VB Forum / Controls / September 2008

As Paul said, no other way than to either use a lib with "long-math"
support (there should be many of these libs out there - and IIRC
e.g. Java has this builtin) - or to write your own, based on Strings
or ByteArrays...

It's somewhat like you've learned to multiply or divide in
school  ... to get an idea, what optimizations can be done
"on top" of such a scool-like algo (you don't need to process
each digit on a 10th-base)...
Here's a (somewhat rusty) LongMul-routine, wich I wrote ca.
10 years ago. It is able to multiply two 600-digit-numbers in
under 4 msec on an old PIII 500.

'****Into a Form
Option Explicit
Private Declare Function QueryPerformanceFrequency& Lib "kernel32" (X@)
Private Declare Function QueryPerformanceCounter& Lib "kernel32" (X@)

Private Sub Form_Click()
Dim R$, Factor$: Const L& = 600
 Factor = String(L, "9")
 R = LongMul(Factor, Factor)
 'small test for "pure" 9'er
 If R = String(L - 1, "9") & 8 & String(L - 1, "0") & 1 Then
   Print "correct result for 'square-niners'"
 End If
End Sub

Private Function LongMul$(S1$, S2$)
Dim i&, j&, d&, m&, JArr$(), L1&(), L2&(), Sum@(), S, t#
 t = HPTimer
 If S1 = "" Then S1 = "0"
 If S2 = "" Then S2 = "0"
 If Left(S1, 1) = "-" Then S1 = Mid(S1, 2): S = True
 If Left(S2, 1) = "-" Then S2 = Mid(S2, 2): S = Not S

'Convert to Long-Array
 m = Len(S1) + 1: d = 4: i = 0
 Do: m = m - 4: If m <= 0 Then d = 4 + m - 1: m = 1
   ReDim Preserve L1(i): L1(i) = Mid(S1, m, d): i = i + 1
 Loop Until m = 1
 m = Len(S2) + 1: d = 4: i = 0
 Do: m = m - 4: If m <= 0 Then d = 4 + m - 1: m = 1
   ReDim Preserve L2(i): L2(i) = Mid(S2, m, d): i = i + 1
 Loop Until m = 1
Print "Transform to LongArr: " & CLng((HPTimer - t) * 1000)

'Multiply and Group-accumulation
t = HPTimer
 ReDim Sum(UBound(L1) + UBound(L2))
 For i = 0 To UBound(L1)
   For j = 0 To UBound(L2)
     Sum(i + j) = Sum(i + j) + L1(i) * L2(j)
   Next j
 Next i
Print "Multiply: " & CLng((HPTimer - t) * 1000)

'Reconvert to String-Parts
t = HPTimer
 ReDim JArr(UBound(Sum))
 For i = 0 To UBound(Sum) - 1
   JArr(UBound(Sum) - i) = Right("000" & Sum(i), 4)
   Sum(i + 1) = Sum(i + 1) + Int(Sum(i) / 10000)
 Next i
 JArr(UBound(Sum) - i) = IIf(S, "-", "") & Sum(i)
Print "Prepare for Output: " & CLng((HPTimer - t) * 1000)

'Join&Exit
t = HPTimer
 LongMul = Join(JArr, "")
Print "Join&Exit: " & CLng((HPTimer - t) * 1000)
End Function

Private Function HPTimer#()
Dim X@: Static Frq@
 If Frq = 0 Then QueryPerformanceFrequency Frq
 If QueryPerformanceCounter(X) Then HPTimer = X / Frq
End Function

Olaf

Use logs Bob, e.g the following calculates your 1000!

Dim i As Integer, fRes As Double

i = 1000
Do While i > 1
   fRes = fRes + Log(i) / Log(10)
   i = i - 1
Loop
Debug.Print "Answer = " & 10 ^ (fRes - Int(fRes)) & "E" & Int(fRes)

   Tony Proctor