Question

I am trying inversion counting in Scala with merge sort and not able to make progress because one of the method is throwing IndexOutOfBoundsException for passing empty List as a parameter.

def sortAndCountInv(il: List[Int]): Int = {

def mergeAndCountInv(ll: List[Int], rl: List[Int]): (List[Int], Int) = {
  println("mergeAndCountInv : ")
  if (ll.isEmpty && !rl.isEmpty) (rl, 0)
  if (rl.isEmpty && !ll.isEmpty) (ll, 0)
  if (ll.isEmpty && rl.isEmpty) (List(), 0)
  if (ll(0) <= rl(0)) {

    val x = mergeAndCountInv(ll.tail, rl)//*passing empty list, ll.tail invoking IndexOutOfBoundsException*//


    (ll(0) :: x._1, x._2)
  } else {
    val y = mergeAndCountInv(ll, rl.tail)
    (rl(0) :: y._1, 1 + y._2)
  }
}

def sortAndCountInv(l: List[Int], n: Int): (List[Int], Int) = {
  if (n <= 1) (l, 0)
  else {
    val two_lists = l.splitAt(n / 2)
    val x = sortAndCountInv(two_lists._1, n / 2)
    val y = sortAndCountInv(two_lists._2, n / 2)
    val z = mergeAndCountInv(x._1, y._1)
    (z._1, x._2 + y._2 + z._2)
  }
}

val r = sortAndCountInv(il, il.length)
println(r._1.take(100))
r._2

}

Answer 1

I would suggest using else in mergeAndCountInv when you check if left or right or both of lists are empty. Because instead of return you ignore a tuple with your calculations.