Combines multiple recursive generator vba [duplicate]

Question

I have run into an overflow error in Excel VBA and cannot find my way around it. While Microsoft's documentation indicates that the range for doubles should reach ~1.8E308, I am receiving an overflow error for numbers significantly lower than that threshold. My code is as follows:

Public Function Fixed_Sample_Nums(ByVal n As Long, seed As Long) As Double()

    Dim x() As Double, y() As Double, i As Long
    ReDim y(1 To n)
    ReDim x(1 To n)

    x(1) = (CDbl(48271) * seed) Mod CDbl(2 ^ 31 - 1)

    For i = 2 To n
        x(i) = (CDbl(48271) * CDbl(x(i - 1))) Mod (CDbl(2 ^ 31 - 1))
        y(i) = CDbl(x(i)) / CDbl(2 ^ 31 - 1)
    Next i

    Fixed_Sample_Nums = y

End Function

'I receive the error in the first iteration of the for loop with 
'seed equal to any value >= 1 (i.e. w/ seed = 1): 

Debug.Print((CDbl(48271) * CDbl(48271)) Mod (CDbl(2 ^ 31 - 1))) 

'results in an overflow error 

I am attempting to create a pseudo-random number generator that can take in any 'seed' value up to and including 2 ^ 31 - 1. The for loop should be able to iterate at least 9,999 times (i.e. n = 10000). If the overflow error is not encountered within the first few iterations, it most likely will not be encountered for any subsequent iteration.

As you can see, I am converting each integer to a double before any calculation. I am aware of the fact that arrays substantially increase the byte size of the calculation, but that does not appear to be the current issue as I directly copied the example calculation above into the immediate window and still received the overflow error. My attempts to find a solution online have resulted in no avail, so I would really appreciate any input. Thanks in advance!

Answer 1

Try using Chip Pearson's XMod function:

x(i) = XMod((CDbl(48271) * seed), CDbl(2 ^ 31 - 1))

As he notes:

You can also get overflow errors in VBA using the Mod operator with very large numbers. For example,

Dim Number As Double
Dim Divisor As Double
Dim Result As Double

Number = 2 ^ 31
Divisor = 7
Result = Number Mod Divisor ' Overflow error here.

Code for the function:

Function XMod(ByVal Number As Double, ByVal Divisor As Double) As Double
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' XMod
' Performs the same function as Mod but will not overflow
' with very large numbers. Both Mod and integer division ( \ )
' will overflow with very large numbers. XMod will not.
' Existing code like:
'       Result = Number Mod Divisor
' should be changed to:
'       Result = XMod(Number, Divisor)
' Input values that are not integers are truncated to integers. Negative
' numbers are converted to postive numbers.
' This can be used in VBA code and can be called directly from 
' a worksheet cell.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
    Number = Int(Abs(Number))
    Divisor = Int(Abs(Divisor))
    XMod = Number - (Int(Number / Divisor) * Divisor)
End Function

Additional details:

http://www.cpearson.com/excel/ModFunction.aspx