日志
Scala中下划线的用途
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存在性类型：
def foo(l: List[Option[_]]) =
def f(m: M[_])
高阶类型参数：
case class A[K[_],T](a: K[T])
def f[M[_]]
临时变量：
val _ = 5
临时参数：
List(1, 2, 3) foreach { _ => println("Hi") } //List(1, 2, 3) foreach { t => println("Hi") }
通配模式：
Some(5) match { case Some(_) => println("Yes") }
match {
case List(1,_,_) => " a list with three element and the first element is 1"
case List(_*) => " a list with zero or more elements "
case Map[_,_] => " matches a map with any key type and any value type "
case _ =>
}
val (a, _) = (1, 2)
for (_ <- 1 to 10)
通配导入：
// Imports all the classes in the package matching
import scala.util.matching._
// Imports all the members of the object Fun (static import in Java).
import com.test.Fun._
隐藏导入：
// Imports all the members of the object Fun but renames Foo to Bar
import com.test.Fun.{ Foo => Bar , _ }
// Imports all the members except Foo. To exclude a member rename it to _
import com.test.Fun.{ Foo => _ , _ }
连接字母和标点符号：
def bang_!(x: Int) = 5
占位符：
( (_: Int) + (_: Int) )(2,3)
val nums = List(1,2,3,4,5,6,7,8,9,10)
nums map (_ + 2)
nums sortWith(_>_)
nums filter (_ % 2 == 0)
nums reduceLeft(_+_)
nums reduce (_ + _)
nums reduceLeft(_ max _)
nums.exists(_ > 5)
nums.takeWhile(_ < 8)
偏函数：
def fun = {
// Some code
}
val funLike = fun _
List(1, 2, 3) foreach println _
1 to 5 map (10 * _)
//List("foo", "bar", "baz").map(_.toUpperCase())
List("foo", "bar", "baz").map(n => n.toUpperCase())
初始化默认值:
var d:Double = _
var i:Int = _
参数序列：
//Range转换为List
List(1 to 5:_*)
//Range转换为Vector
Vector(1 to 5: _*)
//可变参数中
def capitalizeAll(args: String*) = {
args.map { arg =>
arg.capitalize
}
}
val arr = Array("what's", "up", "doc?")
capitalizeAll(arr: _*)
作为参数名：
//访问map
var m3 = Map((1,100), (2,200))
for(e<-m3) println(e._1 + ": " + e._2)
m3 filter (e=>e._1>1)
m3 filterKeys (_>1)
m3.map(e=>(e._1*10, e._2))
m3 map (e=>e._2)
//元组
(1,2)._2
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