VBA loop until function is true, changing passed value each time - ms-access

Question

I have a form that adds new users. I'm trying to auto generate the username, which needs to be unique. I have a function checkUsername that takes the string passed and checks the db table to see if that username exists. It returns false if the username exists and true if not.
What I'm trying to do is if the username already exists then add a number on to the end and check again. I want to keep looping until a unqiue username is found. Heres my attempt at a loop below, however its returning usernames that already exist. Also I prefer it to increment the number rather than just adding it to the end. Currently the pattern ends up being username1, username12, username123.
'Generate username
Dim Username As String
Username = generateUsername(LCase(Left(FirstName, 1) & LastName))
Function generateUsername(Username As String) As String
Dim i As Integer
i = 1
Do While checkUsername(Username)
If checkUsername(Username) Then
Exit Do
Else
Username = Username & i
i = i + 1
End If
Loop
generateUsername = Username
End Function

Answer 1

What I would do is, when the username exists already (see Andre's answer for the code that will accomplish this a different way):
check what the previous "i" number was, as a string, to see how many characters I need to cut off the end of username
set username equal to a substring of username, which will cut off the last X characters (X being the number of characters the previous "i" was)
add the new "i" to the end username
That will solve your naming issue.
For your issue of looking for unique names, no one will be able to help with that until we can see what the code looks like for the "checkUsername" function. Make sure that that function is returning the value you are expecting, first. I suspect the problem is in there.
EDIT:
I also don't understand why you have an "If/Else" statement in your loop. You already determined that it was true. See below for simplified version (again, assuming I got the code right for VB...I usually do it in C#):
Do While Not checkUsername(NewName)
NewName = Username & i
i = i + 1
Loop

Answer 2

I assume checkUsername() returns true if the username exists?
Then your If condition is wrong - you want to exit if it doesn't exist.
Your loop gets clearer if you use two variables:
Function generateUsername(origUsername As String) As String
Dim i As Integer
Dim Username As String
i = 1
Username = origUsername
Do While checkUsername(Username)
' If Not checkUsername(Username) Then
' Exit Do
' Else
Username = origUsername & i
i = i + 1
' End If
Loop
generateUsername = Username
End Function
Edit: as pointed out by Joel, the additional checkUsername() in the loop isn't needed. Since it probably involves a DLookup or similar, it is actually harmful to performance.
I commented it out above.

Answer 3

Sorry, but this is the corrected version!
Change your code to this instead:
Dim Username As String
Username = generateUsername(LCase(Left(FirstName, 1) & LastName))
Function generateUsername(Username As String) As String
Dim NewName as String
Dim i As Integer
i = 1
NewName = Username
Do While checkUsername(NewName)
If checkUsername(NewName) Then
Exit Do
Else
NewName = Username & i
i = i + 1
End If
Loop
generateUsername = NewName
End Function