Scala学习之三Scala_ 下划线占位符语法

Scala学习之三Scala_  下划线占位符语法：2017年8月13日12:46:11

先看例子

  * _占位符的理解：  */ 他们输出的是1234567   List(1,2,3,4,5,6,7).foreach( x=> print(x))    List(1,2,3,4,5,6,7).foreach( print(_))   List(1,2,3,4,5,6,7).foreach( print _)    List(1,2,3,4,5,6,7).foreach( print )    List(1,2,3,4,5,6,7).foreach{ print }

 println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(x=>x.split(" "))可以的

 println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_ =>_.split(" "))这个是不行的

println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_.split(" "))这个就可以

理解一：在flatMap(输入=>输出)

有了占位符_就可以不要输入了

println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(x=>x.split(" ")).map((_,1)).map(_._2).reduce((x,y)=>x+y))这个也可以 println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(x=>x.split(" ")).map((_,1)).map(_._2).reduce(_+_))可以的

 分析

reduce((x,y)=>x+y))和reduce(_+_))不要输入，分别用占位符代替

println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_.split(" ")).map(x=>(x,1)).map(_._2).reduce(_+_))println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_.split(" ")).map((_,1)).map(_._2).reduce(_+_))//map((_,1)).和map(x=>(x,1))

println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_.split(" ")).map((_,1)).map(x=>x._2).reduce(_+_))println(List[String]("I am into Spark so much  ","scala is powerful").flatMap(_.split(" ")).map((_,1)).map(_._2).reduce(_+_))map(x=>x._2)和map(_._2)

占位符

一个下划线代表了一个输入参数。   上列中   _  分别代表了1 , 2 ,3 ....
多个下划线指代多个参数：第一个下划线代表第一个参数，第二个下划线代表第二个。。。。

使用条件：每个参数在函数中最多出现一次时