Week 5
More on the Selection Structure
1

Nested, If/ElseIf/Else, and Case Selection Structures
Lesson A Objectives
After completing this lesson, you will be able to:
 Include a nested selection structure in pseudocode and in a flowchart
 Code a nested selection structure
 Desk-check an algorithm
 Recognize common logic errors in selection structures
2

Nested, If/ElseIf/Else, and Case Selection Structures
Lesson A Objectives
 Code an If/ElseIf/Else selection structure
 Include a Case selection structure in pseudocode and in a flowchart
 Code a Case selection structure
 Write code that uses the Is, TypeOf…Is, and Like comparison operators
3

Nested Selection Structures
When either a selection structure’s true path or its false path contains another selection structure, the inner selection structure is referred to as a nested selection structure , because it is contained
(nested) within the outer selection structure
Message
You are too young to vote
You can vote
You need to register before you can vote
Criteria
Person is younger than 18
Person is at least 18 years old
AND
Is registered to vote
Person is at least 18 years old
AND
Is not registered to vote
4

Pseudocode and Example of Nested Selection Structure
If your age < 18
Message is “You are too young to vote”
Else
If you are registered
Message is “You can vote
Else
Message is “You need to register before you can vote”
End If
End If
If intAge < 18 Then strMessage = “You are too young to vote”
Else
If blnRegistered Then strMessage = “You can vote
Else strMessage = “You need to register before you can vote”
End If
End If
5

Logic Errors in Selection Structures
 Logic errors commonly made when writing selection structures are a result of one of the following mistakes:
1.
Using a logical operator when a nested selection structure is needed
2.
Reversing the primary and secondary decisions
3.
Using an unnecessary nested selection structure
 An algorithm is simply the set of step-by-step instructions that accomplish a task
 Desk-checking , also called hand-tracing , means that you use sample data to walk through each of the steps in the algorithm manually, just as if you were the computer
6

An Algorithm
Algorithm for determining amount of vacation
1. Declare variables
2. Get employment status and years
3. If the employment status is full-time
If the years is greater than 5 display “3-weeks” else display “2-weeks” end if end if
7

Using a Logical Operator Rather Than a Nested Selection Structure
Incorrect algorithm
1. Declare variables
2. Get employment status and years
3. If the status is full-time AndAlso the years is greater than 5 display “3-weeks” else display “2-weeks” end if
8

Reversing the Primary and Secondary
Decision
Incorrect algorithm
1. Declare variables
2. Get employment status and years
3. If the years is greater than 5
If the status is full-time display “3-weeks” else display “2-weeks” end if end if
9

Using an Unnecessary Nested
Selection Structure
Inefficient algorithm
1.
Declare variables
2.
Get employment status and years
3. If the status is full-time
If the years is greater than 5 display “3-weeks” else if the years are less than or equal to 5 display “2-weeks” end if end if endif
10

The If/ElseIf/Else Selection Structure
Determination of grade
Dim strGrade As String strGrade = UCase(InputBox("Grade?", "Grade"))
If strgrade = "A" Then
Me.MsgLabel.Text = "Excelent"
ElseIf strgrade = "B" Then
Me.MsgLabel.Text = "Above Average"
ElseIf strGrade = "C" Then
Me.MsgLabel.Text = "Average"
ElseIf strGrade = "D" OrElse strGrade = "F" Then
Me.MsgLabel.Text = "Below Average"
Else
Me.MsgLabel.Text = "Error"
End If
11

The Case Selection Structure
Select Case selectorExpression
[Case expressionList1
[instructions for the first Case]]
[Case expressionList2
[instructions for the second Case]]
[Case expressionListn
[instructions for the nth Case]]
[Case Else
[instructions for when the
selectorExpression does not match any of the expressionLists]]
End Select
12

The Case Selection Structure
Determination of grade
Dim strGrade As String = InputBox("Grade?", "Grade").ToUpper
Select Case strGrade
Case "A"
Me.MsgLabel.Text = "Excelent"
Case "B"
Me.MsgLabel.Text = "Above Average"
Case "C"
Me.MsgLabel.Text = "Average"
Case "D", "F"
Me.MsgLabel.Text = "Below Average"
Case Else
Me.MsgLabel.Text = "Error"
End Select
13

To and Is Keywords
 To keyword: specifies a range of minimum and maximum values
Case 1 To 5
 Is keyword: specifies a range of values when you know only one value, either the minimum or the maximum
Case Is > 10
14

The Is, TypeOf…Is, and Like
Comparison Operators
 The Is operator determines whether two object references refer to the same object
 An object reference is a memory address within the computer’s internal memory; it indicates where in memory the object is stored
 The TypeOf…Is operator determines whether an object is a specified type
 The Like operator uses pattern matching to determine whether one string is equal to another string
15

The Is Operator
Is operator
Syntax: objectReference1 Is objectReference2
Private Sub CalcComm(ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles Calc2Button.Click, Calc4Button.Click, Calc7Button.Click
Dim sngSales, sngComm As Single sngSales = Val(Me.SalesTextBox.Text)
If sender Is Me.Calc2Button Then sngComm = sngSales * 0.02
Handles
ElseIf sender Is Me.Calc4Button Then sngComm = sngSales * 0.04
ElseIf sender Is Me.Calc7Button Then sngComm = sngSales * 0.07
End If
End Sub
Compares memory addresses
16

The TypeOf…Is Operator
TypeOf … Is operator
Syntax: TypeOf objectReference1 Is objectType
Private Sub DisplayMessage(ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Compares
Handles SalesTextbox.TextChanged, ClearButton.Click
object to
Select Case True object type
Case TypeOf sender Is TextBox
Me.MsgLabel.Text = “The text box invoked procedure”
Case Else
Me.MsgLabel.Text = “The button invoked procedure”
End Select
End Sub
17

The Like Comparison Operator
Like comparison operator
Syntax: string Like pattern
Pattern Matches
?
Any single character
*
#
Zero or more characters
Any single digit
[charlist] Any character in charlist
[!charlist] Any character not in charlist
18

The Math Practice Application
Lesson B Objectives
After completing this lesson, you will be able to:
 Include a group of radio buttons in an interface
 Designate a default radio button
 Include a check box in an interface
 Create a user-defined Sub procedure
19

The Math Practice Application
Lesson B Objectives
 Generate random numbers using the Random object and the Random.Next method
 Call a user-defined Sub procedure
 Invoke a radio button control’s Click event procedure from code
 Process code when a form is first loaded into the computer’s memory
20

Completing the User Interface
 The application should display the addition or subtraction problem on the screen, then allow the student to enter the answer and then verify that the answer is correct
 If the student’s answer is not correct, the application should give him or her as many chances as necessary to answer the problem correctly
21

Adding a Radio Button to the Form
 RadioButton Control:
 Used to limit the user to only one choice in a group of options
 Should have a unique access key, which allows the user to select the button using the keyboard
 The selected button is called the default radio button and is either the radio button that represents the user’s most likely choice or the first radio button in the group
 You designate a radio button as the default radio button by setting the button’s Checked property to the Boolean value True
22

Adding a CheckBox
Control to the Form
 Checkbox Controls:
 Used to add a check box control to the interface
 Work like radio buttons in that they are either selected or deselected only; but that is where the similarity ends
 You use check box controls to allow the user to select any number of choices from a group of one or more independent and nonexclusive choices
 Any number of check boxes on a form can be selected at the same time
23

User-Defined Procedures
 A user-defined Sub procedure is a collection of code that can be invoked from one or more places in an application
 Enter Private Sub SubName() just above the End
Class statement
 The rules for naming a user-defined Sub procedure are the same as those for naming variables and constants
 The End Sub will automatically be generated
24

Generating Random Numbers
 Visual Studio provides a pseudo-random number generator, which is a device that produces a sequence of numbers that meet certain statistical requirements for randomness
 Dim GeneratorRandom As New Random()
 Methods
 Next(minValue, maxValue)
 NextDouble()
25

Coding the Grade1RadioButton and Grade2RadioButton
Click Event Procedures

You can use the Visual Basic Call statement , whose syntax is Call
procedurename([argumentlist]), to call
(invoke) a user-defined Sub procedure
 The square brackets in the syntax indicate that the argumentlist is optional
26

Coding the AdditionRadioButton and SubtractionRadioButton
Click Event Procedures
 When the user clicks either the
AdditionRadioButton control or the
SubtractionRadioButton control, the control’s
Click event procedure should display the appropriate mathematical operator (either a plus sign or a minus sign) in the OperatorPictureBox control and then generate and display two random numbers in the Num1Label and
Num2Label controls
27

Coding the Form’s
Load Event Procedure
 Instructions entered in the form’s Load event procedure are processed when the application is started and the form is loaded into memory
 The latter statement uses the
RandomButton.PerformClick method , whose syntax is radiobutton.
PerformClick (), to invoke the Addition radio button’s Click event, which causes the code in the Click event procedure to be processed by the computer
28

Completing the Math
Practice Application
Lesson C Objectives
After completing this lesson, you will be able to:
 Select the existing text in a text box control
 Code a check box control’s Click event procedure
 Display and hide a control
29

Coding the CheckAnswerButton
Click Event Procedure
 You still need to code the Click event procedures for the CheckAnswerButton and the DisplaySummaryCheckBox controls
 The pseudocode for the CheckAnswerButton control’s Click event procedure is shown in
Figure 5-49 in the textbook
30

31

Coding the SummaryCheckBox
Click Event Procedure
 The SummaryCheckBox control’s Click event procedure is responsible for both displaying and hiding the SummaryGroupBox control
 The procedure should display the group box control when the user selects the check box, and it should hide the group box control when the user deselects the check box
32