' IsPrime Example

' Input a number.  Output whether the number is prime or not.
' Recall that a number is prime if its only divisors are 1 and
' the number itself.

Public Class Form1
    Inherits System.Windows.Forms.Form

#Region " Windows Form Designer generated code "

    Public Sub New()
        MyBase.New()

        'This call is required by the Windows Form Designer.
        InitializeComponent()

        'Add any initialization after the InitializeComponent() call

    End Sub

    'Form overrides dispose to clean up the component list.
    Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing Then
            If Not (components Is Nothing) Then
                components.Dispose()
            End If
        End If
        MyBase.Dispose(disposing)
    End Sub

    'Required by the Windows Form Designer
    Private components As System.ComponentModel.IContainer

    'NOTE: The following procedure is required by the Windows Form Designer
    'It can be modified using the Windows Form Designer.  
    'Do not modify it using the code editor.
    Friend WithEvents btnCheck As System.Windows.Forms.Button
    Friend WithEvents Label1 As System.Windows.Forms.Label
    Friend WithEvents lblOutput As System.Windows.Forms.Label
    Friend WithEvents txtInt As System.Windows.Forms.TextBox
    <System.Diagnostics.DebuggerStepThrough()> Private Sub InitializeComponent()
        Me.txtInt = New System.Windows.Forms.TextBox
        Me.btnCheck = New System.Windows.Forms.Button
        Me.Label1 = New System.Windows.Forms.Label
        Me.lblOutput = New System.Windows.Forms.Label
        Me.SuspendLayout()
        '
        'txtInt
        '
        Me.txtInt.Font = New System.Drawing.Font("Microsoft Sans Serif", 14.25!, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, CType(0, Byte))
        Me.txtInt.Location = New System.Drawing.Point(24, 16)
        Me.txtInt.Name = "txtInt"
        Me.txtInt.Size = New System.Drawing.Size(144, 29)
        Me.txtInt.TabIndex = 0
        Me.txtInt.Text = ""
        '
        'btnCheck
        '
        Me.btnCheck.Font = New System.Drawing.Font("Microsoft Sans Serif", 14.25!, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, CType(0, Byte))
        Me.btnCheck.Location = New System.Drawing.Point(184, 16)
        Me.btnCheck.Name = "btnCheck"
        Me.btnCheck.Size = New System.Drawing.Size(176, 32)
        Me.btnCheck.TabIndex = 1
        Me.btnCheck.Text = "Check if Prime"
        '
        'Label1
        '
        Me.Label1.Font = New System.Drawing.Font("Microsoft Sans Serif", 12.0!, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, CType(0, Byte))
        Me.Label1.Location = New System.Drawing.Point(64, 56)
        Me.Label1.Name = "Label1"
        Me.Label1.Size = New System.Drawing.Size(72, 24)
        Me.Label1.TabIndex = 2
        Me.Label1.Text = "Integer"
        '
        'lblOutput
        '
        Me.lblOutput.Font = New System.Drawing.Font("Microsoft Sans Serif", 12.0!, System.Drawing.FontStyle.Bold, System.Drawing.GraphicsUnit.Point, CType(0, Byte))
        Me.lblOutput.Location = New System.Drawing.Point(32, 88)
        Me.lblOutput.Name = "lblOutput"
        Me.lblOutput.Size = New System.Drawing.Size(264, 32)
        Me.lblOutput.TabIndex = 3
        '
        'Form1
        '
        Me.AutoScaleBaseSize = New System.Drawing.Size(5, 13)
        Me.ClientSize = New System.Drawing.Size(424, 134)
        Me.Controls.Add(Me.lblOutput)
        Me.Controls.Add(Me.Label1)
        Me.Controls.Add(Me.btnCheck)
        Me.Controls.Add(Me.txtInt)
        Me.Name = "Form1"
        Me.Text = "IsPrime Example"
        Me.ResumeLayout(False)

    End Sub

#End Region

    Private Sub btnCheck_Click(ByVal sender As System.Object, _
        ByVal e As System.EventArgs) Handles btnCheck.Click

        Dim n As Integer

        If Not IsNumeric(txtInt.Text) Then
            lblOutput.Text = "Not numeric."
            Exit Sub
        End If

        If txtInt.Text.IndexOf(".") > -1 Then
            lblOutput.Text = "Decimal point not allowed."
            Exit Sub
        Else
            n = txtInt.Text
        End If

        ' In case of negative input, convert to positive.
        n = Math.Abs(n)

        If IsPrime(n) Then
            lblOutput.Text = "Is prime."
        Else
            lblOutput.Text = "Is not a prime."
        End If
    End Sub

    Private Function IsPrime(ByVal x As Integer) As Boolean
        Dim factor As Integer

        ' Zero and one are not considered prime.
        If x = 0 Or x = 1 Then
            Return False
        End If

        ' Check possible factors of x.  If a factor is found,
        ' x is not prime.
        For factor = 2 To x - 1
            If x Mod factor = 0 Then
                Return False
            End If
        Next
        Return True
    End Function
End Class