_在scala语言中的使用

_在scala中使用的非常巧妙，有时候也不好理解，现在总结如下：

1._可以用来指定foreach循环体中当前循环变量的值。

 val s = 1.to(10).map({ _ * 2 })

 println(s)

2.作为偏函数的参数占位符使用：

 val sum = (a: Int, b: Int, c: Int) => a + b + c

 val f = sum(1, 2, _: Int)

 println(f(3))

3.在match表达式中使用

 def echoWhatYouGaveMe(x: Any): String = x match {

    // constant patterns

    case 0                 => "zero"

    case true              => "true"

    case "hello"           => "you said 'hello'"

    case Nil               => "an empty List"

    // sequence patterns

    case List(0, _, _)     => "a three-element list with 0 as the first element"

    case List(1, _*)       => "a list beginning with 1, having any number of elements"

    case Vector(1, _*)     => "a vector starting with 1, having any number of elements"

    // tuples

    case (a, b)            => s"got $a and $b"

    case (a, b, c)         => s"got $a, $b, and $c"

    // constructor patterns

    // typed patterns

    case s: String         => s"you gave me this string: $s"

    case i: Int            => s"thanks for the int: $i"

    case f: Float          => s"thanks for the float: $f"

    case a: Array[Int]     => s"an array of int: ${a.mkString(" , ")}"

    case as: Array[String] => s"an array of strings: ${as.mkString(" , ")}"

    

    case list: List[_]     => s"thanks for the List: $list"

    case m: Map[_, _]      => {

      m.toString

    }

    

    case Person(first, "Alexander" ) => s"found an Alexander, first name = $first"

    case Dog("Suka" ) => "found a dog named Suka"

    

    // the default wildcard pattern

    case _                 => "Unknown"

  }

 

case list: List[_] => s"thanks for the List: $list"
case m: Map[_, _] => m. toString

_ 表示具体存什么值我不关心，只是有值存在集合里面

 

4.为变量设置默认值

var num : Int = _

println(s"num=$num")

打印输出num=0,  _会根据数据类型选择默认值

 

5._表示导入包中的所有类型，相当于java导入中的*

import akka.actor._

import scala.concurrent.duration._