How to split a string given a list of positions in Scala

Question

How would you write a funcitonal implementation for split(positions:List[Int], str:String):List[String], which is similar to splitAt but splits a given string into a list of strings by a given list of positions?

For example

• split(List(1, 2), "abc") returns List("a", "b", "c")
• split(List(1), "abc") returns List("a", "bc")
• split(List(), "abc") returns List("abc")

Answer 1

def lsplit(pos: List[Int], str: String): List[String] = {
  val (rest, result) = pos.foldRight((str, List[String]())) {
    case (curr, (s, res)) =>
      val (rest, split) = s.splitAt(curr)
      (rest, split :: res)
  }
  rest :: result
}

Answer 2

Something like this:

def lsplit(pos: List[Int], s: String): List[String] = pos match {
  case x :: rest => s.substring(0,x) :: lsplit(rest.map(_ - x), s.substring(x))
  case Nil => List(s)
}

(Fair warning: not tail recursive so will blow the stack for large lists; not efficient due to repeated remapping of indices and chains of substrings. You can solve these things by adding additional arguments and/or an internal method that does the recursion.)