Programming with
Microsoft Visual Basic 2008
Fourth Edition
Chapter Four
The Selection Structure
Previewing the Monthly Payment
Calculator Application
• The Monthly Payment Calculator application
uses the selection structure
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Previewing the Monthly Payment
Calculator Application (continued)
Figure 4-2: Monthly payment amount shown in the interface
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Lesson A Objectives
After studying Lesson A, you should be able to:
• Write pseudocode for the selection structure
• Create a flowchart to help you plan an
application’s code
• Write an If...Then...Else statement
• Write code that uses comparison operators and
logical operators
• Change the case of a string
• Determine the success of the TryParse method
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The Selection Structure
• Selection structure:
– Chooses one of two paths based on condition
– Also called a decision structure
• Example:
– If employee works over 40 hours, add overtime
pay
• Condition: Expression evaluating to true or false
• Four selection structures in Visual Basic:
– If, If/Else, If/ElseIf/Else, and Case
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The Selection Structure (continued)
Figure 4-3: Selection structures you might use today
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Writing Pseudocode for If
and If/Else Selection Structures
• If selection structure:
– Contains only one set of instructions
– Instructions are processed if the condition is true
• If/Else selection structure:
– Contains two sets of instructions
– True path: Instruction set following true condition
– False path: Instruction set following false
condition
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Writing Pseudocode for If
and If/Else Selection Structures
(continued)
Figure 4-4: Examples of the If and If/Else selection structures
written in pseudocode
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Flowcharting the If and If/Else
Selection Structures
• Flowchart:
– Uses standardized symbols showing steps to be
taken to accomplish a task
•
•
•
•
Oval: Start/stop symbol
Rectangle: Process symbol
Parallelogram: Input/output symbol
Diamond: Decision symbol
– Used in both selection and repetition structures
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Coding the If and
If/Else Selection Structures
• If…Then…Else statement: Used to code If and
If/Else selections structures
• Syntax:
If condition Then
statement block for true path
[Else
statement block for false path]
End If
– condition must be a Boolean expression that
evaluates to True or False
– Else clause is optional
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Flowcharting the If and If/Else
Selection Structures (continued)
Figure 4-5: Examples of the If and If/Else selection
structures drawn in flowchart form
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Comparison Operators
• Comparison (relational) operators:
– Used to test two items for equality or types of nonequality
– Always result in a True or False value
• Rules for comparison operators
– They do not have an order of precedence
– They are evaluated from left to right
– They are evaluated after any arithmetic operators
in the expression
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Comparison Operators (continued)
Figure 4-8: Evaluation steps for an expression containing arithmetic and
comparison operators
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Using Comparison Operators—
Swapping Numeric Values
• Sample application displays the lowest and
highest of two numbers entered by the user
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Using Comparison Operators—
Swapping Numeric Values
(continued)
Figure 4-9: Sample run of the Lowest and Highest application
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Using Comparison Operators—
Swapping Numeric Values (continued)
Figure 4-10: Display button’s pseudocode showing the If
selection structure
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Using Comparison Operators—
Swapping Numeric Values (continued)
Figure 4-11: Display button’s flowchart showing the If selection structure
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Using Comparison Operators—
Swapping Numeric Values (continued)
Figure 4-12: Display button’s Click event procedure showing the
If selection structure
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Using Comparison Operators—
Swapping Numeric Values
(continued)
• Values input by the user are stored in variables
with procedure scope
• A temporary variable is used when values must
be swapped
– Declared within statement block
• Block scope: Restricts use of variable to
statement block in which it is declared
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Using Comparison Operators—
Swapping Numeric Values
(continued)
Figure 4-13: Illustration of the swapping concept
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Using Comparison Operators—
Displaying the Sum or Difference
• Sample application that displays the sum or
difference of two numbers entered by the user
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Using Comparison Operators—
Displaying the Sum or Difference
(continued)
Figure 4-14: Sample run of the Addition and Subtraction application
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Using Comparison Operators—
Displaying the Sum or Difference
(continued)
Figure 4-15: Calculate button’s pseudocode showing the If/Else
selection structure
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Using Comparison Operators—
Displaying the Sum or Difference
(continued)
Figure 4-16: Calculate button’s flowchart showing the If/Else
selection structure
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Using Comparison Operators—
Displaying the Sum or Difference
(continued)
Figure 4-17: Calculate button’s Click event procedure showing the
If/Else selection structure
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Logical Operators
• Logical operators:
– Used to create compound conditions
– Also called Boolean operators
• Six logical operators in Visual Basic:
–
–
–
–
–
–
And
Or
Not
AndAlso
OrElse
Xor
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Logical Operators (continued)
Figure 4-18: Listing and examples of logical operators
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Logical Operators (continued)
• Truth tables: Show how logical operators are
evaluated
• Short circuit evaluation: Bypasses evaluation of
condition when outcome can be determined without
it
– Operators using technique: AndAlso, OrElse
• Example:
– If state = "TN" AndAlso sales > 50000D Then…
– If state is not TN, no need to evaluate sales >
50000D
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Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators
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Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators
(continued)
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Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators
(continued)
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Using the Truth Tables
• Scenario: Calculate a bonus for a salesperson
– Bonus condition: “A” rating and sales > $10,000
– Appropriate operators: And, AndAlso (more
efficient)
– Both conditions must be true to receive bonus
– Sample code: rating = "A" AndAlso sales > 10000
• Precedence of logical operators:
– Evaluated after any arithmetic or comparison
operators in the expression
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Using Logical Operators: Calculating
Gross Pay
• Data validation: Verifying that input data is
within expected range
• Scenario: Calculate and display employee gross
pay
• Requirements for application:
– Verify hours are within range (>= 0.0 and <= 40.0)
– If data is valid, calculate and display gross pay
– If data is not valid, display error message
• Can accomplish this using AndAlso or OrElse
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Comparing Strings Containing
Letters
• Scenario:
– Display “Pass” if ‘P’ is entered in txtLetter control
– Display “Fail” if ‘F’ is entered in txtLetter control
• Can use the OrElse or the AndAlso operator
• Note that ‘P’ is not the same as ‘p’
– They have different Unicode values
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Comparing Strings Containing
Letters (continued)
Figure 4-23: Visual Basic code showing string comparisons in the
If...Then...Else statement’s condition
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Comparing Strings Containing
Letters (continued)
Figure 4-23: Visual Basic code showing string comparisons in the
If...Then...Else statement’s condition (continued)
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Converting a String
to Uppercase or Lowercase
• String comparisons are case sensitive
• CharacterCasing property:
– Three case values: Normal (default), Upper,
Lower
• ToUpper method: Converts string to upper case
• ToLower method: Converts string to lower case
• Example: If strLetter.ToUpper = “p" Then
– Note that strLetter’s value is not permanently
converted
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Converting a String to Uppercase or
Lowercase (continued)
Figure 4-24: Syntax and examples of the ToUpper and ToLower methods
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Converting a String to Uppercase or
Lowercase (continued)
Figure 4-24: Syntax and examples of the ToUpper and ToLower methods
(continued)
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Using the ToUpper and ToLower
Methods: Displaying a Message
• Procedure requirements:
– Display message “We have a store in this state”
– Valid states: IL, IN, KY
– Must handle case variations in the user’s input
• Can use ToLower or ToUpper
• Can assign a String variable to the input
textbox’s value converted to upper case
Dim strState As String
strState = txtState.Text.ToUpper
• Use If/Else to test value and display message
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Comparing Boolean Values
• Boolean variable: Contains either True or False
• Naming convention: “Is” denotes Boolean type
– Example: blnIsInsured
• When testing for a True value, it is not necessary
to include the “= True”
– Examples:
If blnIsInsured = True Then
or
If blnIsInsured Then
• Use Not logical operator to test for False value
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Comparing Boolean Values
(continued)
Figure 4-27: Examples of comparing Boolean values in an
If…Then…Else statement’s condition
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Comparing Boolean Values:
Determining Whether a String Can
Be Converted to a Number
• Determining whether a string can be converted to
a number:
– TryParse method returns a numeric value after
converting the string, or 0 if it cannot be converted
– TryParse also returns a Boolean value indicating
success or failure of the conversion attempt
• Use Boolean value returned by TryParse method
in an If…Then…Else statement
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Comparing Boolean Values:
Determining Whether a String Can
be Converted to a Number
(continued)
Figure 4-28: Syntax and example of using the Boolean value returned by
the TryParse method
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Lesson A Summary
• Arithmetic operators are evaluated first, then
comparison operators, and finally logical operators
• If...Then...Else statement: Selection structure with
a true path and a false path
• Use comparison operators to compare two values
• Use a temporary variable to swap values
contained in two variables
• Use logical operators to create a compound
condition
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Lesson A Summary (continued)
• Use text box’s CharacterCasing property to
change text to upper or lower case
• Use ToUpper and ToLower to temporarily modify
the case of input text
• Use Boolean return value of TryParse method to
determine whether string was successfully
converted to numeric value
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Lesson B Objectives
After studying Lesson B, you should be able to:
• 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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Creating the Monthly Payment
Calculator Application
• Program requirement: Calculate monthly
payment on car loan
• To do so, 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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Adding a Group Box to the Form
• Group box: Container control for other controls
• GroupBox tool:
– Used to add group box control to interface
• Group box control provides:
– Visual separation of related controls
– Ability to manage the grouped controls by
manipulating the group box control
• Lock controls to ensure that they are not moved
• Be sure to set TabIndex after placement of
controls
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Coding the Monthly Payment
Calculator Application
• Procedures required according to TOE chart:
– Click event procedure code for the two buttons
– Code for TextChanged, KeyPress, and Enter
events for text boxes
• Procedures that are already coded:
– btnExit Click event and TextChanged events for
the text boxes
• Procedure to code in Lesson B:
– btnCalc button’s Click event procedure
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Coding the Monthly Payment
Calculator Application (continued)
Figure 4-33: TOE chart for the Monthly Payment Calculator application
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Coding the btnCalc Control’s
Click Event Procedure
• Tasks for btnCalc button’s Click event procedure:
– Calculate monthly payment amount
– Display result in lblPayment control
• Two selection structures needed
– If and If/Else
• Determine interest rate and term
• Determine need for named constants and
variables within procedure
– Constants: Items that do not change with each call
– Variables: Items will likely change with each call
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Coding the btnCalc Control’s
Click Event Procedure (continued)
Figure 4-34: Pseudocode for the btnCalc control’s Click event procedure
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Coding the btnCalc Control’s
Click Event Procedure (continued)
Figure 4-35: Partially completed Click event procedure
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Using the Financial.Pmt Method
• Calculates periodic payment on loan or
investment
• Must ensure that interest rate and number of
periods are expressed in same units (months or
years)
– Convert an annual interest rate to monthly rate by
dividing by 12
– Convert an annual term to a monthly term by
multiplying by 12
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Using the Financial.Pmt Method
(continued)
Figure 4-36: Basic syntax and examples of the Financial.Pmt method
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Using the Financial.Pmt Method
(continued)
Figure 4-37: Selection structure’s true path coded in the procedure
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The MessageBox.Show Method
• MessageBox.show method: Displays message
box with text message, caption, button(s), and
icon
• Use sentence capitalization for text message
• Use book title capitalization for caption
• Icons:
– Exclamation or Question: Indicates user must
make a decision before continuing
– Information: Indicates informational message
– Stop: Indicates serious problem
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The MessageBox.Show Method
(continued)
Figure 4-41: Values returned by the MessageBox.Show method
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The MessageBox.Show Method
(continued)
Figure 4-41: Values returned by the MessageBox.Show method
(continued)
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Lesson B Summary
• Group box is container control that treats its
contents as one unit
• Use Financial.Pmt method to calculate loan or
investment payments
• MessageBox.Show method displays message
box with text, one or more buttons, and icon
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Lesson C Objectives
After studying Lesson C, you should be able to:
• Prevent the entry of unwanted characters in a
text box
• Select the existing text in a text box
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Coding the KeyPress Event
Procedures
• KeyPress event:
– Occurs when key is pressed while control has
focus
– Character corresponding to pressed key is sent to
KeyPress event’s e parameter
• KeyPress event can be used to prevent users
from entering inappropriate characters
– Use e parameter’s KeyChar property to
determine pressed key
– Use Handled property to cancel key if needed
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Coding the KeyPress Event
Procedures (continued)
Figure 4-47: Code template for the txtPrincipal’s KeyPress event
procedure
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Coding the KeyPress Event
Procedures (continued)
Figure 4-48: Examples of using the KeyChar and Handled properties in
the KeyPress event procedure
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Coding the KeyPress Event
Procedures (continued)
Figure 4-49: Completed CancelKeys procedure
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Coding the Enter Event Procedure
• Enter event:
– Occurs when text box receives focus
– If text is selected, user can replace existing text by
pressing key
• Can use Enter event to select all of text
• SelectAll method: Selects all text contained in
text box
– Add to each text box’s Enter event procedure
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Coding the Enter Event Procedure
(continued)
Figure 4-50: Syntax and an example of the SelectAll method
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Lesson C Summary
• KeyPress event occurs when user presses key
• Use KeyPress event to cancel unwanted key
entered by user
• Enter event occurs when text box receives focus
• Use Enter event to process code when control
receives focus
• Use SelectAll method to select all contents of text
box
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