S99 Problem 16

S9916.scala

/*
 *  S99 P16 http://aperiodic.net/phil/scala/s-99/
 *  (**) Drop every Nth element from a list.
 *
 *  Example:
 *
 * scala> drop(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
 * res0: List[Symbol] = List('a, 'b, 'd, 'e, 'g, 'h, 'j, 'k)
 *
 */

object S9916 extends App {

  def drop[A](n: Int, lst: List[A]): List[A] = {
    lst.foldLeft((1, List[A]())) {
      case ((aidx, alst), e) if aidx % n != 0 => (aidx + 1, e :: alst)
      case ((aidx, alst), _)                  => (aidx + 1, alst)
    }._2.reverse
  }

  val r = drop(3, List('a, 'b, 'c, 'd, 'e, 'f, 'g, 'h, 'i, 'j, 'k))
  println(r)
  assert( r == List('a, 'b, 'd, 'e, 'g, 'h, 'j, 'k) )

}