为什么我的 VB if 语句不起作用？
Why isn't my VB if statement working?
我正在为我的视觉基础编程开发我的第一个项目class。
我试图让这个条件语句说明折扣率为 .1 或 (10%)，然后添加到获得折扣的客户数 (msngFrequentFlyers) 并添加到跟踪给定总折扣的累加器 (msngTotalDiscounts) ).
我对这一切都很陌生 - 请帮忙！项目今晚到期。
这是我的代码：
Public Class frmTaxiConsole
'Modular Variable Declaration Section
'Declares msngDiscountRate and sets inital value to -1 for testing user input
Dim msngDiscountRate As Single = -1
'Declares all counter variables
Dim msngNumberOfRides As Single
Dim msngNumberOfFrequentFlyers As Single
'Declares all accumulator variables
Dim msngRevenue As Single
Dim msngTotalDiscounts As Single
Dim msngBillableMiles As Single
如果选中折扣单选按钮：
Private Sub radFrequentFlyer_CheckedChanged(sender As Object, e As EventArgs) Handles radFrequentFlyer.CheckedChanged
msngDiscountRate = 0.1 'Sets discount rate to 10% upon selection of radio button
End Sub
这是无效的 if 语句，在 "Process Transaction" 单击事件中：
If msngDiscountRate = "0.1" Then 'Checks to see if this transaction had any discount
msngNumberOfFrequentFlyers += 1 'If so, adds 1 to the number of discounts given
msngTotalDiscounts = msngTotalDiscounts + (sngSubTotal * msngDiscountRate) 'Also adds the discount amount to the total discounts accumulator (without making it negative)
End If
下面是 "Process Transaction" 点击事件的完整代码：
Private Sub btnProcessTx_Click(sender As Object, e As EventArgs) Handles btnProcessTx.Click
'Local Variable Declaration Section
Dim sngMilesDriven As Single
Dim dblOdometerStart As Double
Dim dblOdometerEnd As Double
Dim sngInitialFee As Single
Dim sngPerMileFee As Single
Dim sngMileageCharge As Single
Dim sngSubTotal As Single
Dim sngDiscount As Single
Dim sngTotalDue As Single
'Data Input + Testing Section
'Changes all text box backcolors white, in case they had been turned red due to an error
txtOdometerStart.BackColor = Color.White
txtOdometerEnd.BackColor = Color.White
txtInitialFee.BackColor = Color.White
txtPerMileFee.BackColor = Color.White
'Try/Catch validates user input for Inital Fee
Try
'Attempts to convert user input to Single and store as a local variable
sngInitialFee = CSng(txtInitialFee.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid initial fee.", "Invalid Initial Fee", MessageBoxButtons.OK)
txtInitialFee.BackColor = Color.Red
txtInitialFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for Per-Mile Fee
Try
'Attempts to convert user input to Single and store as a local variable
sngPerMileFee = CSng(txtPerMileFee.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid per-mile fee.", "Invalid Per-Mile Fee", MessageBoxButtons.OK)
txtPerMileFee.BackColor = Color.Red
txtPerMileFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for starting milage
Try
'Attempts to convert user input to Double and store as a local variable
dblOdometerStart = CDbl(txtOdometerStart.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid starting milage.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.BackColor = Color.Red
txtOdometerStart.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for ending milage
Try
'Attempts to convert user input to Double and store as a local variable
dblOdometerEnd = CDbl(txtOdometerEnd.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid ending milage.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerEnd.BackColor = Color.Red
txtOdometerEnd.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'If statement ensures Inital Fee is a positive number
If sngInitialFee < 0 Then
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Initial Fee cannot be negative.", "Invalid Inital Fee", MessageBoxButtons.OK)
txtInitialFee.BackColor = Color.Red
txtInitialFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End If
'If statement ensures Per-Mile Fee is a positive number
If sngPerMileFee < 0 Then
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Per-Mile Fee cannot be negative.", "Invalid Per-Mile Fee", MessageBoxButtons.OK)
txtPerMileFee.BackColor = Color.Red
txtPerMileFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End If
'If statement checks to make sure starting milage is smaller number than ending milage
If dblOdometerEnd <= dblOdometerStart Then
'If ending milage is smaller number than starting milage, displays a messagebox and exits sub.
MessageBox.Show("Your ending mileage cannot be less than or equal to your starting mileage. Please check your odometer readings.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.Focus() 'Focuses in starting odometer reading text box so user can fix
Exit Sub
End If
'If statement checks to make sure both odometer readings are positive numbers.
If dblOdometerEnd < 0 Or dblOdometerStart < 0 Then
'If either odometer reading is negative, displays a messagebox and exits sub.
MessageBox.Show("Both your odometer readings must be positive numbers. Please check your odometer readings.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.Focus() 'Focuses in starting odometer reading text so user can fix
Exit Sub
End If
'If statement checks to ensure user has seleted one of the two radio buttons
If msngDiscountRate = -1 Then
'If msngDiscountRate is the same as the initial value, neither option has been chosen, an error message is shown, and sub exited.
MessageBox.Show("Please choose a frequent flyer status.", "Frequent Flyer?", MessageBoxButtons.OK)
Exit Sub
End If
'Calculations Section
sngMilesDriven = CSng(dblOdometerEnd - dblOdometerStart) 'Subtracts starting mileage from ending mileage, converts to single, stores as var sngMilesDriven
sngMileageCharge = sngMilesDriven * sngPerMileFee 'Multiplies the miles driven by the per-mile fee and stores as var sngMileageCharge
sngSubTotal = sngMileageCharge + sngInitialFee 'Adds the milage charge to the initial fee, stores as var sngSubTotal
sngDiscount = sngSubTotal * msngDiscountRate * -1 'Multiplies subtotal by discount rate, makes negative, stores as var sngDiscount
sngTotalDue = sngSubTotal + sngDiscount 'Subtracts discounts from subtotal, stores as var sngTotalDue
'Counter and Accumulator Operations
msngNumberOfRides += 1 'Adds 1 to the number of rides given
If msngDiscountRate = "0.1" Then 'Checks to see if this transaction had any discount
MsgBox(msngDiscountRate)
msngNumberOfFrequentFlyers += 1 'If so, adds 1 to the number of discounts given
msngTotalDiscounts = msngTotalDiscounts + (sngSubTotal * msngDiscountRate) 'Also adds the discount amount to the total discounts accumulator (without making it negative)
End If
msngRevenue = msngRevenue + sngTotalDue 'Adds the total due for current transaction to revenue accumulator
msngBillableMiles = msngBillableMiles + sngMilesDriven 'Adds miles from this transaction to running total
'Output Section
'Displays above calculations in respective labels and formats as currency if neccecary.
lblMilesDrivenOutput.Text = sngMilesDriven
lblSumInitialFeeOutput.Text = FormatCurrency(sngInitialFee) 'Formats sngInitialFee as currency and displays in lblSumInitialFeeOutput
lblSumMilageChargeOutput.Text = FormatCurrency(sngMileageCharge)
lblSumSubTotalOutput.Text = FormatCurrency(sngSubTotal)
lblSumDiscountOutput.Text = FormatCurrency(sngDiscount)
lblSumTotalDueOutput.Text = FormatCurrency(sngTotalDue)
'Displays all counter and accumulator variables after they are updated
lblTotalRidesOutput.Text = msngNumberOfRides
lblFrequentFlyersOutput.Text = msngNumberOfFrequentFlyers
lblRevenueOutput.Text = FormatCurrency(msngRevenue)
lblTotalDiscountsOutput.Text = FormatCurrency(msngTotalDiscounts)
lblBillableMilesOutput.Text = msngBillableMiles
End Sub
首先，如上所述，您需要删除 if 语句中 0.1 周围的引号，这将不起作用，因为它将数字与字符串进行比较。
我不是浮点运算方面的专家，但正如 Plutonix 所暗示的那样，我认为当你尝试不带引号时的问题是 0.1 你正在比较你的 msngDiscountRate在 if 语句中默认为 Double，而您已将变量声明为 Single，因此 if 语句的计算结果为 false。
您可以将 msngDiscountRate 变量声明为 Double，或者将 0.1 转换为 Single 来绕过它。这些简单的例子可能会有所帮助 -
'Variable declared as a Single, compared to 0.1 (a Double) - If statement evaluates to false
Dim msngDiscountRateExample1 As Single = -1
msngDiscountRateExample1 = 0.1
If msngDiscountRateExample1 = 0.1 Then
Debug.Print(msngDiscountRateExample1.ToString)
End If
'Variable declared as a Double, compared to 0.1 (another Double) - If statement evaluates to true
Dim msngDiscountRateExample2 As Double = -1
msngDiscountRateExample2 = 0.1
If msngDiscountRateExample2 = 0.1 Then
Debug.Print(msngDiscountRateExample2.ToString)
End If
'Variable declared as a Single, compared to 0.1 (a Double) which is *cast* as a Single - If statement evaluates to true
Dim msngDiscountRateExample3 As Single = -1
msngDiscountRateExample3 = 0.1
If msngDiscountRateExample3 = CSng(0.1) Then
Debug.Print(msngDiscountRateExample3.ToString)
End If
我正在为我的视觉基础编程开发我的第一个项目class。 我试图让这个条件语句说明折扣率为 .1 或 (10%)，然后添加到获得折扣的客户数 (msngFrequentFlyers) 并添加到跟踪给定总折扣的累加器 (msngTotalDiscounts) ).
我对这一切都很陌生 - 请帮忙！项目今晚到期。
这是我的代码：
Public Class frmTaxiConsole
'Modular Variable Declaration Section
'Declares msngDiscountRate and sets inital value to -1 for testing user input
Dim msngDiscountRate As Single = -1
'Declares all counter variables
Dim msngNumberOfRides As Single
Dim msngNumberOfFrequentFlyers As Single
'Declares all accumulator variables
Dim msngRevenue As Single
Dim msngTotalDiscounts As Single
Dim msngBillableMiles As Single
如果选中折扣单选按钮：
Private Sub radFrequentFlyer_CheckedChanged(sender As Object, e As EventArgs) Handles radFrequentFlyer.CheckedChanged
msngDiscountRate = 0.1 'Sets discount rate to 10% upon selection of radio button
End Sub
这是无效的 if 语句，在 "Process Transaction" 单击事件中：
If msngDiscountRate = "0.1" Then 'Checks to see if this transaction had any discount
msngNumberOfFrequentFlyers += 1 'If so, adds 1 to the number of discounts given
msngTotalDiscounts = msngTotalDiscounts + (sngSubTotal * msngDiscountRate) 'Also adds the discount amount to the total discounts accumulator (without making it negative)
End If
下面是 "Process Transaction" 点击事件的完整代码：
Private Sub btnProcessTx_Click(sender As Object, e As EventArgs) Handles btnProcessTx.Click
'Local Variable Declaration Section
Dim sngMilesDriven As Single
Dim dblOdometerStart As Double
Dim dblOdometerEnd As Double
Dim sngInitialFee As Single
Dim sngPerMileFee As Single
Dim sngMileageCharge As Single
Dim sngSubTotal As Single
Dim sngDiscount As Single
Dim sngTotalDue As Single
'Data Input + Testing Section
'Changes all text box backcolors white, in case they had been turned red due to an error
txtOdometerStart.BackColor = Color.White
txtOdometerEnd.BackColor = Color.White
txtInitialFee.BackColor = Color.White
txtPerMileFee.BackColor = Color.White
'Try/Catch validates user input for Inital Fee
Try
'Attempts to convert user input to Single and store as a local variable
sngInitialFee = CSng(txtInitialFee.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid initial fee.", "Invalid Initial Fee", MessageBoxButtons.OK)
txtInitialFee.BackColor = Color.Red
txtInitialFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for Per-Mile Fee
Try
'Attempts to convert user input to Single and store as a local variable
sngPerMileFee = CSng(txtPerMileFee.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid per-mile fee.", "Invalid Per-Mile Fee", MessageBoxButtons.OK)
txtPerMileFee.BackColor = Color.Red
txtPerMileFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for starting milage
Try
'Attempts to convert user input to Double and store as a local variable
dblOdometerStart = CDbl(txtOdometerStart.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid starting milage.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.BackColor = Color.Red
txtOdometerStart.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'Try/Catch validates user input for ending milage
Try
'Attempts to convert user input to Double and store as a local variable
dblOdometerEnd = CDbl(txtOdometerEnd.Text)
Catch ex As Exception
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Please enter a valid ending milage.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerEnd.BackColor = Color.Red
txtOdometerEnd.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End Try
'If statement ensures Inital Fee is a positive number
If sngInitialFee < 0 Then
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Initial Fee cannot be negative.", "Invalid Inital Fee", MessageBoxButtons.OK)
txtInitialFee.BackColor = Color.Red
txtInitialFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End If
'If statement ensures Per-Mile Fee is a positive number
If sngPerMileFee < 0 Then
'If error occurs, displays a messagebox, changes background color of relavent text box, and exits sub.
MessageBox.Show("Per-Mile Fee cannot be negative.", "Invalid Per-Mile Fee", MessageBoxButtons.OK)
txtPerMileFee.BackColor = Color.Red
txtPerMileFee.Focus() 'Focuses in text box where error occured so user can fix
Exit Sub
End If
'If statement checks to make sure starting milage is smaller number than ending milage
If dblOdometerEnd <= dblOdometerStart Then
'If ending milage is smaller number than starting milage, displays a messagebox and exits sub.
MessageBox.Show("Your ending mileage cannot be less than or equal to your starting mileage. Please check your odometer readings.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.Focus() 'Focuses in starting odometer reading text box so user can fix
Exit Sub
End If
'If statement checks to make sure both odometer readings are positive numbers.
If dblOdometerEnd < 0 Or dblOdometerStart < 0 Then
'If either odometer reading is negative, displays a messagebox and exits sub.
MessageBox.Show("Both your odometer readings must be positive numbers. Please check your odometer readings.", "Invalid Odometer Reading", MessageBoxButtons.OK)
txtOdometerStart.Focus() 'Focuses in starting odometer reading text so user can fix
Exit Sub
End If
'If statement checks to ensure user has seleted one of the two radio buttons
If msngDiscountRate = -1 Then
'If msngDiscountRate is the same as the initial value, neither option has been chosen, an error message is shown, and sub exited.
MessageBox.Show("Please choose a frequent flyer status.", "Frequent Flyer?", MessageBoxButtons.OK)
Exit Sub
End If
'Calculations Section
sngMilesDriven = CSng(dblOdometerEnd - dblOdometerStart) 'Subtracts starting mileage from ending mileage, converts to single, stores as var sngMilesDriven
sngMileageCharge = sngMilesDriven * sngPerMileFee 'Multiplies the miles driven by the per-mile fee and stores as var sngMileageCharge
sngSubTotal = sngMileageCharge + sngInitialFee 'Adds the milage charge to the initial fee, stores as var sngSubTotal
sngDiscount = sngSubTotal * msngDiscountRate * -1 'Multiplies subtotal by discount rate, makes negative, stores as var sngDiscount
sngTotalDue = sngSubTotal + sngDiscount 'Subtracts discounts from subtotal, stores as var sngTotalDue
'Counter and Accumulator Operations
msngNumberOfRides += 1 'Adds 1 to the number of rides given
If msngDiscountRate = "0.1" Then 'Checks to see if this transaction had any discount
MsgBox(msngDiscountRate)
msngNumberOfFrequentFlyers += 1 'If so, adds 1 to the number of discounts given
msngTotalDiscounts = msngTotalDiscounts + (sngSubTotal * msngDiscountRate) 'Also adds the discount amount to the total discounts accumulator (without making it negative)
End If
msngRevenue = msngRevenue + sngTotalDue 'Adds the total due for current transaction to revenue accumulator
msngBillableMiles = msngBillableMiles + sngMilesDriven 'Adds miles from this transaction to running total
'Output Section
'Displays above calculations in respective labels and formats as currency if neccecary.
lblMilesDrivenOutput.Text = sngMilesDriven
lblSumInitialFeeOutput.Text = FormatCurrency(sngInitialFee) 'Formats sngInitialFee as currency and displays in lblSumInitialFeeOutput
lblSumMilageChargeOutput.Text = FormatCurrency(sngMileageCharge)
lblSumSubTotalOutput.Text = FormatCurrency(sngSubTotal)
lblSumDiscountOutput.Text = FormatCurrency(sngDiscount)
lblSumTotalDueOutput.Text = FormatCurrency(sngTotalDue)
'Displays all counter and accumulator variables after they are updated
lblTotalRidesOutput.Text = msngNumberOfRides
lblFrequentFlyersOutput.Text = msngNumberOfFrequentFlyers
lblRevenueOutput.Text = FormatCurrency(msngRevenue)
lblTotalDiscountsOutput.Text = FormatCurrency(msngTotalDiscounts)
lblBillableMilesOutput.Text = msngBillableMiles
End Sub
首先，如上所述，您需要删除 if 语句中 0.1 周围的引号，这将不起作用，因为它将数字与字符串进行比较。
我不是浮点运算方面的专家，但正如 Plutonix 所暗示的那样，我认为当你尝试不带引号时的问题是 0.1 你正在比较你的 msngDiscountRate在 if 语句中默认为 Double，而您已将变量声明为 Single，因此 if 语句的计算结果为 false。
您可以将 msngDiscountRate 变量声明为 Double，或者将 0.1 转换为 Single 来绕过它。这些简单的例子可能会有所帮助 -
'Variable declared as a Single, compared to 0.1 (a Double) - If statement evaluates to false
Dim msngDiscountRateExample1 As Single = -1
msngDiscountRateExample1 = 0.1
If msngDiscountRateExample1 = 0.1 Then
Debug.Print(msngDiscountRateExample1.ToString)
End If
'Variable declared as a Double, compared to 0.1 (another Double) - If statement evaluates to true
Dim msngDiscountRateExample2 As Double = -1
msngDiscountRateExample2 = 0.1
If msngDiscountRateExample2 = 0.1 Then
Debug.Print(msngDiscountRateExample2.ToString)
End If
'Variable declared as a Single, compared to 0.1 (a Double) which is *cast* as a Single - If statement evaluates to true
Dim msngDiscountRateExample3 As Single = -1
msngDiscountRateExample3 = 0.1
If msngDiscountRateExample3 = CSng(0.1) Then
Debug.Print(msngDiscountRateExample3.ToString)
End If