Form1.vb

Public Class Form1
    'Global variables
    Public value1 As Double
    Public value2 As Double
    Public previousOperator As Char
    Public currentOperator As Char
    Public operated As Boolean
    Public answerPresented As Boolean
    Public buttonClicked As String 'keep trak of the currently clicked button

    'Initializing variables and user interface
    Public Sub New()
        InitializeComponent()
        value1 = 0
        value2 = 0
        operated = False
        answerPresented = False
        previousOperator = "N"
        currentOperator = "N"
    End Sub

    Private Sub numButtons_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button0.Click, Button1.Click, Button2.Click, Button3.Click, Button4.Click, Button5.Click, Button6.Click, Button7.Click, Button8.Click, Button9.Click
        ShowValue(sender)
    End Sub

    'Clear the text box
    Private Sub ShowValue(ByVal button As Button)
        buttonClicked = button.Text 'update the currently clicked button
        If answerPresented Then
            Double.TryParse(Me.TextBox1.Text, value1)
            Me.TextBox1.Text = button.Text
            answerPresented = False
        Else
            Me.TextBox1.Text = Val(Me.TextBox1.Text & button.Text)
        End If
    End Sub

    Private Sub ButtonClear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonClear.Click
        ClearAll()
    End Sub
   
    Private Sub optBtns_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonPlus.Click, ButtonMinus.Click, ButtonMultiply.Click, ButtonDivide.Click
       If isPreviouslyClicked(sender) Then
            Return
        End If

        If currentOperator <> "N" Then
            previousOperator = currentOperator
            currentOperator = DirectCast(sender, Button).Text
            Double.TryParse(Me.TextBox1.Text, value2)
            calculate(value1, value2, previousOperator)
            clearTextBox_UpdateDisplay(currentOperator)
            Me.TextBox1.Text = value1
            answerPresented = True
            Return
        End If

        Double.TryParse(Me.TextBox1.Text, value1)
        If Me.TextBox1.Text <> "" And value1 <> 0 Then
            currentOperator = DirectCast(sender, Button).Text
            clearTextBox_UpdateDisplay(currentOperator)
        End If
    End Sub
    'Calclation of 4 major mathematics operators
    Public Sub calculate(ByRef part1 As Double, ByVal part2 As Double, ByVal [operator] As String)
        Select Case [operator]
            Case "+"
                part1 += part2
            Case "-"
                part1 -= part2
            Case "/"
                part1 /= part2
            Case "*"
                part1 *= part2
        End Select
        value2 = 0
    End Sub

    Private Sub ButtonRoot_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonRoot.Click
        If isPreviouslyClicked(sender) Then
            Return
        End If
        If Me.TextBox1.Text <> "" Then
            Double.TryParse(Me.TextBox1.Text, value2)
            Me.TextBox1.Text = Math.Sqrt(value2)
        End If
    End Sub

    Private Sub ButtonOneoverX_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonOneoverX.Click
        If isPreviouslyClicked(sender) Then
            Return
        End If
        If Me.TextBox1.Text <> "" Then
            Double.TryParse(Me.TextBox1.Text, value2)
            Me.TextBox1.Text = 1 / value2
            answerPresented = True
        End If
    End Sub

    Private Sub ButtonDot_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonDot.Click
        If isPreviouslyClicked(sender) Or Me.TextBox1.Text.Contains(".") Then
            Return
        Else
            Me.TextBox1.Text = Me.TextBox1.Text + "."
        End If
    End Sub
    'this method clear the text box and updates the results on the lable
    Private Sub clearTextBox_UpdateDisplay(ByVal currentOperator As Char)
        Dim s As String = Me.Label1.Text.ToString()
        If s.Length = 0 And Me.TextBox1.Text <> "" Then
            Me.Label1.Text = Me.Label1.Text & Me.TextBox1.Text & " " & currentOperator
            Me.TextBox1.Text = ""
            Return
        End If

        If Me.TextBox1.Text <> "" Then
            Me.Label1.Text = Me.Label1.Text & Me.TextBox1.Text & " " & currentOperator
            Me.TextBox1.Text = ""
        End If
    End Sub

    Private Sub ButtonEquals_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ButtonEquals.Click
        If isPreviouslyClicked(sender) Then
            Return
        End If

        If currentOperator <> "N" Then
            previousOperator = currentOperator
            Double.TryParse(Me.TextBox1.Text, value2)
            calculate(value1, value2, previousOperator)
            Me.Label1.Text = ""
            Me.TextBox1.Text = value1
            answerPresented = True
        Else
            Me.Label1.Text = ""
            Double.TryParse(Me.TextBox1.Text, value1)
        End If
        resetVariables()
    End Sub
    'Reset all the global variables and clear text
    Private Sub ClearAll()
        Me.TextBox1.Text = ""
        Me.Label1.Text = ""
        operated = False
        answerPresented = False
        value1 = 0
        value2 = 0
        currentOperator = "N"
        previousOperator = "N"
        buttonClicked = ""
    End Sub
    'Rest application variables without clearing the displayed output
    Private Sub resetVariables()
        operated = False
        answerPresented = False
        value1 = 0
        value2 = 0
        currentOperator = "N"
        previousOperator = "N"
        buttonClicked = ""
    End Sub

    'check whether the button clicked is the same as previous 
    'if yes return true
    Private Function isPreviouslyClicked(ByVal sender As Button) As Boolean
        If buttonClicked = sender.Text Then
            buttonClicked = sender.Text 'update the currently clicked button
            Return True
        Else
            buttonClicked = sender.Text 'update the currently clicked button
            Return False
        End If
    End Function
End Class