4-1

Chapter 4

The Selection Process in VB .NET

4-2

Learning Objectives

Understand the importance of the selection process in programming.

Describe the various types of decisions that can be made in a computer program.

Discuss the statement forms used in VB .NET to make decisions.

Use the list box control to select from a list of alternatives.

4-3

Learning Objectives (continued)

Work with complex comparison structures and nested decisions to handle more sophisticated selection processes.

Use the scroll bar to input integer values. Use the Form_Load event to execute a

procedure when a form is loaded. Work with the debugging toolbar to find

program errors.

4-4

The Selection process

Do a comparison Result of comparison has a boolean value

– If True, execute one set of statements– If False, execute another set of statements

Example: If x=y Then

‘one set of statementsElse

‘another set of statementsEnd If

4-5

Types of decisions

Binary decision, only two alternatives– Yes – No– True – False

Multiple alternative– Choice is based on value of a certain variableSelect MyVariable

Condition 1: execute code for choice 1 Condition 2: execute code for choice 2

Condition 3: execute code for choice 3 End Selection

4-6

The Two Alternative Decision

Simple IfIf condition Then

‘Run code corresponding to true conditionEnd If

If-Then-ElseIf condition Then

‘Run code corresponding to true conditionElse

‘Run code corresponding to false conditionEnd If

4-7

Comparison Operators

4-8

Example: Find pay based on hours of work

If Hours > 40 Then

Pay = 40*PayRate+1.5*(Hours-40)*Payrate

Else

Pay = Hours*Payrate

End If

4-9

Step-by-Step 4-1: Creating a Payroll Project

Demo

4-10

Step-by-Step 4-2:Continuing Vintage DVDs Project

Demo

4-11

Multiple Alternative Decision:Compounded If’s

If condition1 Then‘Run code corresponding to condition1

ElseIf condition2 Then‘Run code corresponding to condition2

ElseIf condition3 Then‘Run code corresponding to condition3

Else‘Run code not corresponding to any of ‘the previous conditions

End If

4-12

Example: Calculate Grade

Private Sub btnLetter_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) _ Handles

btnLetter.Click

Dim intAverage As Integer, strLetterGrade As String intAverage = CInt(txtAverage.Text)

If intAverage >= 90 Then strLetterGrade = "A"

ElseIf intAverage >= 80 Then strLetterGrade = "B"

ElseIf intAverage >= 70 Then strLetterGrade = "C"

ElseIf intAverage >= 60 Then strLetterGrade = "D"

Else strLetterGrade = "F"

End if

txtLetter.Text = strLetterGrade End Sub

4-13

Step-by-Step 4-3:Creating the Letter Grade Project

Demo

4-14

Multiple Alternative Decision:The Select Case Decision Structure

Select Case expressionCase condition1

‘Code for condition1Case condition2

‘Code for condition2Case condition3

‘Code for condition 3Case Else

‘Code for other conditionsEnd Select

4-15

Comparison conditions

4-16

Example: Calculate Grade

Private Sub btnLetter_Click(ByVal sender As System.Object,_ ByVal e As System.EventArgs) Handles btnLetter.Click

Dim intAverage As Integer Dim strLetterGrade As String intAverage = CInt(txtAverage.Text) Select Case intAverage Case Is >= 90

strLetterGrade = "A" Case Is >= 80

strLetterGrade = "B" Case Is >= 70

strLetterGrade = "C" Case Is >= 60

strLetterGrade = "D" Case Else

strLetterGrade = "F" End Select

txtLetter.Text = strLetterGrade

End Sub

4-17

Step-by-Step 4-4:Revisiting Vintage DVDs Project

Demo

4-18

Application to Vintage DVD’s

Three types of DVD’s– Kids : $0.99– Regular : $1.99– Classic : $ 2.99

Feature to add: Indicate type of DVD and compute the price automatically

Need a way to enter DVD type

4-19

Application to Vintage DVD’sHow to input the DVD type

Naive – Use InputBox– Input errors are very likely

Smarter – Supply user with a “list” to choose from– Less input error– More guidance for the user

4-20

The ListBox Control

Like an extended TextBox Displays multiple text entries in a list Properties

– Items : the list of “items” displayed– SelectedItem : the item being selected

“Main Event”– SelectedIndexChanged : fires when user changes

the selection

4-21

Application to Vintage DVD’sCode

Code for lstTypes SelectedIndex-Changed event

Private Sub lstTypes_SelectedIndexChanged(ByVal sender As _ System.Object, ByVal e As System.EventArgs) Handles _ lstTypes.SelectedIndexChanged Dim strDVDType As String, decPrice As Decimal strDVDType = lstTypes.SelectedItem Select Case strDVDType Case "Kids"

decPrice = 0.99 Case "Regular"

decPrice = 1.99 Case "Classic"

decPrice = 2.99 End Select txtDVDPrice.Text = Format(decPrice, "c")

End Sub

4-22

More Complex Decisions

Nested Decisions Compound Decisions

4-23

Nested Decisions

When one of the decision branch contains another decision structure

Nested decision structure must be “nested”, i.e. the “sub” decision must be completely contained inside of the outer decision branch

No overlap of decision structures

4-24

Compound Decisions

When the condition contains more than one expression (comparison)– If This And That Then …..– If This Or That Then …..– If This And Not That Then …..

4-25

Logical Operators in Compound Decisions

4-26

The Form Load Event

Occurs when the form is loaded Ideal place for setting up initial conditions

4-27

Step-by-Step 4-5:Using the frmVintage_Load Event

Demo

4-28

VB .NET Debugging Tools

The Debug Toolbar

4-29

VB .NET Debugging Tools

Breakpoints

4-30

Copyright 2004 John Wiley & Sons, Inc.

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