首页 > GAME > 游戏 > 正文

抚州颗星略新能源有限公司,厦门恐庸市场营销有限公司,防城港付捣沧健身服务中心

在钢铁海龙兽要对其他孩子发动攻击的时候战斗暴龙兽以极快的速度冲向钢铁海龙兽,一撞之下,大家都是数码合金,大家的力量都十分的强横,虽然战斗暴龙兽有前冲的优势,但是也仅仅只是能做到将钢铁海龙兽的攻击打断,将他巨大的身体击退而已。

网络兼职赚钱2628

当泰坦听到唐三说唐昊自残两肢归还魂骨时,不禁老泪纵横,情绪极为激动。
他的**强度比不上天狗忍。被飞天大侠一头锤撼击中。就倒的不起了……至于猩猩理查德也是一样。他扑过来时。被飞天大侠一捉住。就完全失去战斗力。表情跟肉霸王卡拉奇诺同样恐惧无比。最后让飞天大侠放倒在的。完全像一块肥肉般躺着。毫无反手之力。普通观众根本看不出来。就算是放慢镜头。也完全看不出来。大家看见飞天大侠的华丽攻击。只是拼命的鼓掌。拼命的欢呼。

“老李,那千佛洞真的有一千尊佛像吗?”白杨这时忍不住好奇打断李文道。

Scala入门系列(十一):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。

类似于Java中的switch case语法,但是模式匹配的功能要比它强大得多,switch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

而且在Spark源码中也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源码,学好模式匹配是非常重要的。

基础语法

match case的语法如下:变量 match { case 值 => 代码}。如果值为下划线,则代表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。(这一点与java不同,java的switch case需要用break阻止)

对变量值进行匹配

// 案例:成绩评价
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")
}
}
judgeGrade: (grade: String)Unit
scala> judgeGrade("E")
you need work harder
scala> judgeGrade("B")
Good

在模式匹配中使用if守卫

// 案例:成绩评价(升级版)
def judgeGrade(name: String, grade: String){
grade match {
case "A" => println("Excellent")
case "B" => println("Good")
case "C" => println("Just so so")
case _ if name == "leo" => println(name + ", you are a good boy, come on")
case _ => println("you need to work harder")
}
}
judgeGrade: (name: String, grade: String)Unit
scala> judgeGrade("leo", "E")
leo, you are a good boy, come on
scala> judgeGrade("sparks", "E")
you need to work harder

在模式匹配中进行变量赋值

模式匹配中可以将默认情况即下划线替换为一个变量名,这样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:成绩评价(升级版)
def judgeGrade(grade: String){
grade match {
case "A" => println("you got A grade, excellent!")
case "B" => println("you got B grade, good")
case "C" => println("you got C grade, so so")
case badGrade => println("you got " + badGrade + "grade, i hope that you can get C next time")
}
}
judgeGrade: (grade: String)Unit
scala> judgeGrade("E")
you got Egrade, i hope that you can get C next time
scala> judgeGrade("F")
you got Fgrade, i hope that you can get C next time

类型匹配

Scala的模式匹配强大之处就在于可以直接匹配类型而不仅仅是值,这也是Java中switch case绝对做不到的。

匹配类型的语法为case 变量 : 类型 => 代码

// 案例:异常处理
import java.io._
def processException(e: Exception){
e match {
case e1:IllegalArgumentException => println("you passed illegal argument, exception is : " +e1)
case e2:IOException => println("you got an error while doing IO operation!" + e2)
case _:Exception => println("cannot know which exception you have")
}
}
processException: (e: Exception)Unit
// 测试能否匹配类型
scala> processException(new IllegalArgumentException("expect two arguments, but found only one"))
you passed illegal argument, exception is : java.lang.IllegalArgumentException: expect two arguments
, but found only one
scala> processException(new ArrayIndexOutOfBoundsException("array is null."))
cannot know which exception you have

对Array和List的元素进行匹配

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, may I know you name? "+girl1+" "+girl2+" "+
girl3)
case Array("Leo", _*) => println("Hi, leo, please introduce your friends to me.")
case _ => println("hey, who are you?")
}
}
greeting: (arr: Array[String])Unit
// 测试
scala> greeting(Array("Leo"))
Hi, Leo!
scala> greeting(Array("jen", "Alice", "lory"))
Hi, girls, may I know you name? jen Alice lory
scala> greeting(Array("sparks"))
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。

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 + " " + gir
l2 + " " + girl3)
case "Leo" :: tail => println("Hi, Leo, please introduce your friends to me.")
case _ => println("hey, who are you?")
}
}
greeting: (list: List[String])Unit
// 测试
scala> greeting(List("Leo"))
Hi, Leo!
scala> greeting(List("Marry", "Alice", "lory"))
Hi, girls, nice to meet you.Marry Alice lory
scala> greeting(List("Sparks"))
hey, who are you?

case class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val或var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来定义)

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。

// 定义case class
class Person
case class Teacher(name: String, subject: String) extends Person
case class Student(name: String, classroom: String) extends Person
case class Worker(name: String) extends Person
// 案例:学校门禁
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 " + classro
om)
case _ => println("Illegal access, please go out of the school")
}
}
defined class Person
defined class Teacher
defined class Student
defined class Worker
judgeIdentify: (p: Person)Unit
// 测试匹配效果
scala> val leo: Person = Student("leo", "class1")
leo: Person = Student(leo,class1)
scala> val tom: Person = Teacher("tom", "Math")
tom: Person = Teacher(tom,Math)
scala> val jack: Person = Worker("jack")
jack: Person = Worker(jack)
scala> judgeIdentify(leo)
Student, name isleo, classroom is class1
scala> judgeIdentify(tom)
Teacher, name is tom,subject is Math
scala> judgeIdentify(jack)
Illegal access, please go out of the school

Option匹配

Scala中有一种特殊的类型,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。

Spark源码中大量地使用了Option,比如Some(a)、None这种语法。

// 案例:成绩查询
val grades = Map("Leo" -> "A", "Jack" -> "B", "Sparks" -> "C")
def getGrade(name: String) {
声明:所发布的内容均来源于互联网,目的在于传递信息,但不代表本站赞同其观点及立场,版权归属原作者,如有侵权请联系删除。
国内知名的网络赚钱平台

赚钱宝

下班在家兼职做什么好