Question

I want to choose an element and then remove it from a mutable list in O(1) time.
In C++ I could do

std::list<Foo> lst;
std::list<Foo>::iterator it = //some choice
//And then, possibly in another function,
lst.erase(it);

Can I have equivalent code in Scala, or do I have to do filter or diff?

Edit: to clarify, I want to separate choice and removal. I want to mark an element so that it can be later quickly accessed, modified and possibly removed. It would be great if I could insert another element after the selected one too. That’s the functionality C++ iterators give.

Answer 1

If you want to do O(1) removal, I think your only option is to search the respective inner linked lists (with next) and keep a reference.

If you use a mutable.DoubleLinkedList things will become a bit easier:

val li = DoubleLinkedList(1,2,3,4,5,6)
val elem = li.next.next.next // O(< n) until you find what you want
elem.remove() // O(1)

li == DoubleLinkedList(1,2,3,5,6)

But even then, you won’t have a complete mirror of C++s mutable iterator interface.

I am not sure there is a proper Scala list which would support something like that. An alternative suggestion would be to use a Zipper which also provides O(1) for operations at the position of the zipper.