scala冷门分享

上周给同事分享一些不常用但很有用的知识点，这里把做的材料也分享出来

zipWithIndex

(1 to 10).zipWithIndex

call by Name and call By value

def something() = {
  println("calling something")
  1 // return value
}

//call by value version side-effect of the passed-in function call only hadppend once ,however call-by-name version side-effect happended twice 
//This is because call-by-value functions compute the passed-in expression's value before calling the function, thus the same value is accessed every time. However, call-by-name functions recompute the passed-in expression's value every time it is accessed.
def callByValue(x: Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

def callByName(x: => Int) = {
  println("x1=" + x)
  println("x2=" + x)
}

partition && span

val x = (1 to 10 )
x.parition( _ % 2 == 1) 
x.span( _ % 2 == 1 )

def quickSort(xs: List[Int]): List[Int] = xs match {
  case Nil => Nil
  case head :: tail =>
    val (before, after) = tail.partition(_ < head)
    quickSort(before) ++ (head :: quickSort(after))
}

tuppled

def mult(d1:Double,d2:Double,d3:Double) = d1 * d2 * d3

val x = (2.2,2.3,2.4)
mult(x._1,x._2,x._3)
//cool
val multTupled = Function.tupled(mult _)
multTupled(x)
val multUntupled = Function.untupled(multTupled)

grouped

val x = (1 to 10 )
 x.grouped(2).toList

fold

(0 /: x) (_ + _) //
x.foldLeft(0)(_ + _)

  implicit def getHadoopConf(resources: Seq[String]): Configuration = {

    val config = resources.foldLeft(new Configuration()) {
      case (conf, path) ⇒ conf.addResource(new Path(path))
        conf
    }
    config

  }

make && fill

List.make(2,"10")
List.fill(2)("10")
List.tabulate(5)(_ * 2 )

mkString

x.mkString("[","$$","]")

transpose

 val x = Seq(Seq(1,2,3),Seq(4,5,6))
xs.transpose

implicit

implicit val x = "a"
implicitly[String]
implicitly[Int]