Scala中的空列表头

Question

I've made this recursive metod in Scala that returns a list made of all distinct elements of another list.

object es20 extends App{

    def filledList:List[Int]=List()

    @scala.annotation.tailrec
    def distinct(l:List[Int]):List[Int] ={
        if (l.isEmpty) filledList
        if (filledList.forall(_!=l.head)) l.head::filledList

    distinct(l.tail)
    }
println(distinct(List(1,1,5,6,6,3,8,3))) //Should print List(1,5,6,3,8)
}

However, when I compile the code and then I run it, there's this exception:

java.util.NoSuchElementException: head of empty list

I thought that this exception was handle by the condition if (l.isEmpty ).

How can I fix the code?

Answer 1

In Scala method returns last expression of the block. In your case you have three expressions: two if-expressions which result in unit and call to distinct, so checks will be executed every time you call distinct, no matter if the list is empty or not.

To fix it you can use if / else construct, or pattern match on input list, or make operation on headOption.

Anyway I doubt if this code correct: you trying to check something on 'filledList' which is always empty

Answer 2

You can fix this particular error by inserting else before the second if . However, as mentioned in the other answer, your code isn't correct, and won't work anyway, you need to rewrite it.

Also, I understand, that you are just trying to write this function as an exercise (if not, just do list.distinct ), but I submit, that implementing quadratic solutions to trivially linear problems is never a good exercise to begin with.