1 week 5 more on the selection structure. 2 nested, if/elseif/else, and case selection structures...

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Week 5 More on the Selection Structure

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Nested, If/ElseIf/Else, and Case Selection Structures

Lesson A ObjectivesAfter 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

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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

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Nested Selection Structures

Message Criteria

You are too young to vote Person is younger than 18

You can vote Person is at least 18 years old

AND

Is registered to vote

You need to register before you can vote

Person is at least 18 years old

AND

Is not registered to vote

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

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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

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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

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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


end if

end if

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Using a Logical Operator Rather Than a Nested Selection Structure

Incorrect algorithm



3. If the status is full-time AndAlso the years is greater than 5


else


end if

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Reversing the Primary and Secondary Decision

Incorrect algorithm



3. If the years is greater than 5

If the status is full-time


else


end if

end if

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Using an Unnecessary Nested Selection Structure

Inefficient algorithm



3. If the status is full-time

If the years is greater than 5


else

if the years are less than or equal to 5


end if

end if

endif

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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

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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

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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

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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

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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

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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

ElseIf sender Is Me.Calc4Button Then


ElseIf sender Is Me.Calc7Button Then


End If

End Sub

Handles

Comparesmemory

addresses

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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) _

Handles SalesTextbox.TextChanged, ClearButton.Click

Select Case True

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

Comparesobject to

object type

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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

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The Math Practice ApplicationLesson 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

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The Math Practice ApplicationLesson 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

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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

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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

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Adding a CheckBoxControl 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

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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

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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()

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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

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Coding the AdditionRadioButtonand 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

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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

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Completing the MathPractice ApplicationLesson 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

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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

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Coding the SummaryCheckBoxClick 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

1 week 5 more on the selection structure. 2 nested, if/elseif/else, and case selection structures...

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