I've reworked this utility as follows:

A bug in the AllowDupes feature is fixed
Prompts have been moved to the caller
Parameters are passed to the function via an array
Function supports:
- removing matches or non-matches
- returning a list with or without duplicate values


<code - watch for line wrapping

The new caller:
Sub CompareCols_FilterMatches()
Dim bSuccess As Boolean, lMatchesFound As Long
Dim vAns As Variant, vCriteria(5) As Variant, sMsg As String

'Get the label of the columns to act on
Const MSG As String = "Please enter the label of the column"

tryagain:
'Column to filter
sMsg = MSG & " to be filtered": vAns = Application.InputBox(sMsg,
Type:=2)
If vAns = False Or vAns = "" Then Beep: Exit Sub
vCriteria(0) = Range(vAns & "1:" & vAns & Cells(Rows.Count,
vAns).End(xlUp).Row).Address
'Output goes in the column being filtered unless specified otherwise
below
vCriteria(2) = UCase$(vAns)

'Column to be checked
sMsg = MSG & " to check for matches": vAns =
Application.InputBox(sMsg, Type:=2)
If vAns = False Or vAns = "" Then Beep: Exit Sub
vCriteria(1) = Range(vAns & "1:" & vAns & Cells(Rows.Count,
vAns).End(xlUp).Row).Address

'Make sure lists contain more than 1 item
If Not Range(vCriteria(0)).Cells.Count 1 _
Or Not Range(vCriteria(1)).Cells.Count 1 Then
sMsg = "Columns MUST have more than one value!" & vbLf & vbLf
sMsg = sMsg & "Please try again with a different set of
columns"
MsgBox sMsg, vbCritical: GoTo tryagain
End If

'Column to receive the results
sMsg = MSG & "where the new list is to go" & vbLf _
& "(Leave blank or click 'Cancel' to use column '" &
vCriteria(2) & "')"
vAns = Application.InputBox(sMsg, Type:=2)
If Not (vAns = False) And (vAns < "") Then vCriteria(2) =
UCase$(vAns)

'Return or remove matches?
sMsg = "Do you want to return the matches found instead of removing
them?"
vAns = MsgBox(sMsg, vbYesNo + vbQuestion)
If (vAns = vbYes) Then vCriteria(3) = 1 Else vCriteria(3) = 0

'Return a unique list?
sMsg = "Do you want only unique items in the returned list?" & vbLf &
vbLf & "(No duplicates)"
vAns = MsgBox(sMsg, vbYesNo + vbQuestion) '//YES = no dupes allowed
If (vAns = vbYes) Then vCriteria(4) = 0 Else vCriteria(4) = 1
bSuccess = FilterMatches(lMatchesFound, vCriteria())

If lMatchesFound = 0 Then MsgBox "No matches found!": Exit Sub
If lMatchesFound < 0 Then
sMsg = "Both columns must have more than 1 item!"
sMsg = sMsg & vbLf & vbLf & "Please try again: specify different
columns!"
MsgBox sMsg, vbExclamation: Exit Sub
End If 'lMatchesFound < 0

If bSuccess Then
sMsg = Format(CStr(lMatchesFound), "#,##0") _
& " Matches were found"
If vAns = vbYes Then _
sMsg = sMsg & " (including non-match duplicates)"
MsgBox sMsg '//comment out if using option below

'Optional: Ask to run a process on the new list
' sMsg = sMsg & vbLf & vbLf _
' & "Do you want to process the new list?"
'
' vAns = MsgBox(sMsg, vbYesNo + vbQuestion)
' If vAns = vbYes Then
' 'Code... ('Call' a process to act on the new list)
' End If 'vAns = vbYes
Else
MsgBox "An error occured!"
End If 'bSuccess
End Sub


The new function:
Function FilterMatches(Matches As Long, Criteria() As Variant) As
Boolean
' Compares 2 user-specified cols and filters matches found.
' User can also specific target col to receive resulting list.
' Optionally supports returning a unique list or allow duplicates.
' Optionally supports returning matches or non-matches.
'
' Args In: Matches: ByRef var to return number of matches found to
the caller.
'
' vCriteria(): A variant array containing the filtering
parameters.
' Criteria(0) - Address of the values to be filtered
' Criteria(1) - Address of the values to check
' Criteria(2) - Label of the column to put the filtered
list
' Criteria(3) - Numeric value to determine if we return
matches or non-matches
' Criteria(4) - Numeric value to determine if we return a
unique list or allow dupes
'
' Returns: True if matches found and no error occurs;
' False if a: matches not found --OR-- error occurs;

Dim i&, j& 'as long
Dim vFilterRng, vCheckRng, vResult, vaMatches(), vaNoMatches(),
vaDataOut() 'as variant
Dim bReturnMatches As Boolean, bMatch As Boolean, bDupesAllowed As
Boolean
Dim cItemsToCheck As New Collection, sMsg$, sRngOut$ 'as string

'Load the filtering criteria
vFilterRng = Range(Criteria(0)): vCheckRng = Range(Criteria(1)):
sRngOut = Criteria(2)
bReturnMatches = (Criteria(3) = 1): bDupesAllowed = (Criteria(4) = 1)
ReDim vaMatches(UBound(vFilterRng)): ReDim
vaNoMatches(UBound(vFilterRng)): j = 0

'Load the Collection with the values to be checked.
'Collections only allow unique keys so use OERN (no need to check if
they already exist)
Set cItemsToCheck = New Collection: On Error Resume Next
For i = LBound(vCheckRng) To UBound(vCheckRng)
cItemsToCheck.Add Key:=CStr(vCheckRng(i, 1)), Item:=vbNullString
Next 'i
Err.Clear

'Check the Collection for matches
On Error GoTo MatchFound
For i = LBound(vFilterRng) To UBound(vFilterRng)
bMatch = False '..reset
cItemsToCheck.Add Key:=CStr(vFilterRng(i, 1)), Item:=vbNullString
If bMatch Then
If bReturnMatches Then vaMatches(j) = vFilterRng(i, 1): j = j + 1
Else
vaNoMatches(j) = vFilterRng(i, 1): j = j + 1
cItemsToCheck.Remove CStr(vFilterRng(i, 1)) '..so dupes of it
don't get counted
End If 'bMatch
Next 'i

'Initialize the return list
If bReturnMatches Then vResult = vaMatches Else vResult = vaNoMatches

'Return a list of unique values?
If Not bDupesAllowed Then
On Error GoTo UniqueList
Dim cUniqueList As New Collection
For i = LBound(vResult) To UBound(vResult)
cUniqueList.Add Key:=CStr(vResult(i)), Item:=vbNullString
Next 'i
End If 'Not bDupesAllowed
Err.Clear: On Error GoTo ErrExit

'Make the list to return contiguous.
ReDim vaDataOut(UBound(vResult), 0): j = 0
For i = LBound(vResult) To UBound(vResult)
If Not vResult(i) = "" Then vaDataOut(j, 0) = vResult(i): j = j + 1
Next 'i

If Matches 0 Then '..only write if Matches 0
Columns(sRngOut).ClearContents
With Range(sRngOut & "1").Resize(UBound(vaDataOut) + 1, 1)
.Value = vaDataOut
.NumberFormat = "0000000000000" '..optional
.EntireColumn.AutoFit '..optional
End With
End If 'Matches 0


ErrExit:
' If bReturnMatches Then Matches = UBound(vResult) ' + 1
FilterMatches = (Err = 0): Exit Function

MatchFound:
bMatch = True: Matches = Matches + 1: Resume Next

UniqueList:
vResult(i) = "": Matches = Matches + 1: Resume Next

End Function 'FilterMatches()

--
Garry

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