Frequently used collection methods in Scala

object Test {
  // copy elements from a list to an array
  val x = new Array[Int](5)
  val y = List(1, 3, 5)
  y.copyToArray(x, 0, 2)
  x
  // does it have definite size?
  x.hasDefiniteSize
  y.hasDefiniteSize

  implicit class ListSafeTailInit[A](val list: List[A]) {
    def tailOption: Option[List[A]] = {
      if (list.isEmpty) None else Some(list.tail)
    }

    def initOption: Option[List[A]] = {
      if (list.isEmpty) None else Some(list.init)
    }
  }


  // safe head and tail
  List[Int]().headOption.getOrElse(-9)
  List[Int]().lastOption.getOrElse(-9)
  // List[Int]().tail // UnsupportedOperationException
  Nil.initOption // None
  List[Int]().tailOption // None

  // find first elem that satisfies predicate
  List(1, 2, 3).find(_ > 1) // Some(2)
  List(1, 2, 3, 4, 5).slice(0, 3) // from index 0 (inclusive) to 3 (exclusive)
  List(1, 2, 3, 4, 5).span(_ < 3) // combines takeWhile and dropWhile
  List(1, 2, 3, 4, 5).partition(_ % 2 == 0) // combines filter and filterNot
  List(1, 2, 3, 4, 5).groupBy(x => if(x % 2 == 0) "even" else "odd") // returns a map

  // string methods
  val sb = new StringBuilder()
  List(1, 2, 3).addString(sb, "'", ",", "'")
  List(4, 5, 6).addString(sb, "'", ",", "'")
  sb.toString() // '1,2,3''4,5,6'
  List(1, 2, 3).mkString("'", ",", "'") // '1,2,3'
  List(1, 2, 3).stringPrefix // List

  def time[R](block: => R): R = {
    val t0 = System.nanoTime()
    val result = block    // call-by-name
    val t1 = System.nanoTime()
    println("Elapsed time: " + (t1 - t0)/1e9 + "s")
    result
  }

  // collection views
  // A view on a collection makes all transformations lazy, which makes it possible to combine multiple transformations into one.
  // The non-lazy (eager) version:
  time {
    (1 to 1e7.toInt).map(_ + 2).map(x => {
      if(x % 2 != 0) -x else x
    }).sum
  }
  // The lazy version using a view
  time {
    (1 to 1e7.toInt).view.map(_ + 2).map(x => {
      if(x % 2 != 0) -x else x
    }).force.sum
  }

}