Question from Mike:

Need help working a code called car loan. The book is the Introduction to Programming Using Visual Basic 2012. Programming project no. 3, p. 162. I think my math is totally wrong.

Description:

Write a program to analyze a car loan. See figure below. The user should enter the amount of the loan, the annual percentage rate of interest, and the duration of the loan in months. When the user clicks on the button, the information that was entered should be checked to make sure that it is reasonable. If bad data has been supplied, the user should be so advised. Otherwise, the monthly payment and the total amount of interest paid should be displayed. The formula for the monthly payment is

where p is the amount of the loan, r is the monthly interest rate (annual rate divided by 12) given as a number between 0 (for 0%) and 1 (for 100%), and n is the duration of the loan in months. The formula for the total interest paid is

Here is an example of what the user interface might look like:

Suggested Control Names and Attributes:

Name Property	Text Property	Control Type	Notes
frmCarLoan	Car Loan	Form	Holds Controls
txtLoanAmt	N/A	TextBox	Captures loan amount
txtIntRate	N/A	TextBox	Captures interest rate
txtNumOfMonths	N/A	TextBox	Captures number of months
btnCalc	Calculate	Button	Calculates and outputs results
txtMonthlyPmt	N/A	TextBox	Displays monthly payment
txtTotalInt	N/A	TextBox	Displays total interest

Hints:

• The equations are probably the trickiest part of this program.
• Since the second equation above is dependent upon the answer from the first equation above, the order you place them in the program is important.
• Equation 1: payment = (loanAmt * (intRate / 12)) / (1 – (1 + (intRate / 12)) ^ (-months))
• Equation 2: totalInt = months * payment – loanAmt

Write the Code:

Public Class frmCarLoan
Private Sub btnCalc_Click(sender As Object, e As EventArgs) Handles btnCalc.Click
‘ Declare local variables
Dim loanAmt As Double = 0
Dim intRate As Double = 0
Dim months As Integer = 0
Dim payment As Double = 0
Dim totalInt As Double = 0

‘ Collect data from user and store in variables
If (CDbl(txtLoanAmt.Text) < 0) Then
MessageBox.Show(“Loan Amount can’t be a negative number.”)
txtLoanAmt.Clear()
txtLoanAmt.Focus()
Else
loanAmt = CDbl(txtLoanAmt.Text)
End If

If (CDbl(txtIntRate.Text) < 0) Then
MessageBox.Show(“Interest Rate can’t be a negative number.”)
txtIntRate.Clear()
txtIntRate.Focus()
ElseIf (CDbl(txtIntRate.Text) > 50) Then
MessageBox.Show(“You should probably shop around for a better interest rate.”)
txtIntRate.Clear()
txtIntRate.Focus()
Else
intRate = CDbl(txtIntRate.Text) / 100
End If

If (CDbl(txtNumOfMonths.Text) < 0) Or (CDbl(txtNumOfMonths.Text) > 100) Then
MessageBox.Show(“Loan duration should be between 1 and 100 months.”)
txtNumOfMonths.Clear()
txtNumOfMonths.Focus()
Else
months = CInt(txtNumOfMonths.Text)
End If

‘ Perform calculations
payment = (loanAmt * (intRate / 12)) / (1 – (1 + (intRate / 12)) ^ (-months))
totalInt = months * payment – loanAmt

‘ Output results
txtMonthlyPmt.Clear()
txtTotalInt.Clear()
If (loanAmt > 0) And (intRate > 0) And (months > 0) Then
txtMonthlyPmt.Text = payment.ToString(“C”)
txtTotalInt.Text = totalInt.ToString(“C”)
End If
End Sub
End Class