--
   Jim Mack
   Twisted tees at http://www.cafepress.com/2050inc
   "We sew confusion"

-----BEGIN CODE
Private Function IsPrime(ByVal dblNumberToTest As Double) As Boolean

    Dim dblDivisor As Double
    Dim bResult As Boolean

    'We can eliminate non-whole numbers immediately.
    'Of course, we could specify the data type of the passed
    'value to be a Long, but then we can only determine primes
    'up to 2,147,483,647 whereas a double allows up to
    '1.79769313486231E308 (see Data Type Summary in VB's Help).
    'This also means that there is the potential of being able
    'to pass a non-whole number, and that's not good, since,
    'by definition, a prime number must be a whole number.
    If Not IsWholeNumber(dblNumberToTest) Then
        IsPrime = False
        Exit Function
    End If

    'We can also immediately eliminate all even numbers
    'greater than 2.  This will allow us to reduce the
    'iterations in the loop below by half.
    If dblNumberToTest = 2 Then
        IsPrime = True
        Exit Function
    Else
        If dblNumberToTest Mod 2 = 0 Then
            IsPrime = False
            Exit Function
        End If
    End If

    'Now loop through all odd numbers up to the square root
    'of the dividend.  This means the primes for any given range
    'can be determined SIGNIFICANTLY faster.  Want to see this for
    'yourself?  Uncomment the 2nd FOR statement below and comment
    'the 1st one.  Both yield the exact same results but be prepared
    'to wait a little longer using the 2nd FOR statement.

    For dblDivisor = 3 To Int(Sqr(dblNumberToTest)) Step 2
    'For dblDivisor = 3 To dblNumberToTest - 1 Step 2
        bResult = (dblNumberToTest Mod dblDivisor = 0)
        'As soon as a number divides evenly, we know it can't be
        'prime, so return false and exit
        If bResult = True Then
            IsPrime = False
            Exit Function
        End If
    Next

    'If we get completely through the loop, we've found nothing
    'that divides evenly into the number which we're testing.
    IsPrime = True

End Function

Private Function IsWholeNumber(ByVal dblNum As Double) As Boolean

    'There's probably a better way to do this using
    'math functions, but I didn't feel like figuring
    'it out.  :)

    Dim sNumber As String

    sNumber = CStr(dblNum)

    If InStr(sNumber, ".") Then
        IsWholeNumber = False
    Else
        IsWholeNumber = True
    End If

End Function
-----END CODE

Now, just loop through the range of numbers you want to determine are prime
and call IsPrime for each one.

--
Mike

<cut>

--
Mike

Dim testnumber As Long
Dim divisor As Long
Dim isprime As Boolean
Dim range As Long
Dim start As Long

start = Timer

range = 1000000

Debug.Print 2
Debug.Print 3
Debug.Print 5
Debug.Print 7

For testnumber = 9 To range Step 2

    divisor = 2
    isprime = True
    Do While divisor * divisor <= testnumber
        If testnumber Mod divisor = 0 Then
            isprime = False
            Exit Do
        End If
        divisor = divisor + 1
    Loop

    If isprime = True Then
        Debug.Print testnumber
        isprime = False
    End If
Next testnumber
Debug.Print "total time = " + Str(Int(Timer - start)) + " seconds"

You can extend this by only using previous primes as divisors as well, this
would speed things up dramatically, however, you'd need to keep these in an
array or something. With bigger numbers you could run out of ram.

--
HK

To make the comparison/test w/ a Double one would need to perform the
division explicitly in VB rather than relying on the Mod operation that
isn't valid for more precision than a Long can hold.

--

--
Mike

--

Dim testnumber As Long
Dim divisor As Long
Dim isprime As Boolean
Dim range As Long
Dim start As Long
Dim primearray() As Long
Dim newsize As Long
Dim arrayindex As Long

start = Timer
'highest number to test

range = 1000000
'get these out of the way right off
Debug.Print 2
Debug.Print 3
Debug.Print 5
Debug.Print 7

'put these first primes in the array
ReDim Preserve primearray(3)
primearray(0) = 2
primearray(1) = 3
primearray(2) = 5
primearray(3) = 7

'test only odd numbers
For testnumber = 9 To range Step 2
    'set first divisor to the first prime = 2
    arrayindex = 0
    divisor = primearray(arrayindex)
    isprime = True
    'check only to the sqare root of the testnumber (divisor * divisor) is
way faster
    ' than sqr(testnumber)
    Do While divisor * divisor <= testnumber
        If testnumber Mod divisor = 0 Then
            isprime = False
            Exit Do
        End If
        'index the array by one
        arrayindex = arrayindex + 1
        'get next prime to use as divisor
        divisor = primearray(arrayindex)
    Loop

    If isprime = True Then
        'show the prime value
        Debug.Print testnumber
        'increase array size by 1 and store next prime in new array index
        newsize = UBound(primearray, 1) + 1
        ReDim Preserve primearray(newsize)
        primearray(newsize) = testnumber
    End If
Next testnumber
'display elapsed time
Debug.Print "total time = " + Str(Int(Timer - start)) + " seconds"
--
HK

--
Mike

--

--
Mike