Scala adding immutable list to mutable one

Question

I am trying to add some user input Strings to an immutable List with tail recursion and then add this immutable List to a MutableList which is defined. Somehow this immutable List isn't getting prepended to my MutableList. Why is that so?

EDIT: Updated the code. Now its working!

val list = scala.collection.mutable.MutableList[String]()

  def exerciseSelector() {
    val user = scala.io.StdIn.readLine("go:")
    user match {
      case "add" => val tempList = scanInput(List[String]())
                    if (!tempList.isEmpty) list ++= tempList.get ; exerciseSelector()
      case "print" => println(list)
      case "exit" => sys.exit()
    }
  }

    def scanInput(acc: List[String]): Option[List[String]] = {
    val input = scala.io.StdIn.readLine("User input:")
    input match {
    case "stop" => Option(acc)
    case input: String => scanInput(input :: acc)
  }
}

exerciseSelector()

Shadowlands · Accepted Answer · 2015-09-19 09:53:12Z

3

You have two mutually-recursive functions exerciseSelector and scanInput, which will only ever result in list being updated if scanInput returns, which ultimately can only happen after an input of "exit", which calls sys.exit(), quitting altogether before the return from scanInput can happen, and so before list ever gets updated.

Any input of "print" will be printing out an empty list, as list never gets a chance to be updated without sys.exit() being called first.

Depending on what exactly you are trying to achieve, I suspect just dropping out the co-recursive call to exerciseSelector() on receiving a "stop" input will fix your code (or get you a lot further towards your intended result).

answered Sep 19, 2015 at 9:53

Shadowlands

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5 Comments

bajro Over a year ago

Thank you very much now its working just fine!! So let me just summarize this up again: If i remove the exerciseSelector() call at the "stop" match case then the list can be returned, else the list isnt even returned and I am leaving the list behind in the scanInput() because i first called the exerciseSelector() and then returned the list???

Shadowlands Over a year ago

@ArmaGeddon You are "leaving the list behind", as you put it, in scanList, because the call to exerciseSelector() in front of the to-be-returned Option(acc) will inevitably result in a call to sys.exit() - ending the program - before completing. Also, you probably shouldn't edit your question so that the code now works, or people reading it may not understand what the problem was nor how the solution fixed it (at least, not without looking at the history of edits).

bajro Over a year ago

Okay, so I missunderstood the behaviour of the case here becuase I thought even when I am calling the exerciseSelector() and returning the list back it will do both things but now its clear to me thanks! Yes I edited it but last time another user told me not to "answer my question" or did he meant only not to do that if I am having further questions?

Shadowlands Over a year ago

@ArmaGeddon I'm not sure what (s)he might have meant.

bajro Over a year ago

Never mind... For the next time, I will do it by just posting another answer than editing my question. Thank you for your help!

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