var k = -1
for (i <- 0 until array.length)
  if ((i < array.length - 1) && array(i) < array(i + 1))
    k = i

scala> val xs = List(5, 4, 2, 3, 1)
xs: List[Int] = List(5, 4, 2, 3, 1)

scala> val tail = xs.tail
tail: List[Int] = List(4, 2, 3, 1)

scala> xs.zip(tail)
res0: List[(Int, Int)] = List((5,4), (4,2), (2,3), (3,1)

scala> res0.indexWhere { case (x, y) => x < y }
res1: Int = 2

val k = (array zip array.tail) lastIndexWhere { case (x, y) => x < y }

scala> Array(1,2,2,4,5,6, 6).sliding(2).toList
res12: List[Array[Int]] = List(Array(1, 2), Array(2, 2), Array(2, 4), Array(4, 5), Array(5, 6), Array(6, 6))

Array(1,2,2,4,5,6, 6).sliding(2).indexWhere { case Array(x1, x2) => x1 == x2 }

Array(1,2,2,4,5,6, 6)
  .sliding(2)     //splits each in to pairs
  .zipWithIndex   //attaches the current index to each pair
  .collect { case (Array(x1, x2), index) if (x1 == x2) => index }  //collect filters out non-matching pairs AND transforms them to just the inde