P03 (*) Find the Kth element of a list.

P03.scala

package com.learner.s99

object P03 {
  def nth(n: Int, l: List[Any]): Any = {
    require(n >= 0, "n must be greater than or equal to zero")
    l match {
      case List() => None
      case head :: tail if n == 0 => head
      case head :: tail if n > 0 => nth(n - 1, tail)
    }
  }
}

P03Spec.scala

package com.learner.s99

import com.learner.s99.P03._
import org.scalatest.{MustMatchers, FlatSpec}

class P03Spec extends FlatSpec
with MustMatchers {
  behavior of "A List"
  it must "return 0 in List(0,1,2,3) as 0th element" in {
    nth(0, List(0, 1, 2, 3)) mustBe 0
  }
  it must "return 3 in List(0,1,2,3) as 3rd element" in {
    nth(3, List(0, 1, 2, 3)) mustBe 3
  }
  it must "return None in List(0,1,2,3) as 10th element" in {
    nth(10, List(0, 1, 2, 3)) mustBe None
  }
  it must "throw an IllegalArgumentException for negative index in list" in {
    an [IllegalArgumentException] must be thrownBy nth(-10, List(0, 1, 2, 3))
  }
}