Visual Basic计算器的逻辑

Question

I'm working on a simple calculator program. I know that I am missing something. I've followed steps that have been provided, but I have already pointed out several inconsistencies previously so I am sure that this is the case again or information is just plain "missing". I have two issues. One, is that when I press a second number before pressing an operator button, the second (and all consecutive number buttons pressed) concatenates twice. Meaning, if I press "1" then "2", I get "122". If I then press "3" I get "12233". The second problem I have is that I cannot figure out how to code the equal button. I thought that the variable dblCurrentNumber.ToString() would be what I assign to the Equals_click . But, that does not work. I'm not looking for anyone to tell me the answer or complete my assignment for me. That would defeat the purpose of learning. I'm just looking for a clue as to what I'm missing.

Option Explicit On
Option Strict On
Imports System.Math

Public Class Calculator
    'declare the global variables here
    Dim dblResult As Double 'Stores the result of an operation
    Dim dblCurrentNumber As Double 'Stores the current number in the display
    Dim dblMemory As Double 'Stores a value placed in memory by the user
    Dim blnStartNewNumber As Boolean 'Determines whether a new number should be started in the display
    Dim strLastMathOperator As String 'Stores the operation selected by the user; possible values include Clear, Add, Subtract, Multiply, Divide or Equals


'''<summary>
'''clears the values of all variables
'''required to reset the calculator
'''</summary>
'''<remarks>none</remarks>
Private Sub resetCalculator()
    Me.lblResult.Text = "0"
    dblResult = 0
    strLastMathOperator = "Clear"
    blnStartNewNumber = True
    dblCurrentNumber = 0
End Sub

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    resetCalculator()
End Sub
Private Sub btnPower_Click(sender As Object, e As EventArgs) Handles btnPower.Click
    'Clears the values of all variables
    resetCalculator()
End Sub

Private Sub buildNumber(ByVal strNumber As String)
    lblResult.Text = lblResult.Text & strNumber
    dblCurrentNumber = Convert.ToDouble(lblResult.Text)
    'Should we start a new number or add to an existing number?
    If blnStartNewNumber Then
        'start a new number
        lblResult.Text = strNumber
    Else
        'append to the current number
        lblResult.Text = lblResult.Text & strNumber
    End If
    blnStartNewNumber = False
End Sub

''' <summary>
''' Applies the last operator to result using current number
''' </summary>
''' <param name="strOperation">the math operation to 
'''     perform +, -, *, / or clear
'''</param>

Private Sub handleOperator(ByVal strOperation As String)
    strLastMathOperator = strOperation
    Select Case strLastMathOperator.ToUpper
        Case "ADD"
            dblResult = dblResult + dblCurrentNumber
        Case "SUBTRACT"
            dblResult = dblResult - dblCurrentNumber
        Case "MULTIPLY"
            dblResult = dblResult * dblCurrentNumber
        Case "DIVIDE"
            dblResult = dblResult / dblCurrentNumber
        Case Else
            dblResult = dblCurrentNumber
    End Select
    'dblCurrentNumber = dblResult
    Me.lblResult.Text = dblCurrentNumber.ToString
    Me.blnStartNewNumber = True
    btnDecimal.Enabled = True

End Sub

Private Sub btn0_Click(sender As Object, e As EventArgs) Handles btn0.Click
    buildNumber("0")
End Sub

Private Sub btn1_Click(sender As Object, e As EventArgs) Handles btn1.Click
    buildNumber("1")
End Sub

Private Sub btn2_Click(sender As Object, e As EventArgs) Handles btn2.Click
    buildNumber("2")
End Sub

Private Sub btn3_Click(sender As Object, e As EventArgs) Handles btn3.Click
    buildNumber("3")
End Sub

Private Sub btn4_Click(sender As Object, e As EventArgs) Handles btn4.Click
    buildNumber("4")
End Sub

Private Sub btn5_Click(sender As Object, e As EventArgs) Handles btn5.Click
    buildNumber("5")
End Sub

Private Sub btn6_Click(sender As Object, e As EventArgs) Handles btn6.Click
    buildNumber("6")
End Sub

Private Sub btn7_Click(sender As Object, e As EventArgs) Handles btn7.Click
    buildNumber("7")
End Sub

Private Sub btn8_Click(sender As Object, e As EventArgs) Handles btn8.Click
    buildNumber("8")
End Sub

Private Sub btn9_Click(sender As Object, e As EventArgs) Handles btn9.Click
    buildNumber("9")
End Sub

Private Sub btnSquareRoot_Click(sender As Object, e As EventArgs) Handles btnSquareRoot.Click
    dblCurrentNumber = Sqrt(dblCurrentNumber)
    Me.lblResult.Text = Str(dblCurrentNumber)
End Sub

Private Sub btnPercent_Click(sender As Object, e As EventArgs) Handles btnPercent.Click
    Me.lblResult.Text = Str(dblCurrentNumber / 100)
    dblCurrentNumber = dblCurrentNumber / 100
    blnStartNewNumber = True
End Sub

Private Sub btnPlusOrMinus_Click(sender As Object, e As EventArgs) Handles btnPlusOrMinus.Click
    dblCurrentNumber = (dblCurrentNumber * (-1))
    Me.lblResult.Text = dblCurrentNumber.ToString()
End Sub

Private Sub btnDivide_Click(sender As Object, e As EventArgs) Handles btnDivide.Click
    handleOperator("DIVIDE")
End Sub

Private Sub btnMultiply_Click(sender As Object, e As EventArgs) Handles btnMultiply.Click
    handleOperator("MULTIPLY")
End Sub

Private Sub btnSubtract_Click(sender As Object, e As EventArgs) Handles btnSubtract.Click
    handleOperator("SUBTRACT")
End Sub

Private Sub btnAdd_Click(sender As Object, e As EventArgs) Handles btnAdd.Click
    handleOperator("ADD")
End Sub

Private Sub btnEquals_Click(sender As Object, e As EventArgs) Handles btnEquals.Click
    dblCurrentNumber.ToString()
End Sub
End Class

Answer 1

First problem is an easy one:

 1 Private Sub buildNumber(ByVal strNumber As String)
 2     lblResult.Text = lblResult.Text & strNumber
 3     dblCurrentNumber = Convert.ToDouble(lblResult.Text)
 4     'Should we start a new number or add to an existing number?
 5     If blnStartNewNumber Then
 6         'start a new number
 7         lblResult.Text = strNumber
 8     Else
 9         'append to the current number
10         lblResult.Text = lblResult.Text & strNumber
11     End If
12     blnStartNewNumber = False
13 End Sub

Have a look at lines 2, 7, and 10.

In the case where you're appending a digit, line 2 appends it regardless of state (first or subsequent digit). Then line 10 appends it again!

That has no effect for the first digit (when blnStartNewNumber is true) because, while line 2 appends the digit, that's thrown away when setting the value in line 7.

You should probably be asking yourself about the utility of line 2 in that code (nudge, nudge, wink, wink).

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In terms of what to do in the equals subroutine, have a look at the difference between it and (for example) the square root handler. You'll see the latter actually does something with the result rather than just calculating it (and presumably throwing that result away).