CHAPTER FIVE
Specifying Alternate Courses of
Action: Selection Statements
5- 2
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.
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Objectives
• 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- 4
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.
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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:
1. The condition.
2. The statements to perform for the first
outcome.
3. The statements to perform for the second
outcome.
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5- 6
5.2 The If…Then...Else Statement (cont.)
• Syntax and Action of If…Then…Else
– The If…Then…Else statement has the following
syntax:
• If condition Then
– Statementblock1
• Else
– Statementblock2
• End If
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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
statementblock1.
• False: the computer skips to Else and executes the
statements in statementblock2.
– Meta Statements
• Describe compound statement with a single
phrase.
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5- 8
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
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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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5.2 The If…Then...Else Statement (cont.)
– If…Then
• Sometimes only one of two outcomes requires
processing.
– If condition Then
statementblock
– End If
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5.3 Nested If Statements
• If statements may contain one within another.
If (X<Y) And (Y<Z) Then
AscOrDesc = “ascending”
Else
If (X>Y) And (Y>Z) Then
AscOrDesc = “descending”
Else
AscOrDesc = “neither ascending nor descending”
End If
End If
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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
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5.4 The MsgBox() Function (cont.)
• MsgBox() Button Combinations
–
–
–
–
–
–
MsgBoxStyle.AbortRetryIgnore (2)
MsgBoxStyle.OKOnly (0)
MsgBoxStyle.OKCancel (1)
MsgBoxStyle.RetryCancel (5)
MsgBoxStyle.YesNo (4)
MsgBoxStyle.YesNoCancel (3)
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5.4 The MsgBox() Function (cont.)
• MsgBox() Return Values
–
–
–
–
–
–
–
vbOK (1)
vbCancel (2)
vbAbort (3)
vbRetry (4)
vbIgnore (5)
vbYes (6)
vbNo (7)
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5.5 The RadioButton Control
• 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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5.5 The RadioButton Control (cont.)
– Properties
Appearance
CheckAlign
Checked
Enabled
FlatStyle
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Image
ImageAlign
Text
TextAlign
Visible
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5- 17
5.5 The RadioButton Control (cont.)
– Events
• CheckChanged event.
– Occurs when the value of the Checked property
changes.
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5- 18
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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5.6 The GroupBox Control (cont.)
– Properties
•
•
•
•
Enabled
FlatStyle
Text
Visible
– Events
• Click
• CheckChanged
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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.
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5.7 The CheckBox Control (cont.)
– Properties and Events
• Similar to the RadioButton.
• CheckState property gets or sets the state of the
CheckBox.
• ThreeState property will allow the user to select an
Indeterminate state.
• CheckBox can respond to a CheckedChanged
event.
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5.7 The CheckBox Control (cont.)
– Properties of the CheckBox control
•
•
•
•
•
•
•
•
CheckAlign
Checked
CheckState
Enabled
Text
TextAlign
ThreeState
Visible
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5.8 The Select Case Statement
• Can handle conditions with multiple outcomes.
– Syntax and Action of Select Case
Select Case testexpression
Case expressionlist1
statementblock1
Case expressionlist2
statementblock2
…
Case expressionlistN
statementblockN
Case Else
statementblock
End Select
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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.
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5.8 The Select Case Statement (cont.)
• 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.
– Ranges
• Two variations
– Use the keyword “To”
» Ex. 25 To 37
– Use the keyword “Is”
» Ex. Is >= 77
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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.
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5.9 The Exit Sub Statement
• 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.
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Chapter Summary (cont.)
• 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.
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