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General Tip

How to Determine Whether a Specified Number is Prime Visual Basic Code
Tested with VB6
Courtesy of Marshall Ellis

A prime number is one that only divides evenly by 1 and itself.  For instance, 3, 5 and 11 are all primes.  The key to determining whether a number is prime is to find every factor, which is time-consuming when you get into larger numbers. The example given here is a slow, "brute force" method, and there are alternatives for larger numbers. I am aware of that and leave it as an exercise for the reader.

The sample project described below centers around creating a single function that returns true or false after testing the number passed in.  Using the same function, we'll have a command button that checks a single number, and a command button that lists all primes from 1 to the specified number.

1 Create a new standard VB project.

2 In the General Declarations section of the form code, or in a module, place the following:

Private Function AmIPrime(CheckMe As Long) As Boolean

  AmIPrime = True

  '1 is always prime so let's handle it here
  If
CheckMe = 1 Then
    AmIPrime = True
    Exit Function
  End If

  'Positive numbers only
  If CheckMe <= 0 Then
    AmIPrime = False
    Exit Function
  End If

  'Here's where the real action takes place
  'Note that we go from 2 to 1 less than the source
  'This is because 1 and the source are known to be _
   factors already.  Non-prime means there are others.

  For i = 2 To CheckMe - 1
    If CheckMe Mod i = 0 Then
      AmIPrime = False
     
'Get out as soon as we know it's not prime
      'This speeds things dramatically

      Exit Function

    End If
  Next

End Function

3 On the form place a textbox, two command buttons, a label, and a listbox.  We'll assume the default names for them.

4 The code for the first command button's click event should look something like this:

Private Sub Command1_Click()
  'Grab the number to test from the textbox
  Dim lngSource As Long
  lngSource = Text1.Text

  'Get the result by sending the number _
   to test to the AmIPrime function.

  Dim boolResult As Boolean
  boolResult = AmIPrime(lngSource)

  'This just displays the result on the label. _
  You could also use a msgbox, or simply use _
  the result behind the scenes without _
  displaying it.

  If boolResult = True Then
    Label1.Caption = Text1.Text & " IS Prime!"
  Else
    Label1.Caption = Text1.Text & " isn't Prime!"
  End If

End Sub



5 The code for the second command button's click event should look something like this:

Private Sub Command2_Click()
  Dim lngCounter As Long
  Dim lngSource As Long
  Dim boolResult as Boolean

  'Get the upper value of the range to test for primes. _
   Lower bound is 1 here.  You could change the code to _
   check a range starting with something other than 1.

  lngSource = Text1.Text

  'Uncommenting the two debug lines in the IDE will _
   allow you optionally to check how long it takes.
  'Debug.Print "Start time: " & Now


  'Pass every value in the range to test for prime.
  For
lngCounter = 1 To lngSource
    boolResult = AmIPrime(lngCounter)
    If boolResult = True Then
      'If it's prime we'll toss it in the list.
      List1.AddItem lngCounter
      'The DoEvents can be used to ensure results _
       display immediately, but slows things down.
      'DoEvents
    End If
  Next

  'Debug.Print "End time: " & Now

End Sub

6 Run the program, enter a number in the textbox and test Command1.  Now test Command2 to list the primes from 1 to your number in the textbox.  Note that larger numbers will take significant lengths of time to process (1 to 1,000,000 took 7 minutes, 26 seconds on a Pentium III 450 when tested, and returned 11,022 results).  Also note that the program concerns itself only with positive integers.

 

 

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