Visual Basic: How can i display the prime numbers between 1 and the inputted number

Question

Hello everyone so i'm trying to find the prime numbers of any input. I want them to be in a listbox and the input in a text box. I would like to use two arguments but i don't know how to. this is the code i have i need dire help. I am not the best at visual basic i just need some guidance. My code isn't working but display a drop down box when i press display.

Public Class Form1
    Private Sub Button3_Click_1(sender As Object, e As EventArgs) Handles Button3.Click

        Dim prim As Integer
        Dim test As Integer
        Dim imPrime As Boolean = False
        prim = CInt(txtNum.Text)
        test = prim

        If prim = 1 Then
            imPrime = False
            MessageBox.Show("Enter a number greater than one please")
        Else
            Do While prim >= 2
                For i As Integer = 2 To prim
                    If prim Mod i = 0 Then
                        imPrime = False
                        Exit For
                    Else
                        imPrime = True
                        lstPrime.Items.Add(prim)
                    End If
                Next
            Loop
        End If
        If imPrime = True Then
            lstPrime.Items.Add(prim)
        End If

    End Sub
End Class

John Kennedy M. Aquino · Answer 1 · 2017-07-05T11:36:32.467

This is my fastest VBA code to generate prime numbers between two numbers.

The generated prime numbers are put in clipboard. You will need to open your ms office word and type Ctrl+V to view all the generated prime numbers.

Sub generateprimenumbersbetween()
Dim starting_number As Long
Dim last_number As Long
Dim primenumbers As Variant
Dim a As Long
Dim b As Long
Dim c As Long
starting_number = 1 'input value here
last_number = 1000000 'input value here
primenumbers = ""
For a = starting_number To last_number
    c = Round(Sqr(a)) + 1
    For b = 2 To c
        If a = 1 Or (a Mod b = 0 And c <> b) Then
            Exit For
        Else
            If b = c Then
                primenumbers = primenumbers & " " & a
                Exit For
            End If
        End If
    Next b
Next a
Dim answer As DataObject
Set answer = New DataObject
answer.SetText primenumbers
answer.PutInClipboard
End Sub

score 1 · Accepted Answer · edited May 23 '17 at 12:12

I think the while loop is not working as you intend. You need two loops, the first one counting up to the possible prime, and an inner one counting up to the counter in the outer loop.

You can find examples everywhere... here's one implemented in C#, but since your question was specifically about a listbox, I've translated it to VB.

Public Class Form1

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        calculatePrimes()
    End Sub

    Private Sub calculatePrimes()

        Dim prim As Integer
        Dim count As Integer = 0

        prim = CInt(Me.TextBox1.Text)
        If prim < 3 Then
            MsgBox("Please enter a bigger number")
            Return
        End If

        Me.ListBox1.Items.Clear()

        For i As Integer = 1 To prim
            Dim isPrime As Boolean = True
            For j As Integer = 2 To i
                If (i Mod j <> 0) Then count = count + 1
            Next
            If count = (i - 2) Then Me.ListBox1.Items.Add(i)
            count = 0
        Next

    End Sub
End Class

(This assumes you have a textbox for input called TextBox1 and a listbox for display called ListBox1)

To verify a prime, you only need to check up to a square root of the number. [Why do we check upto the square root of a prime number to determine if it is prime?](http://stackoverflow.com/questions/5811151/why-do-we-check-upto-the-square-root-of-a-prime-number-to-determine-if-it-is-pri) — Victor Zakharov, Mar 29 '15 at 19:03
Ah yes. Thanks for the reminder. I was caught up in why the OP's code wasn't working as designed and did it the hard way. — Gojira, Mar 29 '15 at 19:07
Thank you man i understand what i was doing wrong :) ur awesome — Wes, Mar 29 '15 at 20:06

Visual Basic: How can i display the prime numbers between 1 and the inputted number

2 Answers2