UniqueList Class in VBA

Question

I often find myself abusing dictionary objects just for the exist method like this,

For Each x in Something
    If Not dict.Exists(x) Then dict.Add x, False 
Next x

Then just exploit the .Exists(x) method. It happens often enough that I thought it merited it's own class.

Instead of composing Scripting.Dictionary, I decided to just maintain a sorted list of unique items. Usage is simple; unique items are inserted into the collection sorted. Non-unique items are not added.

Attributes

Let's just get this out of the way. Attribute VB_PredeclaredId must be set to True and NewEnum allows the class to be iterable. I still haven't found a way to replicate this using an array instead of a collection. With an array I could offload some of my code to my standard library, but then this wouldn't be standalone or portable.

VERSION 1.0 CLASS
BEGIN
  MultiUse = -1
END
Attribute VB_Name = "UniqueList"
Attribute VB_GlobalNameSpace = False
Attribute VB_Creatable = False
Attribute VB_PredeclaredId = True
Attribute VB_Exposed = False

Option Explicit

Private list As Collection

Public Sub Class_Initialize()
    Set list = New Collection
End Sub
Public sub Class_Terminate()
    Set list = Nothing
End Sub
Public Property Get NewEnum() As IUnknown
Attribute NewEnum.VB_UserMemId = -4

    Set NewEnum = list.[_NewEnum]

End Property

Data Manipulation

Public Sub Add(ByVal element As Variant)

    If IsEmpty Then

        list.Add element
        Exit Sub

    End If

    Dim index As Long
    index = LocateIndex(element)

    If (list(index) > element) Then
        list.Add element, Before:=index
    ElseIf list(index) < element Then
        list.Add element, After:=index
    End If

End Sub
Public Sub Merge(ByVal list_ As Variant)

    Dim element As Variant
    For Each element In list_
        Add element
    Next element

End Sub
Public Sub Remove(ByVal element As Variant)

    Dim index As Long
    index = LocateIndex(element)

    If (list(index) = element) Then list.Remove index

End Sub
Public Sub Clear()

    Set list = Nothing
    Set list = New Collection

End Sub

Introspection

Note I don't implement an Item(x) method. I don't want users to be able to scramble the data.

Public Function Exists(ByVal element As Variant) As Boolean
    Exists = (element = list(LocateIndex(element)))
End Function
Public Function Count() As Long
    Count = list.Count
End Function
Public Function IsEmpty() As Boolean
    IsEmpty = (Count = 0)
End Function

Searching

The only private method is a binary search that returns where the element is or where it should be.

Private Function LocateIndex(ByVal element As Variant) As Long

    Dim upper As Long
    upper = Count

    Dim lower As Long
    lower = 1

    While lower < upper

        Dim middle As Long
        middle = (lower + upper) \ 2

        If list(middle) >= element Then
            upper = middle
        Else
            lower = middle + 1
        End If

    Wend

    LocateIndex = upper

End Function

Answer 1

Note I don't implement an Item(x) method. I don't want users to be able to scramble the data.

Ok. But then why expose a NewEnum to enable For Each loops? Without an Item getter (with VB_UserMemId set to 0 to identify the type's default property), it's not clear how a For Each loop might work. An Item property won't let the client code scramble the data if you don't implement a setter for it.

Searching

I like the idea, I like the implementation, ..but I would have called if IndexOf. Just a couple of points:

If list(middle) >= element Then

If list is empty, this method explodes. You may want to consider raising a custom error in this case, and gracefully handle an empty collection in the public methods.

Also I like that you're declaring variables close to their usage, but this:

While lower < upper

    Dim middle As Long
    middle = (lower + upper) \ 2

Is potentially confusing. Consider moving the declaration outside the loop. It's not like the While block defined a scope anyway!

Dim middle As Long
While lower < upper

    middle = (lower + upper) \ 2

---

The binary search and sorted nature of this UniqueList are nice, but given the goal:

I often find myself abusing dictionary objects just for the exist method

You wrote that code to implement your own Exists method, and you're using Variant which tells me you're using it for more than just Integer items.

Objects

It would have been nice to handle object items, since a Variant can also be an Object; your code is silent about how objects are dealt with.

Actually, if I read your code correctly, this UniqueList will happily allow me to Add a New ADODB.Connection, and then will blow up when I try to add a 2nd item... or if I try to remove it with the Remove method - the Clear method will work.

Because we can't override operators (like >=) in VBA, I'd handle objects with a \$O(n)\$ linear search that performs an equality check on each IsObject(item) item until it finds the existing object reference (or not). You could mitigate this a little, by introducing an IComparable interface that custom classes could implement, and then you could call comparable.CompareTo(element) and have custom classes that could be handled with a binary search, if they specify how they compare to items of a given type.

There should also be a mechanism for ensuring all items are of the same type: as it stands I could add an Integer, a Date, a Short and a Long to an instance of a UniqueList, and VBA would find a way to sort them, and that's implicit and not pretty.

I think your class is more like some kind of a SortedList<T> (to borrow from c# notation), that doesn't allow duplicates: all items in a SortedList<T> are of type T, whatever that type is.

Answer 2

Just a couple of notes. The biggest of which is that I think you've misunderstood what VB_PredeclaredId does. It creates a default instance of the class. Which allows you to do things like this

UniqueList.Add someItem

Without declaring and setting a new instance of the class. It's a cool feature, but it's not what allows the list to be iterable and you don't need it here. The [_NewEnum] is what allows the For Each syntax.

The Clear method doesn't need to Set list = Nothing. Drop that line and just set it to a new instance of Collection.

Lastly, I find it really odd that you have a list with no way to get an element back out of the list. We can add and remove items, but not retrieve them. I would rethink implementing an Item method.