How would be a functional approach to shifting certain array elements?

Question

I have a Scala app with a list of items with checkboxes so the user select some, and click a button to shift them one position up (left). I decided to write a function to sh

Answer 1

Here's a purely functional implementation

def shiftElements[A](l: List[A], pred: A => Boolean): List[A] = {
  def aux(lx: List[A], accum: List[A]): List[A] = {
    lx match {
      case Nil => accum
      case a::b::xs if pred(b) && !pred(a) => aux(a::xs, b::accum)
      case x::xs => aux(xs, x::accum)
    }
  }
  aux(l, Nil).reverse
}

And here's one that uses mutability on the inside to be faster

import scala.collection.mutable.ListBuffer
def shiftElements2[A](l: List[A], pred: A => Boolean): List[A] = {
  val buf = new ListBuffer[A]
  def aux(lx: List[A]) {
    lx match {
      case Nil => ()
      case a::b::xs if pred(b) && !pred(a) => {
        buf.append(b)
        aux(a::xs)
      }
      case x::xs => {
        buf.append(x)
        aux(xs)
      }
    }
  }
  aux(l)
  buf.toList
}