Question : Excel: VBA Function To Sum Combinations

I need a VBA function to sum the product of combinations of numbers. Specifically, with three numbers I need the sum of three products taken two at a time and with four numbers I need the sum of four products taken three at a time.

For example, with (2, 3, 4), the result would be (23) + (24) + (34) = 26. Using (4, 5, 6, 7), the result would be (45*6) + (457) + (467) + (567) = 518.

Best wishes,

John

Answer : Excel: VBA Function To Sum Combinations

Hi John,
Here is a function that reads the data from the range in the argument and puts the numbers in a array. Validation could be added here, for example test that number of cells are within the permitted range or a test that the values are valid numbers.
Then it calls a function that loops through the array and calculates the product of all elements except the one to skip, and sums all the products.

I hope this is what you need.

Kind regards,
Stellan

                       
         
           
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             Function CombProdSum(rngNumbers As Range) As Double
    Dim rngCell As Range
    Dim iNum As Integer
    Dim vaNum() As Variant
    Dim i As Integer
    
    'Read the values of all cells into a variant array
    iNum = rngNumbers.Cells.Count
    ReDim Preserve vaNum(1 To iNum)
    i = 0
    For Each rngCell In rngNumbers.Cells
        i = i + 1
        vaNum(i) = rngCell.Value
    Next rngCell
    
    'Calculate the sum of all combinations of products of iNum-1 values
    CombProdSum = CSP(vaNum)
    
    
End Function
 
 
Private Function CSP(vaData As Variant) As Double
    Dim i As Integer, j As Integer
    Dim iToSkip As Integer
    Dim iIndex As Integer
    Dim iNum As Integer
    Dim dProd As Double
    
    iNum = UBound(vaData) 'assuming base 1 array
    CSP = 0
    For iToSkip = 1 To iNum
        iIndex = iToSkip
        dProd = 1
        For i = 1 To iNum - 1
            j = (iIndex Mod iNum) + 1
            Debug.Print j
            dProd = dProd * vaData(j)
            iIndex = iIndex + 1
        Next i
        CSP = CSP + dProd
    Next iToSkip
    
End Function
           
         

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