
      case App(f, v) =>
        print("(")
        printTerm(f)
        print(" ")
        printTerm(v)
        print(")")
    }
  }
  def isIdentityFun(term: Term): Boolean = term match {
    case Fun(x, Var(y)) if x == y => true
    case _ => false
  }
  val id = Fun("x", Var("x"))
  val t = Fun("x", Fun("y", App(Var("x"), Var("y"))))
  printTerm(t)
  println
  println(isIdentityFun(id))
  println(isIdentityFun(t))
}

5、Option与模式匹配(案例:成绩查询)

Option有两种值,一种是Some,表示有值,一种是None,表示没有值。它用于判断某个变量是有值还是没有值

比如如果按照学号查找学生有成绩就输出成绩(grade有值),如果没有grade值,就代表None

案例:

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