Prime number test Mod overflow error

[whyhellothere]

I'm trying the google, "find the first 10 digit prime in the digits of PI problem" and I'm stuck. I have the logic for it but i keep running into mod errors.
Some of this code is lifted as I crashed while doing my last itteration and am too fried to rewrite. Any help is appreciated.

Sub prime()
Dim N As Double
Dim t As Double
Dim i As Double
Dim m As String
Dim q As Double
i = 0
m = Range("A1")
q = 10
For q = 10 To 100
t = Mid(m, 2, q)
Range("A2") = t

Dim bPrime As Boolean
For N = 2 To t ' Lower and Upper are entered by user
bPrime = True ' continue testing as long as true
Dim F As Integer
For F = 2 To t - 1 ' test all factors from 2 to N-1
If t Mod F = 0 Then
bPrime = False ' divisible, therefore composite
Exit For ' no more testing required
End If
Next
If bPrime = True Then ' If never got a factor, Prime found
Cells(5, 1).Value = t
End If
Next
Next
End Sub

 

[shg]

Welcome to the board.

Here's the function I use for testing primality:

Function FirstFactor(dNum As Double) As Variant
    ' shg 2006-0923
    '     2009-1003 fixed bug so if dNum Mod 15 = 0, returns 3 as a factor, not 5
    ' Returns:      if dNum is:
    '   #VALUE!       < 2
    '   "Too big!"    > 1E+15
    '   #VALUE!       <> Int(dNum)
    '   1st factor    otherwise (=dNum if prime)
 
    Dim dQuo        As Double
    Dim dFac        As Double
 
    If Int(dNum) <> dNum Then
        FirstFactor = CVErr(xlErrValue)
    ElseIf dNum > 1E+15 Then
        FirstFactor = "Too big!"
    ElseIf Int(dNum / 2#) = dNum / 2 Then
        FirstFactor = 2#
    Else
        For dFac = 3# To Int(Sqr(dNum)) Step 2#
            dQuo = dNum / dFac
            If Int(dQuo) = dQuo Then
                FirstFactor = dFac
                Exit Function    '------------------------------------->
            End If
        Next dFac
        FirstFactor = dNum
    End If
End Function

EDIT: You can't use MOD because it converts its arguments to Longs, which will overflow for most 10-digit numbers.

 

[mirabeau]

Hi,

I googled your bit in quotes and didn't come up with anything except your post in this thread.

So I don't know what your specific problem actually is, thus can't offer much towards solving it.

But I put int(pi()*10^14) in Range("A1") and ran your code but didn't attain enlightenment.

The general problem of determining if a 10 digit number is prime or not is pretty straightforward and needn't take long, but I don't know what your problem actually is ...

Like, if you want primes, and prime factors, for numbers from 10^9+101 to 10^9+130, the below took me about 0.05secs to calculate and display. Primes only would take less time.
1000000101 3 333333367
1000000102 2 83 6024097
1000000103 1000000103(prime)
1000000104 2 2 2 3 41666671
1000000105 5 13 15384617
1000000106 2 7 47 521 2917
1000000107 3 3 3 211 257 683
1000000108 2 2 31 8064517
1000000109 20063 49843
1000000110 2 3 5 37 163 5527
1000000111 11 90909101
1000000112 2 2 2 2 17 17 216263
1000000113 3 7 47619053
1000000114 2 500000057
1000000115 5 19 10526317
1000000116 2 2 3 3 27777781
1000000117 103 109 89071
1000000118 2 13 23 233 7177
1000000119 3 6421 51913
1000000120 2 2 2 5 7 3571429
1000000121 3463 288767
1000000122 2 3 11 2089 7253
1000000123 1000000123(prime)
1000000124 2 2 397 629723
1000000125 3 3 5 5 5 67 13267
1000000126 2 12251 40813
1000000127 7 29 41 137 877
1000000128 2 2 2 2 2 2 2 3 2604167
1000000129 17 58823537
1000000130 2 5 827 120919