Accessing Collection causes Subscript out of range error? - excel

Question

I've got an UserForm, which upon an incorrect user input calls the following procedure, which highlights the field and disables the "save changes" button.
Private disabledElems As New Collection
Private Sub disable(ByRef controlName As String)
UserForm1.Controls(controlName).BackColor = &H8080FF
Me.save_button.Enabled = False
Dim i As Byte
If disabledElems.Count <> 0 Then
For i = 1 To disabledElems.Count
If disabledElems(i) = controlName Then
Exit Sub ' we dont want to add duplicates to collection
End If
Next i
End If
disabledElems.Add controlName ' otherwise add to collection
End Sub
If the input is corrected, it calls the enable procedure, which looks like this:
Private Sub enable(ByRef controlName As String)
Me.Controls(controlName).BackColor = &H80000005
Dim i As Byte
For i = 1 To disabledElems.Count
If disabledElems(i) = controlName Then
disabledElems.Remove i ' remove the enabled element upon match
End If
Next i
If disabledElems.Count = 0 Then
save_button.Enabled = True
End If
End Sub
This seems to work just fine when I try this with one Textbox
However, as soon I have multiple incorrect entries, my enable procedure seems to throw a Subscript out of range error seemingly for no reason.
The highlighted line in the debugger is:
If disabledElems(i) = controlName Then
I can't comprehend what could be causing this. Any ideas?

Answer 1

Ah alright, it's one of those classical "when removing a row, loop
from end to beginning"
Basically, the reason why the Subscript out of range was thrown - once the element was removed from the collection via the
disabledElems.Remove i
It reduced the size of the Collection from Collection.Count to Collection.Count - 1, however during the for loop declaration, the i was already hard-set to the previous Collection.Count
In an practical example:
Let's say my Collection looks like this
disabledElems = "button1", "button2"
Upon doing this
controlName = "button1"
For i = 1 to disabledElems.Count ' <= 2
If disabledElems(i) = controlName ' < True for i = 1
disabledElems.Remove i ' < button1 was removed from collection, however it still loops
End If
' will loop to i = 2. However disabledElems(2) no longer exists, because upon removal _
the button2 was shifted to disabledElems(1) - hence Subscript out of range
Next i
A clear case of trying to access an element, which has shifted its position in the queue.
There are two possible fixes (that I can think of):
1. Enforce Exit Sub upon removal
For i = 1 to disabledElems.Count
If disabledElems(i) = controlName
disabledElems.Remove i
Exit Sub
End If
Next i
2. Loop from end to start
Dim i as Integer ' needs to be redeclared to int, because Byte can't -1
For i = disabledElems.Count to 1 Step -1
If disabledElems(i) = controlName
disabledElems.Remove i
End If
Next i