How can I write a UDF in Excel VBA to filter a range of cells? - vba

Question

I've gotten into the habit of marking outlying data by changing cell styles. I'd like to write a UDF in excel to take a Range of cells as input, and return the subset of that range that is not marked as an outlier.
This is what I have tried:
Function ValidCells(rCells As Range) As Range
Dim c As Range
For Each c In rCells
If c.Style <> "Bad" Then
Set ValidCells = Range(c, ValidCells)
End If
Next
End Function
My intent is to be able to do =Sum(ValidCells(A1:D1)), and have it only sum the non-styled data.
However, ValidCells seems to return an empty range every time. What am I doing wrong?

Answer 1

Are you sure it's returning an empty range? When I try running this, VBA raises an error on your 'Set' line. If you're calling the routine as a UDF from the worksheet you won't see the VBA error, but the UDF should stop executing and return #VALUE!.
In any case, you can do what you want, but there is one big caveat. First, the code:
Function ValidCells(rCells As Range) As Range
Dim valid As Range
Dim c As Range
For Each c In rCells
If c.Style <> "Bad" Then
If valid Is Nothing Then
Set valid = c
Else
Set valid = Union(valid, c)
End If
End If
Next
Set ValidCells = valid
End Function
The idea is to build up a multi-area range using VBA's 'Union' method. So, for example, if I put a bad cell in C8, and call ValidCells(B7:D9), this returns the multi-area range $B$7:$D$7,$D$8,$B$8:$B$9,$C$9:$D$9. You can then use the result with SUM just fine.
The caveat is that changing cell styles won't trigger this UDF to recalculate. Normally, you'd be able to add a line like this:
Call Application.Volatile(True)
to your UDF and it would recalc on every change to the workbook. However, it seems like changing a cell style doesn't qualify as a "change" for volatility purposes. So, you can get what you want out of the UDF, but there appears to be no real way to make it work like a "normal" one as far as recalculation goes, even if you mark it as volatile. You'll have to remain aware of that if you use it.