Simple Program for Prime numbers...stumped

[pogge123]

Hey Everyone so basically what im making is a form with just one button and 2 text boxes.
The purpose of it is that when i click the button 2 random PRIME numbers between 100-999 will show up in the boxes.

With my code i have now, the numbers that show up in the boxes arnt always PRIME and i cant figure out how to do this.

My current Code is.......

Public Class Form1

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Randomize()
    Dim p, q, i, j As Integer
    Dim IsPrime As Boolean
    For p = 100 To 999
      IsPrime = True
      For i = 2 To Int(Math.Sqrt(p))
        TextBox1.Text = CStr(Int(Rnd() * p))
        If p Mod i = 0 Then
            IsPrime = False
        End If

      Next
    Next

    If IsPrime Then
      TextBox1.Text = p
    End If


    For q = 100 To 999
      IsPrime = True
      For j = 2 To Int(Math.Sqrt(q))
        TextBox2.Text = CStr(Int(Rnd() * q))
        If q Mod j = 0 Then
          IsPrime = False
        End If
      Next



      If IsPrime Then
        TextBox2.Text = q
      End If
    Next
  End Sub
End Class

ANy help is greatly appreciated
thanks

[dglienna]

First, that looks like VB6 code. Second, use CODE TAGS so we can read your indentations

' Like this

Lastly, looks like two NEXT's with one FOR

[Cimperiali]

here a way to make it.

in a Class named PrimeNumbers

Public Class PrimeNumbers

    Public Shared Function GetARandomPrimeNumber(ByVal lowerLimit As Integer, ByVal upperLimit As Integer) As Integer
        Dim tikcs As Integer = My.Computer.Clock.TickCount
        Dim rd As New Random(tikcs)
        Debug.Print(tikcs.ToString())


        Dim retval As Integer = rd.Next(lowerLimit, upperLimit)
        Do
            If IsPrime(retval) Then Exit Do
            If retval < upperLimit Then
                retval = retval + 1
            Else
                retval = rd.Next(lowerLimit, upperLimit)
            End If

        Loop

        Return retval

    End Function
    Public Shared Function IsPrime(ByVal theNumber As Integer) As Boolean
        'this function came from
        'http://www.devx.com/vb2themax/Tip/19051
        'ever read a book of Francesco Balena, Marco Bellinaso, Dino Esposito,...?
        ' manually test 2 and 3
        If theNumber > 3 Then
            If theNumber Mod 2 = 0 Then Exit Function
            If theNumber Mod 3 = 0 Then Exit Function
        End If
        ' we can now avoid to consider multiples
        ' of 2 and 3. This can be done really simply
        ' by starting at 5 and incrementing by 2 and 4
        ' alternatively, that is:
        '    5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, ...
        Dim divisor As Integer
        Dim increment As Integer
        Dim maxDivisor As Integer

        divisor = 5
        increment = 2
        ' we don't need to go higher than the square
        ' root of the number
        maxDivisor = CInt(Math.Sqrt(theNumber)) + 1

        Do Until divisor > maxDivisor
            If theNumber Mod divisor = 0 Then Exit Function
            divisor = divisor + increment
            ' this modifies 2 into 4 and viceversa
            increment = 6 - increment
        Loop

        ' if we get here, the number is prime
        Return True


    End Function
End Class

in a form, with a label named lblInfo
a textBox named txtPrimeOne
a textbox named txtPrimeTwo
a button named btnGetPrime

Public Class frmPrime

    Private Sub frmPrime_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        lblInfo.Text = "push button and get two random prime numbers between 100 and 999. Note: I did not make this smart enough to give you ALWAYS two different numbers (= you could get 101 as first and as second prime number, but I added some logic to avoid it happeniong too often...)"
    End Sub

    Private Sub btnGetPrime_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnGetPrime.Click
        txtPrimeOne.Text = PrimeNumbers.GetARandomPrimeNumber(100, 999)
        'need to pause at least 20 millisecond, or you will
        'be likely get same numbers...
        System.Threading.Thread.Sleep(25)
        txtPrimeTwo.Text = PrimeNumbers.GetARandomPrimeNumber(100, 999)

    End Sub
End Class

now prepare yourself to teacher questions:
what is "Shared" for?
What is "Math" ? What is "My"?
the way to check if a numer is prime is retrieved from
Euclid's theorem. Can you show it to us?
...and so on...

[dglienna]

Definitely going to stand out in the class... You *will* need to explain it (and googling it will return to this site...)

' 5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, ...

[Cimperiali]

by the way...
the correct "good developer" way to get some numbers when required, when the data basis is few (from 100 to 999 it is few) is: prepare all the possible answers, then choose the two required randomly.
Ie:
load an array of prime numbers from 101 to 997 (100 and 998 and 999 are not prime numbers), only once. Then, on any request, choose two random numbers from that array (that is: choose a random position in the array twice and get the number stored; it is better than choose a random number between 100 and 999 and decide after if it is prime, and discard it if not...)