Visual Basic .NETComprehensive Concepts

and Techniques

Chapter 5

Decision Making

2Chapter 5: Decision Making

Objectives

• Use the ComboBox control

• Use the If…Then…Else structure

• Use a nested If…Then…Else structure

• Use the Select Case structure

• Validate user input

3Chapter 5: Decision Making

Objectives

• Use the MessageBox class

• Use the string concatenation in code

• Use relational operators in code

• Use logical operators in code

4Chapter 5: Decision Making

Modifying the Automobile Loan Calculator Application

5Chapter 5: Decision Making

6Chapter 5: Decision Making

7Chapter 5: Decision Making

Opening the Project and Modifying the User Interface• Open the Automobile Loan Calculator

program you created in Chapter 4

• Select the Form1 form in the main work area, right-click the form, and then click Lock Controls on the shortcut menu to unlock the form

• Increase the form height to 248 pixels

8Chapter 5: Decision Making

Opening the Project and Modifying the User Interface• Select the Label3 control, along with

txtnMonthlyPament, btnComputePayment, and btnReset controls

• Drag the controls down four marks on the positioning grid

• Add a Label and ComboBox control from the Toolbox window and position as shown on the next slide

9Chapter 5: Decision Making

10Chapter 5: Decision Making

Label Control Properties

11Chapter 5: Decision Making

ComboBox Control Properties

12Chapter 5: Decision Making

ComboBox1 Control Properties

13Chapter 5: Decision Making

Setting the Items Property

• Select the Items property in the Properties window

• Click the Collection build button

• Type the five items into the String Collection Editor, as shown on the following slide, and click the OK button

• Upon completion, lock the controls using the Format menu

14Chapter 5: Decision Making

15Chapter 5: Decision Making

Modifying the Comment Header and Adding a Constant

16Chapter 5: Decision Making

Resetting the ComboBox Value

• The SelectedIndex property of the ComboBox control is assigned the value of the item’s index

• When no item is selected, the SelectedIndex property is set to -1

• Add the following line of code to the end of the btnReset_Click Event Procedure:

17Chapter 5: Decision Making

Declaring Additional Variables

• Two additional variables are needed in the btnComputePayment_Click event procedure

• Scroll the code window until line 258 appears

• Enter line 259 as shown below:

18Chapter 5: Decision Making

Decision-Making Control Structures

• Decision making is the process of determining which of one or more paths to take– If…Then…Else structure– Select Case structure

19Chapter 5: Decision Making

If…Then…Else Structure

Select Case Structure

20Chapter 5: Decision Making

If…Then…Else Structure for Auto Loan Calculator

21Chapter 5: Decision Making

The MessageBox Class and the Show() Method• The MessageBox class provides the ability

to display a message to the user in a pop-up window

• The Show() method must be used to display a message with a title bar

• MessageBox.Show(“Please enter the customer’s credit rating in the Credit rating list box.”,”No Credit Rating”)

22Chapter 5: Decision Making

The Focus() Method

• When a user selects a control on a form, the control is said to have focus

• It is often necessary to set the focus to a control during run time

• cmbCreditRating.Focus()

23Chapter 5: Decision Making

If…Then…Else Statement

• Relational expression =, <, >, <=, >=, <>

• Single-line If…Then…Else statement

• Block If…Then…Else statement

• Block-level scoping

• Exit Sub Statement– Terminates execution of a procedure

immediately

24Chapter 5: Decision Making

If…Then…Else Structure

• Enter the following code, starting on line 263:

• Parameter Info window

25Chapter 5: Decision Making

Considering Selection Structures

26Chapter 5: Decision Making

The Nested If…Then…Else Structure

27Chapter 5: Decision Making

Coding a Nested If…Then…Else Structure

28Chapter 5: Decision Making

Coding a Nested If…Then…Else Structure• In the code window, delete lines 240

through 251

• Starting on line 259, enter the following code

29Chapter 5: Decision Making

Logical Operators

• The Not Logical Operator

• The And Logical Operator

• The Or Logical Operator

• The Xor Logical Operator

• Other Logical Operators– AndAlso– OrElse

30Chapter 5: Decision Making

Combining Logical Operators

If X>Y Or T=D And H<3 Or Not Y=R Then

intCount = intCount + 1

End If• Rules of precedence

– Arithmetic operators– Relational operators– Not operators– And/AndAlso operators– Or/OrElse or Xor operators

31Chapter 5: Decision Making

The Effect of Parentheses in the Evaluation of Compound Conditions

• Parentheses can be used to change the order of precedence

• Avoid ambiguity and group conditions with a desired logical operator

(C>D And S=4) Or (X<Y And T=5)

32Chapter 5: Decision Making

Coding with a Logical Operator and String Expressions

33Chapter 5: Decision Making

Coding with a Logical Operator and String Expressions• Concatenation (&) operatorstrErrorMessage = “Customers with this credit rating may only borrow “ & strMoney

• The plus sign (+) can be used instead of the (&) operator

34Chapter 5: Decision Making

Coding with a Logical Operator and String Expressions• Enter the following code, starting on line

272

35Chapter 5: Decision Making

The Select Case Structure

• Used to implement an extension of the If…Then…Else structure

36Chapter 5: Decision Making

37Chapter 5: Decision Making

Coding a Select Case Statement

38Chapter 5: Decision Making

Coding a Select Case Statement

• Enter the following code, starting on line 284

39Chapter 5: Decision Making

Modifying a Parameter in a Function Call• The function call to the Pmt function

currently accepts the dblRate variable as the interest rate parameter

• This does not take into account the customer’s credit rating

• In line 300, click between the letters l and R in the dblRate parameter in the Pmt function call and enter Adjusted to change the parameter name

40Chapter 5: Decision Making

41Chapter 5: Decision Making

Saving and Testing the Project

• Click the Save All button on the Standard toolbar• Click the Start button on the Visual Basic .NET

Standard toolbar• Enter 13000 for the Loan amount. Enter 10.25

for the Current interest rate in the second NumericUpDown control. Click the Compute Payment button– The No Credit Rating message box should display

42Chapter 5: Decision Making

Saving and Testing the Project

• Click the OK button. Click the Credity rating box arrow and then select B – Good in the Credit rating drop-down list. Click the Compute Payment button– The resulting output should display $284.44

• Click the Close button on the Automobile Loan Calculator application window title bar

43Chapter 5: Decision Making

Document the Application and Quit Visual Basic .NET• Prepare printouts of your project, as

demonstrated in Chapter 2

• Click the Visual Basic .NET Close button

44Chapter 5: Decision Making

Summary

• Use the ComboBox control

• Use the If…Then…Else structure

• Use a nested If…Then…Else structure

• Use the Select Case structure

• Validate user input

45Chapter 5: Decision Making

Summary

• Use the MessageBox class

• Use the string concatenation in code

• Use relational operators in code

• Use logical operators in code

Visual Basic .NETComprehensive Concepts

and Techniques

Chapter 5 Complete