Recursive function structure in VBA - vba

Question

I'm trying to write a recursive function so that it calculates the sum of the products of the combinations of values in a dynamic array. Right now I've been trying to make it work for a simpler case, but I really don't quite understand the structure I should follow for a recursive function. In this case there's supposed to be the sum of 28 two factor products, resulting 1.4
Sub SuPC()
Dim k As Long
Dim s As Long
Dim i As Long
Dim j As Long
k = 8
s = 2
HSum i, j, s, k
End Sub
Function HSum(i As Long, j As Long, s, k) As Double
Dim P As Variant
Dim z() As Double
Dim Tot As Double
ReDim z(0 To (k * s) - 1)
P = Array(1 / 2, 1 / 3, 1 / 4, 1 / 5, 1 / 6, 1 / 7, 1 / 8, 1 / 9)
If i <= k Then
HSum i + 1, j, s, k
If j <= s Then
HSum i, j + 1, s, k
If z(i) = 0 Then z(i) = 1
z(i) = P(j) * z(i)
End If
Tot = z(i) + Tot
End If
Range("J11") = Tot
End Function
If s and k were low fixated values, I could use For loops but the point is for them to be variable.

Answer 1

You should try to use tail recursion as this is just a sum of the products,
see here an example for tail recursion factoring.
Public Function fact_tail(n As Double) As Double
'Tail Recursion
'fact 4 = 4 * fact 3
' = 4* 3 * fact 2
' = 4* 3 * 2 * fact 1
' = 4* 3 * 2 * 1
'fact 4 = go(4, 1)
' = go((n - 1), (a * n))
' = go((4-1),(1*4))
' = go(3, 4)
' = go(3-1, 3*4)
' = go(2, 12)
' = go(2-1, 12*2)
' = go(1, 24)
' = 4* 3 * 2 * 1 = 24
fact_tail = go_fact(n, 1)
End Function
Private Function go_fact(n, a)
If n <= 1 Then
go_fact = a
Else
go_fact = go_fact((n - 1), (a * n))
End If
End Function