Lecture Learning programming using Visual Basic Net – Chapter 5 Specifying alternate courses of action Selection statements

Chapter 5 Specifying alternate courses of action Selection statements. Lecture Learning programming using Visual Basic Net – Chapter 5 Specifying alternate courses of action Selection statements Lecture Learning,Learning programming,Lecture Learning programming

Specifying Alternate Courses of Action: Selection Statements

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Introduction

• A need to select an appropriate action from several

alternatives.

• Two statements will make this possible:

– The If…Then…Else statement.

– The Select Case statement.

• Construct programs that select alternative actions.

• Compare the Select Case statement with the If…Then… Else statement.

• Create GUIs using the MsgBox() function, Radio Button, GroupBox, and CheckBox.

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5.1 The Decision-Making Process

• We must be very precise in writing the criterion and

alternative actions for decisions.

• In a program,

– A condition is represented as an expression.

– An outcome is the result of an evaluated condition.

– An appropriate action follows the outcome.

5.2 The If…Then Else Statement

• The If…Then…Else statement enables a program to

handle situations having two outcomes.

• The statement has three parts:

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5.2 The If…Then Else Statement (cont.)

• Syntax and Action of If…Then…Else

– The If…Then…Else statement has the following syntax:

5.2 The If…Then Else Statement (cont.)

– Run Time: The Effect of the If…Then…Else Statement

• True: the computer executes the statements in

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5.2 The If…Then Else Statement (cont.)

– Problem Solving and Pseudocode

• Problem solving is the process of writing code to

perform a required task.

• Pseudocode is an English-like outline of the logical for

program code.

– Ex Request address from user

Receive input in a textbox

5.2 The If…Then Else Statement (cont.)

– Using Logical Expressions in If…Then…Else Statements

• The condition of an If…Then…Else statement may be a

logical expression.

– Ex (YearsExperience>5) And (NumberOfLanguages>=3)

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• If statements may contain one within another.

If (X<Y) And (Y<Z) Then

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5.4 The MsgBox() Function

• Displays a message box on the screen and waits for the user to click one of the button.

• Returns a value that indicates which button the user

clicked.

• Syntax:

MsgBox(message, mbStyle, title) As

MsgBoxResult

• MsgBox() Button Combinations

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5.4 The MsgBox() Function (cont.)

• MsgBox() Return Values

• Ensures that user will select only one option

– Appearance and Use

• Appears as descriptive text next to a circle.

• User selects with the mouse.

• Selection is transferred if user clicks another

radiobutton.

• Developers should limit the number of radiobuttons to

about seven per form.

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– Events

• CheckChanged event.

– Occurs when the value of the Checked property changes.

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5.6 The GroupBox Control

• Allows you to group RadioButtons to correspond to

categories of items.

– Appearance and Use

• Appears as a rectangle surrounding the controls it

groups together.

• Descriptive text can help identify the “group.”

• You may drag a RadioButton in or out of a GroupBox.

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5.7 The CheckBox Control

• Used when a combination of options may be selected.

– Appearance and Use

• Appears as descriptive text next to a square.

• User may select a CheckBox by clicking it with the

mouse.

• User may deselect a CheckBox by clicking it again with

the mouse.

– Properties and Events

• Similar to the RadioButton.

• CheckState property gets or sets the state of the

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5.7 The CheckBox Control (cont.)

– Properties of the CheckBox control

• Can handle conditions with multiple outcomes.

– Syntax and Action of Select Case

Select Case testexpression Case expressionlist1

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5.8 The Select Case Statement (cont.)

– Run Time: The Effect of the Select Case Statement

• First the computer evaluates the test expression.

• Then it tries to match the resulting value.

• The search starts at the top expression list.

• The computer stops at the first match.

• The corresponding statement block is executed.

• Control resumes after the End Select.

• If no match occurs and a Case Else exists, then the Case

Else block is executed.

• Then control resumes after the End Select.

• If no match occurs and no Case Else exists, then control

resumes after the End Select.

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5.8 The Select Case Statement (cont.)

– If versus Select Case

• If a decision has only two outcomes and the condition

can be expressed as a single logical expression, then the If statement is easier to read.

• If a decision has multiple outcomes that depend on a

single expression, then the Select Case is usually easier

to read.

• If the outcomes depend on a number of conditions that

may be independent, then embedded If statements are better.

• For decisions where one of the appropriate actions is to

stop processing.

• Causes execution to skip directly to End Sub.

• Is not limited to If…Then…Else and Select Case

statements.

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Chapter Summary

• To make a decision, we first evaluate a condition and

determine its outcome.

• Programs must imitate the decision-making process.

• The logic of decision-making may be complex.

• Developers use pseudocode to describe the logical

steps in a problem solution.

• A meta statement describes a compound statement with

a single phrase.

• The If…Then…Else statement enables a program to

choose one of two actions based on a condition.

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Chapter Summary (cont.)

• The RadioButton, GroupBox, and CheckBox are three

controls that enable the user to make choices with the

GUI.

• The SelectCase statement handles a single condition

that may have more than one outcome.

• The Exit Sub statement enables a program to stop

execution before a procedure is completed.