Calculating Prime Numbers in Visual Basic 6

Prime numbers have important applications in mathematics and computer programming. I met a mathematician on a return trip from Montreal, Canada on his way to a mathematical society meeting in the "Big Easy". Having studied mathematics a bit (and realizing I could work for 35 years and never publish a new theorem), I asked him what was hot in mathematics. He said cryptography. Large prime numbers are difficult to calculate for super computers, which makes them great as keys for encrypted transactions. And, just recently someone asked me about calculating primes in a way that made it sound like a computer science course project. So, here we are.

I thought of an algorithm and searched on the Internet for a quick answer. It just so happens that my algorithm is a pretty close approximation of the "Sieve of Eratosthenes". Eratosthenes was a respected scholar and librarian in Alexandria around 250 BC. (Having to go back so far for the originator suggests to me that we would all be more productive with a lot less television. Newton discovered Calculus because there were no sitcoms.)

A prime number is a number that is divisible by itself and 1 only. The Sieve of Eratosthenes instructs that a prime number can be derived by testing all primes less than or equal to the square root of a number. This works because all integers are a product of primes and no prime number will be divisible by a number greater than its square root without a number lesser than its square root also being a divisor. Thus if, we test all primes less than the square root of a number then we can determine if a number is prime.

If we seed our list of primes with 2, a known prime, then we can calculate all of the primes between 2 and n. The limitation to our algorithm will be the largest number we can store of a certain type. In Visual Basic 6 we can use a Long Integer which will allow us to test all of the primes up to about 2.1 billion. The simple user interface for the sample application can be seen in figure 1. Listing 1 provides the basic solution. The solution is described after the listing.

Picture missing

Figure 1: The user interface for the Sieve of Eratosthenes algorithm.

Listing 1: A Windows application in VB6 that calculates prime numbers up to n using the Sieve of Eratosthenes.

1:  Option Explicit
2:  Private Primes() As Long
3:  Private Function GetMax() As Long
4:  On Error GoTo Handler
5:    GetMax = CLng(Text1.Text)
6:    Exit Function
7:  Handler:
8:    GetMax = 0
9:    Err.Raise -1, "GetMax", "Not a number"
10: End Function
13: Private Function IsPrime(ByVal Number As Long)
15:   IsPrime = False
16:   Dim I As Long
17:   For I = LBound(Primes) To UBound(Primes)
18:     DoEvents
20:     Call UpdateStatus(Number, Primes(I))
21:     If (Number Mod Primes(I) = 0) Then Exit Function
22:     If (Primes(I) >= Sqr(Number)) Then Exit For
23:   Next
24:   IsPrime = True
25: End Function
27: Private Sub UpdateStatus(ByVal Number As Long, ByVal Prime As Long)
28:   If (Check1.Value = 0) Then Exit Sub
29:     StatusBar1.SimpleText = "Testing " & Number & _
30:       " is divisible by Prime(" & Prime & ") = " & _
31:       (Number Mod Prime = 0)
32: End Sub
34: Private Sub BuildPrimes(ByVal Max As Long)
36:   If (Max < 3) Then Exit Sub
37:     Dim I As Long
39:   For I = 2 To Max
40:     If (IsPrime(I)) Then
41:       ReDim Preserve Primes(UBound(Primes) + 1) As Long
42:       Primes(UBound(Primes)) = I
43:     End If
44:   Next
46: End Sub
48: Private Sub Command1_Click()
49: On Error GoTo Handler
51:   Initialize
52:   Dim Value As Variant
53:   BuildPrimes (GetMax())
55:   For Each Value In Primes
56:     List1.AddItem (Value)
57:   Next
59:   Exit Sub
60: Handler:
61:   MsgBox Err.Description, vbCritical, Err.Source
62: End Sub
64: Private Sub Initialize()
65:   ReDim Primes(0)
66:   Primes(0) = 2
67:   List1.Clear
68: End Sub
70: Private Sub Form_Load()
71:   Initialize
72: End Sub

The sieve is comprised of thirteen lines of code on lines 13 through 25. The rest of the code supports simple user interface interactions. We initialize an array of primes with a single prime, 2, on line 66. The IsPrime test begins by assuming the Number argument is not a prime. We use a single loop, looping through all of the primes less than the square root of Number. (The loop is set up on line 17.) We invoke DoEvents to be kind to the user on line 18; this prevents the GUI from appearing to be frozen. On line 20 we update the StatusBar showing the current test value (see figure 1). On line 21 we divide the Number by the candidate known prime on line 21 and exit if Number is divisible by Primes(I), the test prime. If Number is divisible by any prime than Number is not prime. The second test evaluates the candidate prime against the square root of the Number. If we have reached the square root of the Number without finding a divisor then Number is prime. We branch out of the For Loop and return True.

The Sieve of Eratosthenes works as long as we calculate all primes from 2 to n, which ensures that we have all possible prime divisor candidates to check.

About the Author

Paul Kimmel is a freelance writer for and Look for his recent book, Visual Basic .Net Unleashed, at a bookstore near you. Paul Kimmel is available to help design and build your .NET solutions and can be contacted at


  • not helpful

    Posted by treyee on 01/05/2016 08:01am

    not a helpful website

  • What..?

    Posted by Mitchell on 02/01/2013 12:13pm

    This can be done in 15 lines using two variables and two loops... Dim i As Integer Dim j As Integer Cls For i = 0 To 100 If i = 2 Then Print 2 End If For j = 2 To i - 1 If i Mod j = 0 Then Exit For ElseIf j = (i - 1) Then Print i End If Next j Next i

    • prime numbers between 2 and 50

      Posted by knight on 04/11/2015 10:58pm

      am trying to write between 2 and 50 but this is long . how do i go about it ??

  • wat the heck

    Posted by Wat on 01/10/2013 01:56pm

    wat the heck this is terrible!


    Posted by k2m on 10/20/2004 02:31pm

    Dear Sir, I would like to know the following small program codes in Visual Basis . These are very easy for you but for me it is really hard and I cant think any more . If you have a little bit free time please show me or guide to me how to solve these programs . Yours truly, Mr. KK MYINT 1. Print the series of prme number depends on the mumber of first prime numbers the user want to see. Eg. If user key in 5, the program need to print 2 3 5 7 11 2. Check the number values that user key in is even or odd. 3.Write a program to print palindrome depends on the users key in number. 4.Writ a program to print the value of factorial. Eg. If user key in 5, the program excute like 5*4*3*2*1 and the result value will be 120. 5. Write a program to change the hours, minutes, seconds format for the number of seconds , which is keyed by user. Eg. If user key in 4552 seconds , the printed result should be like : 1 hour :15 min: 52 sec 6.Write a program to find out the data if user key in the start date and the number of days. Eg if user key in 12/10/04 as start date and 90 days for the number of days. The print output should be 10/01/05 7. writ a program to show the pictures in a folder as slide show by using the drive ,directory and file controls in visual basic. 8. write a program which can calculate the basic operation as the calculator as reading values and addition , multiplication, division , and subtraction. There must be one function , which can clear out the value as C in calculator.

    • iuhyh

      Posted by tkhan420 on 03/07/2005 09:56pm


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