Scalaの高次関数

def main(args: Array[String]){
    
    (1 to 9).map("*" * _).foreach(println _)
    (1 to 9).filter(_ % 2 == 0) .foreach(println)
    println((1 to 9).reduceLeft(_ * _))
    "Spark is the most exciting thing happening in big data today".split(" ").
        sortWith(_.length < _.length).foreach(println)
    
    val fun = ceil _ 
    val num = 3.14
    println(fun(num) ) 
   Array(3.14, 1.42, 2.0).map(fun).foreach(println)
    
    val triple = (x: Double) => 3 * x 
    Array(3.14, 1.42, 2.0).map((x: Double) => 3 * x)
    Array(3.14, 1.42, 2.0).map{ (x: Double) => 3 * x }
    
    def high_order_functions(f: (Double) => Double) = f(0.25) //    high_order_functions，        f，f    Double，     Double，       f(0.25)
    println(high_order_functions(ceil _)) //   ceil(0.25)
    println(high_order_functions(sqrt _)) //   sqrt(0.25)
    
    def mulBy(factor: Double) = (x: Double) => factor * x //    mulBy，   x
    val quintuple = mulBy(5)
    println(quintuple(20)) 
    
    println(high_order_functions((x: Double) => 3 * x)) 
    high_order_functions((x) => 3 * x)
    high_order_functions(x => 3 * x)
    
    println(high_order_functions(3 * _))
    
    val fun2 = 3 * (_: Double) 
    val fun3: (Double) => Double = 3 * _ 
  }