Error when Populating values for ComboBox.List - arrays

Question

I'm trying to populate a combo box with UNIQUE values only, no duplicates; which I believe is working fine, but something is wrong with my logic in the second For loop
The below logic goings as follows...
Private Function PopulateComboBoxWeeks()
Dim i As Long
Dim x As Long
Dim LR As Long
Dim ws As Worksheet
Dim SearchNextWeek As String
LR = ws.Cells(ws.Rows.Count, "A").End(xlUp).Row
Set ws = ActiveSheet
With UserForm1.ComboBox1
''' Fill first slot in ComboBox1 with the value of last row in Column "A"
.AddItem ws.Range("A" & LR).Value
''' Loop to search Column "A" for items to fill with, start on the second last row, since the above line fills the first line
For i = LR - 1 To 2 Step -1
''' Loop to search the ComboBox.List() array
For x = 0 To .ListCount
''' Array list starts at 0
If Not (.List(x) = ws.Range("A" & i).Value) Then
.AddItem ws.Range("A" & i).Value
End If
Next x
Next i
End With
End Function
It's checking the Array list properly, but I'm stuck on the second For loop, if I start at index 0 of my array and it's taking into account the total items in the array with .ListCount. Thus it's giving me the below error...
Run-Time error '381':
Could not get the List property. Invalid property array index
Which could only mean I'm referencing an array item outside of the array size. I've tried doing .ListCount - 1 but this gives me an infinite loop. I think all my logic is sound here except this one item and I'm not sure how to get passed this point.

Answer 1

Iterating any collection as you're changing it is always a bad idea.
Don't loop on anything. Just tell it what range you want to use.
If you can't do that, then you need to first get the unique values into an array (single-dimensional), then assign ComboBox1.List = theArray. Done.
There are two things you want to do:
Figure out what the unique values are
Assign the List property
Don't do these two things in one single nested spaghetti loop. Separate them.
Dim allValues As Variant
'get a single-dimensional array with all the values in the column:
allValues = Application.WorksheetFunction.Transpose(ws.Range("A2:A" & LR).Value)
'let's use built-in collection keys to ensure uniqueness:
Dim uniqueValuesColl As Collection
Set uniqueValuesColl = New Collection
Dim currentIndex As Long
For currentIndex = LBound(allValues) To UBound(allValues)
If Not IsError(allValues(currentIndex)) Then
On Error Resume Next
uniqueValuesColl.Add allValues(currentIndex), Key:=CStr(allValues(currentIndex))
If Err.Number <> 0 Then
' we already have that value
Err.Clear
End If
On Error GoTo 0
End If
Next
'now we know what the unique values are - get them into an array:
ReDim uniqueValues(0 To uniqueValuesColl.Count - 1)
Dim currentItem As Variant
currentIndex = 0
For Each currentItem In uniqueValuesColl
uniqueValues(currentIndex) = currentItem
currentIndex = currentIndex + 1
Next
'just assign the list of unique values
ComboBox1.List = uniqueValues
So I'm iterating all values once, and then the unique values once. But you're currently iterating them once for every single item in the non-unique list. So this solution is O(n+m) where n is the number of non-unique items and m is the number of unique items, whereas your nested loop is O(n2) (the big-O notation of your solution is actually more complicated than that, but I'm no big-O expert).