S99 P19

S99P19.scala

/*
 *  S99 P19  http://aperiodic.net/phil/scala/s-99/
 *  
 *  (**) Rotate a list N places to the left.
 *  
 *  Examples:
 *
 *   scala> rotate(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
 *   res0: List[Symbol] = List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'a, 'b, 'c)
 *
 *   scala> rotate(-2, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
 *   res1: List[Symbol] = List('j, 'k, 'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i)
 *   
 *   D. Holbrook
 */
object S9919 extends App {
  
  def rotate[A](n: Int, l: List[A]) : List[A] = n match {
    case n if n >= 0 => (l drop n) ::: (l take n)
    case n           => (l drop (l.size + n)) ::: (l take l.size + n)
  }
  
  val r1 = rotate(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
  println(r1)
  assert(r1 == List('d, 'e, 'f, 'g, 'h, 'i, 'j, 'k, 'a, 'b, 'c))

  val r2 = rotate(-2, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
  println(r2)
  assert(r2 ==  List('j, 'k, 'a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i))
  
}