SumProduct over sets of cells (not contiguous)

Question

I have a total data set that is for 4 different groupings. One of the values is the average time, the other is count. For the Total I have to multiply these and then divid

Answer 1

It might be possible with masterful excel-fu, but even if it can be done, it's not likely to be more readable than your original solution. The problem is that even after 20+ years, Excel still borks discontinuous ranges. Naming them won't work, array formulas won't work and as you see with SUMPRODUCT, they don't generally work in tuple-wise array functions. Your best bet here is to come up with a custom function.

UPDATE

You're question got me thinking about how to handle discontinuous ranges. It's not something I've had to deal with much in the past. I didn't have the time to give a better answer when you asked the question but now that I've got a few minutes, I've whipped up a custom function that will do what you want:

Function gvSUMPRODUCT(ParamArray rng() As Variant)

    Dim sumProd As Integer
    Dim valuesIndex As Integer
    Dim values() As Double

    For Each r In rng()
        For Each c In r.Cells
            On Error GoTo VBAIsSuchAPainInTheAssSometimes
                valuesIndex = UBound(values) + 1
            On Error GoTo 0
            ReDim Preserve values(valuesIndex)
            values(valuesIndex) = c.Value
        Next c
    Next r
    If valuesIndex Mod 2 = 1 Then
        For i = 0 To (valuesIndex - 1) / 2
            sumProd = sumProd + values(i) * values(i + (valuesIndex + 1) / 2)
        Next i
        gvSUMPRODUCT = sumProd
        Exit Function
    Else
        gvSUMPRODUCT = CVErr(xlErrValue)
        Exit Function
    End If

VBAIsSuchAPainInTheAssSometimes:
    valuesIndex = 0
    Resume Next

End Function

Some notes:

• Excel enumerates ranges by column then row so if you have a continuous range where the data is organized by column, you have to select separate ranges: gvSUMPRODUCT(A1:A10,B1:B10) and not gvSUMPRODUCT(A1:B10).
• The function works by pairwise multiplying the first half of cells with the second and then summing those products: gvSUMPRODUCT(A1,C3,L2,B2,G5,F4) = A1*B2 + C3*G5 + L2*F4. I.e. order matters.
• You could extend the function to include n-wise multiplication by doing something like gvNSUMPRODUCT(n,ranges).
• If there are an odd number of cells (not ranges), it returns the #VALUE error.

Answer 2

Note that sumproduct(a, b) = sumproduct(a1, b1) + sumproduct(a2, b2) where range a is split into ranges a1 and a2 (and similar for b)

Answer 3

It might be helpful to create an intermediate table that summarizes the data that you are using to calculate the sum product. That would also make the calculation easier to follow.

Answer 4

I agree with the comment "It might be possible with masterful excel-fu, but even if it can be done, it's not likely to be more readable than your original solution"

A possible solution is to embed the CHOOSE() function within your SUMPRODUCT (this trick actually is pretty handy for vlookups, finding conditional maximums, etc.).

Example: Let's say your data has eight observations and is in two columns (columns B and C) but you don't want to include some observations (exclude observations in rows 4 and 5). Then the SUMPRODUCT code looks like this...

=SUMPRODUCT(CHOOSE({1,2},A1:A3,A6:A8),CHOOSE({1,2},B1:B3,B6:B8))

I actually thought of this on the fly, so I don't know the limitations and as you can see it is not that pretty.

Hope this helps! :)