Why isn't my VB if statement working? - vb.net

Question

I'm working on my first project for my visual basic programming class.
I am trying to have this condition statement say if the discount rate is .1 or (10%), then add to the count of customers who got the discount (msngFrequentFlyers) and add to the accumulator that keeps track of total discounts given (msngTotalDiscounts).
I'm very new to all this- please help! Project is due tonight.
Here's my code:
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
If the radio button for the discount is checked:
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
This is the if statement that's not working, in the "Process Transaction" click event:
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
Here's the entire code for the "Process Transaction" click event:
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

Answer 1

Firstly as mentioned above you'll need to remove the quotes around the 0.1 in your if statement, this won't work as it's comparing a number to a string.
I'm no expert on floating-point arithmetic, but as Plutonix alludes to, I think the problem when you tried it without quotes is that the 0.1 you're comparing your msngDiscountRate in the if statement defaults to a Double, whereas you've declared your variable as a Single, so the if statement evaluates to false.
You could either declare your msngDiscountRate variable as Double instead, or cast the 0.1 to a Single to get around it. These simple examples might help -
'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