scala函数和闭包

Function and closure

//1. method
def processFile(filename: String, width: Int): Unit = {

  val source = Source.fromFile(filename)
  for( line <- source.getLines())
    processLine(filename, width, line)
}
def processLine(filename : String,width: Int, line:String): Unit ={
  if(line.length > width)
    println(filename +" : "+ line.trim)
}

//2. local function
def processFile(filename: String, width: Int): Unit = {

  def processLine(line:String): Unit ={
    if(line.length > width)
      println(filename +" : "+ line.trim)
  }

  val source = Source.fromFile(filename)
  for( line <- source.getLines())
    processLine(line)
}

//3.  函数
/**
 * function and closure
 *
 * scala 的函数时头等公民
 * 1. 不仅仅是 函数可以定义和调用
 * 2. 函数可以写为一个 函数字面量 并且作为值进行传递
 *
 * 函数字面量 在编译阶段会被编译为一个类，在运行阶段会被初始化为一个函数值
 * 区别：
 *  函数字面量 存在于源码中
 *  函数值： 作为一个对象存在于运行时
 *
 */

object FunDemo {

  def main(args: Array[String]): Unit = {
    //函数字面量的定义
    (x: Int) => x+1
  }
}

//函数字面量定义
scala> var inc = (x : Int) => x+2
inc: Int => Int = $$Lambda$776/733224796@24c685e7
//使用
scala> inc(3)
res0: Int = 5

//函数字面量的定义
 val inc = (x: Int) => x + 1
 println(inc(2))
 
 //案例
 val someNumbers = List(-11, -10, -5, 0, 5, 10)
 someNumbers.foreach((x: Int) => print(x+" ")) //-11 -10 -5 0 5 10
 val gt = someNumbers.filter((x: Int) => x > 3)
 println(gt) //List(5, 10)
 
 //化简
 someNumbers.foreach(x => print(x+" "))
 val gt2 = someNumbers.filter(x => x > 3) // List(5, 10)
 
 // PLACEHOLDER  SYNTAX
 someNumbers.foreach( _ => print _)
 someNumbers.foreach(print _)
 someNumbers.foreach(print)
 val gt3 = someNumbers.filter( _ > 3)
 
 //案例
 val f = (_: Int) + (_: Int)
 println(f(1,2))

object FunDemo {

  def main(args: Array[String]): Unit = {

    //案例
    val someNumbers = List(-11, -10, -5, 0, 5, 10)
    //someNumbers.foreach(print _) //print _ 其实是一个部分应用函数

    val res = sum _
    val b = res(1, _: Int, 3)
    println(b(2)) // 这里实际调用b.apply, 进而调用了sum(1,2,3)

    //闭包
    val more = 1
    val addMore = (x: Int) => x + more

    println(addMore(3))


    //可变参数
    def fun(arr: String*): Unit = {
      arr.foreach(println)
    }

    fun("a", "b", "c")

    val arr = Array("aa", "bb", "cc")
    fun(arr: _*)

    //带名字参数
    def fun02(time: Int, s: Float): Float = {
      s / time
    }

    println(fun02(s = 100.0F, time = 10)) //10.0

    //缺省参数
    def fun03(out: java.io.PrintStream = Console.out): Unit = {
      out.println("time "+ System.currentTimeMillis())
    }

    fun03()
  }

  def sum(a: Int, b: Int, c: Int): Int = {
    a + b + c
  }
}