Overwrite duplicate values in Scala List

Question

I have the following Comet server:

object UserServer extends LiftActor with ListenerManager {

    private var users: List[UserItem] = Nil

    def createUpdate = users

    override def lowPriority = {

        case UserItem(user, room, active, stamp) => {

            users :+= UserItem(user, room, active, stamp);
            updateListeners()

        }

    }

}

Currently each time the form is submitted a new UserItem is added to the users list. What I'm trying to do is each time the server receives a new user, instead of concatenating to the list it should overwrite an existing item with the same user and room handle.

So if the list contains the following:

UserItem("jam_2323", "demo-room", "James", "1320073365")
UserItem("jim_4533", "demo-room", "Jim", "1320073365")

The next time these users submit the form the above two items in the list will be replaced with the new stamp value:

UserItem("jam_2323", "demo-room", "James", "1320073435")
UserItem("jim_4533", "demo-room", "Jim", "1320073435")

Thanks in advance for any help, much appreciated :)

As a side note, instead of extracting and rebuilding the UserItem, you can simply match on it as an object. You should also either prepend to the List, or use a Vector; appending to a List performs very poorly. case u: UserItem => users ::= u. — leedm777
– leedm777, Commented Oct 31, 2011 at 15:51

Luigi Plinge · Accepted Answer · 2011-10-31 16:36:41Z

2

This sounds like a classic case where you need a map, rather than a list. I don't know about the details of Lift / Comet, but I guess you want something like

    case class User(id: String)
    case class Activity(room: String, active: String, stamp: String)

    var lastUserActivity = Map[User, Activity]()
...
    case UserItem(id, room, active, stamp) => {
        lastUserActivity += User(id) -> Activity(room, active, stamp)
    }

answered Oct 31, 2011 at 16:36

Luigi Plinge

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Comments

Emil Sit · Accepted Answer · 2011-10-31 15:09:45Z

1

If you adjust UserItem from being a straight case class (I assume) to being one where you have overriden equals to ignore the stamp field, then you could make users into a Set.

Alternatively, you could filter the List to remove the old matching values before appending.

answered Oct 31, 2011 at 15:09

Emil Sit

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