Need VBA for SumProduct of Array and Constant raised by Array for Regression

[LRobert]

I am trying to create a UDF for forecasting with polynomial regression. Assistance with the final line of code will be appreciated. The code thus far is as follows:

Function Forecast_PolyTest(X As Single, knownYs As Range, knownXs As Range, Order As Integer)

Dim k, OrdArr, PwrArr
Dim Ord2c, Ord3c, Ord4c, Ord5c, Ord6col
Dim Ord2r, Ord3r, Ord4r, Ord5, Ord6row
Dim Pwr2, Pwr3, Pwr4, Pwr5, Pwr6
Dim Skip As Integer

Ord2c = "{1,2}"
Ord3c = "{1,2,3}"
Ord4c = "{1,2,3,4}"
Ord5c = "{1,2,3,4,5}"
Ord6c = "{1,2,3,4,5,6}"

Ord2r = "{1;2}"
Ord3r = "{1;2;3}"
Ord4r = "{1;2;3;4}"
Ord5r = "{1;2;3;4;5}"
Ord6r = "{1;2;3;4;5;6}"

Pwr2 = "{2,1,0}"
Pwr3 = "{3,2,1,0}"
Pwr4 = "{4,3,2,1,0}"
Pwr5 = "{5,4,3,2,1,0}"
Pwr6 = "{6,5,4,3,2,1,0}"


'SELECT ARRAYS BASED ON ORDER AND WHETHER DATA IS IN COLUMNS OR ROWS

If knownXs.Columns.Count > 1 Then Skip = 4 Else Skip = -1

OrdArr = Application.Choose(Order + Skip, Ord2c, Ord3c, Ord4c, Ord5c, Ord6c, Ord2r, Ord3r, Ord4r, Ord5r, Ord6r)

PwrArr = Application.Choose(Order - 1, Pwr2, Pwr3, Pwr4, Pwr5, Pwr6)

'APPLY LINEST EQUATION

k = Application.Evaluate("=linest(" & knownYs.Address & "," & knownXs.Address & " ^ " & OrdArr & ")")

'FORMULA IN NATIVE FUNCTIONS = SUMPRODUCT(k, X^PwrArr) ALSO = {SUM(k * X^PwrArr)}

'I NEED HELP WITH THIS FINAL STATEMENT:

'Forecast_PolyTest = ?

'NONE OF THESE STATEMENTS WORKED:

'Forecast_PolyTest = Application.Sum(k(1) * X ^ 2, k(2) * X, k(3)) 'works, but is only accurate for Order=2

'Forecast_PolyTest = Application.Evaluate("=Sum(k * X ^ PwrArr)")

'Forecast_PolyTest = Application.Evaluate("=Sum(" & k & " * " & X & " ^ " & PwrArr & ")")

'Forecast_PolyTest = Application.Evaluate("=Sumproduct(k, X ^ PwrArr)")

'Forecast_PolyTest = Application.Evaluate("=Sumproduct(" & k & " ," & X & " ^ " & PwrArr & ")")

'Forecast_PolyTest = Application.WorksheetFunction.SumProduct(" & k & ", " & X & " ^ " & PwrArr & ")

'Forecast_PolyTest = PwrArr 'showed the correct power array is returned and is therefore not the problem

End Function

 

[Rijnsent]

Hi LRobert,

if this works:
'Forecast_PolyTest = Application.Sum(k(1) * X ^ 2, k(2) * X, k(3)) 'works, but is only accurate for Order=2
You can try an if statement:

If Order = 2 Then
    Forecast_PolyTest = Application.Sum(k(1) * X ^ 2, k(2) * X, k(3))
ElseIf Order = 3 Then
    Forecast_PolyTest = Application.Sum(k(1) * X ^ 3, k(2) * X ^ 2, k(3) * X, k(4))
Else
    'etc
End If

Does that work?
Koen