Not quite sure how to write the formula mathematically - excel

Question

So I am a student currently studying VBA. I'm doing ok in the class but still kind of iffy on what how to program. For the question I'm on I have to take numbers in a column and multiply them unless they are less than or equal to zero. Here is an example done by hand of what the result should look like (which are not the same numbers as the actual problem, these are much simpler).
Here is what I have written so far. I'm kinda treating the column as a 1 x 10 array.
Function multpos(C As Variant)
Dim i As Integer
Dim MD As Long
For i = 1 To 9
MD = C(1, i) * C(1, i + 1)
Next i
If C(i, 1) > 0 Then C(i, 1) = C(i, 1) Else C(i, 1) = 1
If C(i + 1, 1) > 0 Then C(i + 1, 1) = C(i + 1, 1) Else C(i + 1, 1) = 1
multpos = MD
End Function
While MD satisfies the equation, it only works for the first two and then doesn't. Intuitively I want to do something like this
MD = C(1, i) * C(1, i)
Next i
Etc but this is also not mathematically correct. So if I had
MD = C(1, i)
how can I get it to multiply by the next value from here? Feel free to look at my other code and correct me as well since that could just as easily be wrong. Thank you for help in advance.

Answer 1

Something like this should work for you. I tried to comment the code for clarity:
Public Function PRODUCTIF(ByVal vValues As Variant, ByVal sCriteria As String) As Double
Dim vVal As Variant
Dim dResult As Double
'Iterate through vValues and evaluate against the criteria for numeric values only
For Each vVal In vValues
If IsNumeric(vVal) Then
If Evaluate(vVal & sCriteria) = True Then
'Value is numeric and passed the criteria, multiply it with our other values
'Note that until a valid value is found, dResult will be 0, so simply set it equal to the first value to avoid a 0 result
If dResult = 0 Then dResult = vVal Else dResult = dResult * vVal
End If
End If
Next vVal
'Output result
PRODUCTIF = dResult
End Function
And you would call the function like this: =PRODUCTIF(A1:A10,">0")

Answer 2

you could exploit AutoFiler() method
Function multpos(C As Range, criteria As String)
Dim MD As Long
Dim cell As Range
With C.Columns(1)
If IsEmpty(.Cells(1, 1)) Then .Cells(1, 1) = "|header|"
.AutoFilter Field:=1, Criteria1:=criteria
If Application.WorksheetFunction.Subtotal(103, .Cells) > 1 Then
MD = 1
For Each cell In C.SpecialCells(xlCellTypeVisible).SpecialCells(xlCellTypeConstants, xlNumbers)
MD = MD * cell.Value
Next cell
End If
If .Cells(1, 1) = "|header|" Then .Cells(1, 1).ClearContents
.Parent.AutoFilterMode = False
End With
multpos = MD
End Function
to be exploited in your main sub like:
MsgBox multpos(Range("A1:A10"), ">0")