Chapter 4: The Selection Structure
Programming with Microsoft Visual Basic
.NET, Second Edition
The If…Then…Else Statement
Lesson A Objectives
• Write pseudocode for the selection structure
• Create a flowchart to help you plan an
application’s code
• Write an If...Then...Else statement
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The If…Then…Else Statement
Lesson A Objectives (continued)
• Write code that uses comparison operators and
logical operators
• Format numbers using the ToString method
• Change the case of a string
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The Selection Structure
• Use the selection structure to make a decision or
comparison and select a particular set of tasks to
perform
• The selection structure is also called the decision
structure
• The condition must result in either a true (yes) or
false (no) answer
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The Selection Structure (continued)
• If the condition is true, the program performs one
set of tasks
• If the condition is false, there may or may not be
a different set of tasks to perform
• Visual Basic .NET provides four forms of the
selection structure: If, If/Else, If/ElseIf/Else, and
Case
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Writing Pseudocode for If
and If/Else Selection Structures
• An If selection structure contains only one set of
instructions, which are processed when the
condition is true
• An If/Else selection structure contains two sets of
instructions:
– One set is processed when the condition is true
– The other set is processed when the condition is
false
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Flowcharting the If and If/Else
Selection Structures
start/stop oval
process rectangle
input/output parallelogram
selection/repetition diamond
symbols are connected by
flowlines
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Flowcharting the If and If/Else
Selection Structures (continued)
T
F
T
F
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Coding the If and
If/Else Selection Structures
If condition Then
statement block containing one or more
statements to be processed when the condition is
true
[Else
statement block containing one or more
statements to be processed when the condition is
false]
End If
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Coding the If and
If/Else Selection Structures
(continued)
• The items in square brackets ([ ]) in the syntax
are optional
• You do not need to include the Else portion
• Words in bold are essential components of the
statement
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Coding the If and
If/Else Selection Structures
(continued)
• Items in italic indicate where the programmer
must supply information pertaining to the current
application
• The set of statements contained in the true path,
as well as the statements in the false path, are
referred to as a statement block
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Comparison Operators
=
Is equal to
>
Is Greater Than
>=
Is Greater Than or Equal to
<
Is Less Than
<=
Is Less Than or Equal to
<>
Is Not Equal to
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Comparison Operators (continued)
• Comparison operators are also referred to as
relational operators
• All expressions containing a relational operator
will result in either a true or false answer only
• Comparison operators are evaluated from left to
right, and are evaluated after any mathematical
operators
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Comparison Operators (continued)
10 + 3 < 5 * 2
7>3*4/2
• 5 * 2 is evaluated first,
giving 10
• 3 * 4 is evaluated first,
giving 12
• 10 + 3 is evaluated
second, giving 13
• 12 / 2 is evaluated
second, giving 6
• 13 < 10 is evaluated
last, giving false
• 7 > 6 is evaluated last,
giving true
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Comparison Operators (continued)
Using a Comparison Operator
Dim first, second As Integer
If (first > second) Then
Dim temp As Integer
temp = first
first = second
first = temp
End If
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Logical Operators
Not
Reverses the truth value of condition; false becomes
true and true becomes false.
1
And
All conditions connected by the And operator must be
true for the compound condition to be true.
2
AndAlso
All conditions connected by the AndAlso operator must
be true for the compound condition to be true.
2
Or
Only one of the conditions connected by the Or
operator needs to be true for the compound condition
to be true.
3
OrElse
Only one of the conditions connected by the OrElse
operator needs to be true for the compound condition
to be true.
3
Xor
One of the conditions connected by Xor must be true
for the compound condition to be true.
4
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Logical Operators (continued)
• Truth table for Not operator
Result = Not Condition
If condition is
Value of Result is
True
False
False
True
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Logical Operators (continued)
• Truth table for And operator
Result = condition1 And Condition2
If condition1 is
And condition2 is Value of Result is
True
True
True
True
False
False
False
True
False
False
False
False
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Logical Operators (continued)
• Truth table for AndAlso operator
Result = condition1 AndAlso Condition2
If condition1 is
And condition2 is Value of Result is
True
True
True
True
False
False
False
(not evaluated)
False
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Logical Operators (continued)
• Truth table for Or operator
Result = condition1 Or Condition2
If condition1 is
And condition2 is Value of Result is
True
True
True
True
False
True
False
True
True
False
False
False
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Logical Operators (continued)
• Truth table for OrElse operator
Result = condition1 OrElse Condition2
If condition1 is
And condition2 is Value of Result is
True
(not evaluated)
True
False
True
True
False
False
False
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Logical Operators (continued)
• Truth table for Xor operator
Result = condition1 Xor Condition2
If condition1 is
And condition2 is Value of Result is
True
True
True
True
False
False
False
True
True
False
False
False
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Logical Operators (continued)
Figure 4-19: Order of
precedence for
arithmetic, comparison,
and logical operators
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Using the ToString Method to Format
Numbers
• Use the ToString method to format a number
• Syntax: variablename.ToString(formatString)
• variablename is the name of a numeric variable
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Using the ToString Method to Format
Numbers (continued)
• formatString is a string that specifies the format
– Must be enclosed in double quotation marks
– Takes the form Axx:
• A is an alphabetic character called the format
specifier
• xx is a sequence of digits called the precision
specifier
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Comparing Strings
• Example 1: Using the OrElse operator
Dim letter As String
letter = Me.uiLetterTextBox.Text
If letter = “P” OrElse letter = “p” Then
Me.uiResultLabel.Text = “Pass”
Else
Me.uiResultLabel.Text = “Fail”
End if
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Comparing Strings (continued)
• Example 2: Using the AndAlso operator
Dim letter As String
letter = Me.uiLetterTextBox.Text
If letter <> “P” AndAlso letter <> “p” Then
Me.uiResultLabel.Text = “Fail”
Else
Me.uiResultLabel.Text = “Pass”
End if
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Comparing Strings (continued)
• Example 3: Correct, but less efficient, solution
Dim letter As String
letter = Me.uiLetterTextBox.Text
If letter = “P” OrElse letter = “p” Then
Me.uiResultLabel.Text = “Pass”
End If
If letter <> “P” AndAlso letter <> “p” Then
Me.uiResultLabel.Text = “Fail”
End if
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Comparing Strings (continued)
• Example 4: Using the ToUpper method
Dim letter As String
letter = Me.uiLetterTextBox.Text
If letter.ToUpper() = “P” Then
Me.uiResultLabel.Text = “Pass”
Else
Me.uiResultLabel.Text = “Fail”
End if
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The Monthly Payment Calculator
Application
Lesson B Objectives
• Group objects using a GroupBox control
• Calculate a periodic payment using the
Financial.Pmt method
• Create a message box using the
MessageBox.Show method
• Determine the value returned by a message box
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Completing the User Interface
• Herman Juarez has asked you to create an
application that he can use to calculate the
monthly payment on a car loan
• To make this calculation, the application needs:
– The loan amount (principal)
– The annual percentage rate (APR) of interest
– The life of the loan (term) in years
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Completing the User Interface
(continued)
Figure 4-31: Sketch of the Monthly Payment Calculator user interface
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Adding a Group Box Control to the
Form
• Use the GroupBox tool in the Toolbox window to
add a group box control to the interface
• A group box control serves as a container for
other controls
• Use a group box control to visually separate
related controls from other controls on the form
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Coding the uiCalcPayButton Click
Event Procedure
• The uiCalcPayButton’s Click event procedure is
responsible for:
– Calculating the monthly payment amount
– Displaying the result in the uiPaymentLabel
control
• Figure 4-37 shows the pseudocode for the
uiCalcPayButton’s Click event procedure
Programming with Microsoft Visual Basic .NET, Second Edition
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Coding the uiCalcPayButton Click
Event Procedure (continued)
Figure 4-37: Pseudocode for the uiCalcPayButton Click event
procedure
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Using the Financial.Pmt Method
• Use the Visual Basic .NET Financial.Pmt method
to calculate a periodic payment on either a loan
or an investment
• Syntax:
Financial.Pmt(Rate, NPer, PV[, FV, Due])
• Rate: interest rate per period
• NPer: total number of payment periods (the term)
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Using the Financial.Pmt Method
(continued)
• PV: present value of the loan or investment; the
present value of a loan is the loan amount,
whereas the present value of an investment is
zero
• FV: future value of the loan or investment; the
future value of a loan is zero, whereas the future
value of an investment is the amount you want to
accumulate; if omitted, the number 0 is assumed
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Using the Financial.Pmt Method
(continued)
• Due: due date of payments; can be either the
constant DueDate.EndOfPeriod or the constant
DueDate.BegOfPeriod; if omitted,
DueDate.EndOfPeriod is assumed
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The MessageBox.Show Method
• Use the MessageBox.Show method to display a
message box that contains text, one or more
buttons, and an icon
• Syntax: MessageBox.Show(text, caption,
buttons, icon[, defaultButton])
• text: text to display in the message box
• caption: text to display in the title bar of the
message box
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The MessageBox.Show Method
(continued)
• buttons: buttons to display in the message box
• icon: icon to display in the message box
• defaultButton: button automatically selected
when the user presses Enter
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Coding the TextChanged Event
• A control’s TextChanged event occurs when the
contents of a control’s Text property have
changed as a result of:
– The user entering data into the control, or
– The application’s code assigning data to the
control’s Text property
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Coding the TextChanged Event
(continued)
• When the user makes a change to the
information entered in the three text box controls,
the Monthly Payment Calculator application
should delete the monthly payment displayed in
the uiPaymentLabel control
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Completing the Monthly
Payment Calculator Application
Lesson C Objectives
• Specify the keys that a text box will accept
• Align the text in a label control
• Handle exceptions using a Try/Catch block
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Coding the KeyPress Event
• Template
Private Sub uiPrincipalTextBox_KeyPress( _
ByVal sender As Object, _
ByVal e As
System.Windows.Forms.KeyPressEventArgs) _
Handles uiPrincipalTextBox.KeyPress
• Setting e.Handled = True will cancel the key
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Aligning the Text in a Label Control
• The TextAlign property controls the placement of
the text in a label control
• The TextAlign property can be set to TopLeft (the
default), TopCenter, TopRight, MiddleLeft,
MiddleCenter, MiddleRight, BottomLeft,
BottomCenter, or BottomRight
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Using a Try/Catch Block
• An exception is an error that occurs while a
program is running
• Use the Try statement to catch (or trap) an
exception when it occurs in a program
• Use a Catch statement to take the appropriate
action to resolve the problem
• A block of code that uses both the Try and Catch
statements is referred to as a Try/Catch block
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Using a Try/Catch Block (continued)
Try
one or more statements that might generate an
exception
Catch [variablename As exceptionType]
one or more statements that will execute when an
exceptionType exception occurs
[Catch [variablename As exceptionType]
one or more statements that will execute when an
exceptionType exception occurs]
End Try
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Summary
• To evaluate an expression containing arithmetic,
comparison, and logical operators, evaluate
arithmetic operators first, then comparison
operators, and then logical operators
• To code a selection structure, use the
If...Then...Else statement
• To create a compound condition, use the logical
operators
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Summary (continued)
• Use the GroupBox tool to add a group box
control to the form; drag controls from the form or
the Toolbox window into the group box control
• To calculate a periodic payment on either a loan
or an investment, use the Financial.Pmt method
• To display a message box that contains text, one
or more buttons, and an icon, use the
MessageBox.Show method
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Summary (continued)
• To allow a text box to accept only certain keys,
code the text box’s KeyPress event
• To align the text in a control, set the control’s
TextAlign property
• To catch an exception, and then have the
computer take the appropriate action, use a
Try/Catch block
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