Mgmt 362a Car Payment Calculator Application

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9

• Mgmt 362a Car Payment Calculator Application
• Introducing the Do While...Loopand Do
  Until...Loop Repetition Statements

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Objectives

• In this tutorial you will learn
• Use the Do While...Loop and Do Until... Loop
  repetition statements to execute statements in an
  application repeatedly.
• Use counter-controlled repetition.
• Display information in ListBoxes.
• Concatenate strings.

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9.1 Test-Driving the Car Payment Calculator
Application

• Typically, banks offer car loans for periods
  ranging from two tofive years (24 to 60 months).
  Borrowers repay the loans in monthly
  installments. The amount of each monthly payment
  is based on the length of the loan, the amount
  borrowed and the interest rate. Createan
  application that allows the customer to enter the
  price of a car, the down-payment amount and the
  annual interest rate of the loan. The application
  should display the loans duration in months and
  themonthly payments for two-, three-, four- and
  five-year loans. Thevariety of options allows
  the user to easily compare repayment plans and
  choose the one that is most convenient.

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9.1 Test-Driving the Car Payment Calculator
Application (Cont.)

• Figure 9.1 Car Payment Calculator application
  before data has been entered.

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9.1 Test-Driving the Car Payment Calculator
Application (Cont.)

• Figure 9.2 Car Payment Calculator application
  after data has been entered.

• Input data
• Click Calculate Button

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9.1 Test-Driving the Car Payment Calculator
Application (Cont.)

• Figure 9.3 Car Payment Calculator application
  displaying calculation results.

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9.2 DoWhile Loop Repetition Statement

• Loop-continuation condition
• While loop-continuation condition remains true,
  loop statement executes its body repeatedly
• When loop-continuation condition becomes false,
  loop terminates
• Code example
• Dim product As Integer 3Do While product lt
  50 product 3Loop

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Common Programming Error

• Provide in the body of every Do While...Loop
  statement an action that eventually causes the
  condition to become false. If you do not,
  therepetition statement never terminates,
  causingan error called an infinite loop. Such an
  errorcauses the application to hang up. When
  aninfinite loop occurs in an application, right
  clickthe application in the task bar and select
  Close.In the dialog box that pops up, click End
  Now.

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9.3 Do UntilLoop Repetition Statement

• Loop-continuation condition
• While loop-continuation condition remains false,
  loop statement executes its body repeatedly
• When loop-continuation condition becomes true,
  loop terminates
• Code example
• Dim product As Integer 3Do Until product gt
  50 product 3Loop

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Common Programming Error

• Failure to provide the body of a Do Until...Loop
  statement with an action that eventually causes
  the condition in the Do Until...Loop to
  becometrue creates an infinite loop.

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9.4 Adding a ListBox to the Car
PaymentCalculator Application (Cont.)

• Figure 9.8 ListBox added to Car Payment
  Calculator applications Form.

• ListBox control
• Change the Name property

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9.4 Using Code to Change a ListBoxs Contents

• Adding code to the event handler
• Clearing the ListBox
• Call method Clear on property Items
• Items property returns an object containing items
  in ListBox

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9.4 Using Code to Change a ListBoxs Contents
(Cont.)

• Figure 9.9 Clearing the contents of a ListBox.

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9.4 Using Code to Change a ListBoxs Contents
(Cont.)

• Adding a header to the ListBox
• Call method Add
• String-concatenation operator ()
• ControlChars.Tab constant

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GUI Design Tip

• Use headers in a ListBox when you aredisplaying
  tabular data. Adding headers improves readability
  by indicating the information that will be
  displayed in the ListBox.

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9.4 Using Code to Change a ListBoxs Contents
(Cont.)

• Figure 9.10 Adding a header to a ListBox.

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9.4 Declaring Variables and Receiving User Input

• Figure 9.11 Variables for the Car Payment
  Calculator application.

Variable declarations
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9.4 Declaring Variables and Receiving User Input
(Cont.)

• Figure 9.12 Retrieving input in the Car Payment
  Calculator application.

• Retrieving input
• Use Val function
• Divide interest rate by 100

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9.4 Declaring Variables and Receiving User Input
(Cont.)

• Calculating values used in the calculation
• Subtract down payment from price
• Monthly interest rate is interest divided by 12

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9.4 Declaring Variables and Receiving User Input
(Cont.)

• Figure 9.13 Determining amount borrowed and
  monthly interest rate.

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9.4 Calculating the Monthly Payment Amounts with
aDo While...Loop Repetition Statement

• Setting the loop-continuation condition
• Counter-controlled repetition
• Uses a counter variable
• Also called definite repetition

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Figure 9.14 Loop-continuation condition.

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Figure 9.15 Converting the loan duration from
  years to months.

• Add the length of the loan in months

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Calculate monthly payment amount
• Use Pmt function
• Returns a Double value
• Type Decimal stores monetary values
• Use the Convert.ToDecimal method

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Figure 9.16 Pmt function returns monthly
  payment.

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Displaying the result
• Use method Add
• Use method String.Format to display values in
  currency format

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Figure 9.17 Displaying the number of months and
  the amount of eachmonthly payment.

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Increment the counter
• Add one to the counter
• Will terminate the loop when value becomes 6

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Calculating the Monthly Payment Amounts with aDo
While...Loop Repetition Statement (Cont.)

• Figure 9.18 Incrementing the counter.

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Outline
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Outline
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