Two balance won't add up if both checkboxes are checked - vb.net

Question

I am writing a program that should calculate the balance with the given information (Which checkbox they choose (Savings (7%), Corporate(5%), or both, then the year they choose (2015, 2016, 2017), and lastly the amount they put in when cued to enter balance (And that is based on what type of account they choose in the first step). I have gotten all of the totals to show up correctly, except for when they choose both account types (Savings and Corporate). To me their should be a line that just adds the two totals if both are chosen, but that isn't the case. At least for me it isn't.
The total I get if I enter the balance of $2000 for savings and choose year 2015 the total is $2289.80 which is correct and then if I choose 2015 for the corporate account and enter $1000 the total is $1102.50. So if I chose both of the accounts and entered the same numbers but checked both checkboxes I get $3434.70, but I should get $3392.30!
Here is the code I have, any help would be much appreciated! ( I think it might literally be just one line of code and it is killing me!)
Public Class Form1
Private Sub btnMediaEstimatedFund_Click(sender As Object, e As EventArgs) Handles btnMediaEstimatedFund.Click
Dim interestRate, initialBalanceSavings, initialBalanceCorporate, finalBalance, year, theYear, subTotal As Double
Dim savingsInterestRate As Double = 0
Dim corporateInterestRate As Double = 0
Dim savingsSubTotal As Double = 0
Dim corporateSubTotal As Double = 0
txtBoxEstimatedBudget.Enabled = False
txtBoxAgenciesNeeded.Enabled = False
If radButtonTraditional.Checked Then
txtBoxAgenciesNeeded.Text = 3
ElseIf radButtonEMedia.Checked Then
txtBoxAgenciesNeeded.Text = 2
End If
If checkBoxSavings.Checked Then
savingsInterestRate = 0.07
ElseIf checkBoxCorporate.Checked Then
corporateInterestRate = 0.05
End If
If checkBoxSavings.Checked = False And checkBoxCorporate.Checked = False Then
MsgBox("Please chose an account type to proceed!", MsgBoxStyle.Critical, "Error")
End If
If radButton2015.Checked Then
theYear = 2015
ElseIf radButton2016.Checked Then
theYear = 2016
ElseIf radButton2017.Checked Then
theYear = 2017
End If
Dim inputtedData As String
If checkBoxSavings.Checked Then
Do
inputtedData = InputBox("Please enter a balance for SAVINGS account between $500.00 and $3000.00", "Initial Savings Balance", "0.00")
If inputtedData = "" Then
MsgBox("User chose to cancel calculation!")
Exit Sub
Else
initialBalanceSavings = CType(inputtedData, Single)
If initialBalanceSavings > 3000 Or initialBalanceSavings < 500 Then MsgBox("Please enter a balance for SAVINGS account equal to or above $500.00 and no more than $3000.00", MsgBoxStyle.Critical, "Error")
End If
Loop Until initialBalanceSavings >= 500 And initialBalanceSavings <= 3000
End If
If checkBoxCorporate.Checked Then
Do
inputtedData = InputBox("Please enter a balance for CORPORATE account between $500.00 and $3000.00", "Initial Corporate Balance", "0.00")
If inputtedData = "" Then
MsgBox("User chose to Cancel calculation!")
Exit Sub
Else
initialBalanceCorporate = CType(inputtedData, Single)
If initialBalanceCorporate > 3000 Or initialBalanceCorporate < 500 Then MsgBox("Please enter a balance for CORPORATE account equal to or above $500.00 and no more than $3000.00", MsgBoxStyle.Critical, "Error")
End If
Loop Until initialBalanceCorporate >= 500 And initialBalanceCorporate <= 3000
End If
savingsSubTotal = initialBalanceSavings
corporateSubTotal = initialBalanceCorporate
For year = 2013 To theYear - 1
If savingsInterestRate > 0 Then
savingsSubTotal = savingsSubTotal * (1 + savingsInterestRate)
End If
If corporateInterestRate > 0 Then
corporateSubTotal = corporateSubTotal * (1 + corporateInterestRate)
End If
Next
finalBalance = savingsSubTotal + corporateSubTotal
txtBoxEstimatedBudget.Text = FormatCurrency(finalBalance)
End Sub

Answer 1

I believe the erroneous logic is in this block of code:
finalBalance = initialBalanceSavings + initialBalanceCorporate
For year = 2013 To theYear - 1
subTotal = finalBalance * (1 + interestRate)
finalBalance = subTotal
Next
Combined with this block of code:
If checkBoxSavings.Checked Then
interestRate = 0.07
ElseIf checkBoxCorporate.Checked Then
interestRate = 0.05
End If
In the above, even if something checks both checkboxes the interest rate is set at 0.07 so in your loop if someone calculates 2015 you will do something like give you a final total of: 3000 * 1.07 * 1.07, whereas you really want (2000 * 1.07 * 1.07) + (1000 * 1.05 * 1.05)
In essence because you only use one variable to hold the interest rate (as well as only having one variable to hold sub-totals) your code cannot account properly for a scenario where someone selects both accounts. I would suggest having 2 variables to hold the interest rates:
Dim savingsInterestRate as Double = 0
Dim corporateInterestRate as Double = 0
And 2 variables to hold the sub-totals:
Dim savingsSubTotal as Double = 0
Dim corporateSubTotal as Double = 0
And in your For loop do:
savingsSubTotal = initialBalanceSavings
corporateSubTotal = initialBalanceCorporate
For year = 2013 To theYear - 1
If savingsInterestRate > 0 Then
savingsSubTotal = savingsSubTotal * (1 + savingsInterestRate)
End If
If corporateInterestRate > 0 Then
corporateSubTotal = corporateSubTotal * (1 + corporateInterestRate)
End If
Next
finalBalance = savingsSubTotal + corporateSubTotal
And change your interest rate check to:
If checkBoxSavings.Checked Then
savingsInterestRate = 0.07
End If
If checkBoxCorporate.Checked Then
corporateInterestRate = 0.05
End If
So you can account for the person's selection (using the ElseIf means that if they check both, it will discard the corporate rate since checkBoxSavings.Checked is already true sp it never falls into the ElseIf block)