Programming with
Microsoft Visual Basic 2010
5th Edition
Chapter Four
The Selection Structure
Previewing the Monthly Payment
Calculator Application
 The Monthly Payment Calculator application uses the
selection structure
2
Figure 4-1
Message box
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
 Include comparison operators and logical operators in a
selection structure’s condition
 Change the case of a string
 Determine the success of the TryParse method
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Making Decisions in a Program
 Three basic control structures
 Sequence
 Selection
 Repetition
 All procedures in an application are written using one of
more of these structures
 Procedures in previous chapters used sequence structure only
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Making Decisions in a Program
(cont’d.)
 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
 Decision expression evaluating to true or false
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Making Decisions in a Program
(cont’d.)
 Single-alternative selection structure
 Tasks performed only when condition is true
 Dual-alternative selection structure
 One set of tasks performed if condition is true
 Called true path
 Different set of tasks performed if condition is false
 Called false path
 If and end if
 Denotes selection structure’s beginning and end
 Else denotes beginning of false path
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Making Decisions in a Program
(cont’d.)
Figure 4-3 Selection structures you might use today
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Making Decisions in a Program
(cont’d.)
 Example: Kanton Boutique
Figure 4-4 Problem specification for Kanton Boutique
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Figure 4-5 Interface for the Kanton Boutique application
Figure 4-6 Pseudocode containing only the sequence structure
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Figure 4-7 Modified problem specification and pseudocode
containing a single-alternative selection structure
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Making Decisions in a Program
(cont’d.)
 Decision symbol
 Diamond shape in a flowchart
 Represents the selection structure’s condition
 Other symbols
 Oval: Start/stop symbol
 Rectangle: Process symbol
 Parallelogram: Input/output symbol
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Figure 4-8 Single-alternative selection structure shown in a flowchart
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Figure 4-9 Modified problem specification and pseudocode containing
a dual-alternative selection structure
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Figure 4-10 Dual-alternative selection structure shown in a flowchart
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Coding Single-Alternative and DualAlternative Selection Structures
 If…Then…Else statement
 Used to code single and dual-alternative selection structures
 Statement block
 Set of statements in each path
 Syntax and examples shown in Figure 4-11 on next slide
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Figure 4-11 Syntax and examples of the If…Then…Else statement (continues)
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Figure 4-11 Syntax and examples of the If…Then…Else statement (cont’d.)
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Comparison Operators
 Comparison operators
 Used to compare two values
 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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Figure 4-12 Listing and examples of commonly used comparison operators
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Comparison Operators (cont’d.)
 Using comparison operators: Swapping numeric values
 Sample application displays the lowest and highest of two
numbers entered by the user
Figure 4-14
Sample run of the Lowest and Highest
application
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Comparison Operators (cont’d.)
Figure 4-15 Pseudocode containing a single-alternative selection structure
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Figure 4-16 Flowchart containing a single-alternative selection structure
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Figure 4-17 Display button’s Click event procedure
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Comparison Operators (cont’d.)
 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-level variable
 Block scope
 Restricts use of variable to statement block in which it is
declared
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Comparison Operators (cont’d.)
Figure 4-18 Illustration of the swapping concept
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Comparison Operators (cont’d.)
 Using comparison operators: Displaying the sum or difference
 Sample application displays the sum or difference of two numbers
entered by the user
Figure 4-19
Sample run of the Addition
and Subtraction application
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Comparison Operators (cont’d.)
Figure 4-20 Pseudocode containing a dual-alternative selection structure
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Figure 4-21 Pseudocode containing a dual-alternative selection structure
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Figure 4-22 Calculate button’s Click event procedure
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Logical Operators
 Logical operators
 Used to create compound conditions
 Expressions evaluate to a Boolean value
 True or False
 Six logical operators in Visual Basic
 Not, And, AndAlso, Or, OrElse, Xor
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Figure 4-23 Listing and examples of logical operators (continues)
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Figure 4-23 Listing and examples of logical operators (cont’d.)
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Logical Operators (cont’d.)
 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 > $5000 Then…
 If state is not TN, no need to evaluate sales > $5000
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Figure 4-24
Truth tables for the logical
operators
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Logical Operators (cont’d.)
 Using the truth tables
 Scenario: Calculate a bonus for a salesperson
 Bonus condition: “A” rating and sales > $9,000
 Appropriate operators: And, AndAlso (more efficient)
 Both conditions must be true to receive bonus
 Sample code: strRating = "A” AndAlso dblSales >
9000
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Logical Operators (cont’d.)
 Using logical operators: Calculating gross pay
 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
 Data validation
 Verifying that input data is within expected range
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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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Figure 4-28
Examples of using string
comparisons in a
procedure
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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 uppercase
 ToLower method: Converts string to lowercase
 Syntax and examples shown in Figure 4-29 on next two slides
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Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (continues)
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Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (cont’d.)
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to Uppercase or Lowercase
(cont’d.)
 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 text box’s value
converted to uppercase
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Figure 4-29
Examples of using the
ToUpper and ToLower
methods in a
procedure
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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 in Figure 4-32 on next slide
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Comparing Boolean Values
(cont’d.)
Figure 4-32 Examples of using Boolean values in a condition
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Comparing Boolean Values
(cont’d.)
 Comparing Boolean values: 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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Figure 4-33 Syntax and example of using the
Boolean value returned by the TryParse method
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Summary of Operators
 Precedence of logical operators
 Evaluated after any arithmetic or comparison operators in the
expression
 Summary listing of arithmetic, concatenation, comparison,
and logical operators in Figure 4-36 in text
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Lesson A Summary
 Single and dual-alternative selection structures
 Use If...Then...Else statement
 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
 Use text box’s CharacterCasing property to change text to
upper- or lowercase
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Lesson A Summary (cont’d.)
 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
 Arithmetic operators are evaluated first, then comparison
operators, and finally logical operators
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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
52
Creating the Monthly Payment
Calculator Application
 Program requirement
 Calculate monthly payment on car loan
 Application needs
 The loan amount (principal)
 The annual percentage rate (APR) of interest
 The life of the loan (term) in years
53
Creating the Monthly Payment
Calculator Application (cont’d.)
 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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Figure 4-42 TOE chart for the Monthly Payment Calculator application
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Coding the Monthly Payment
Calculator Application (cont’d.)
 Coding the btnCalc control’s Click event procedure
 Calculate monthly payment amount
 Display result in lblPayment control
 Determine need for named constants and variables
 Constants: Items that do not change each time procedure
invoked
 Variables: Items will likely change each time
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Coding the Monthly Payment
Calculator Application (cont’d.)
Figure 4-43 Pseudocode for the btnCalc control’s Click event procedure
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Figure 4-44 Partially completed Click event procedure
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Using the Financial.Pmt Method
 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 annual interest rate to monthly rate by dividing by 12
 Convert annual term to monthly term by multiplying by 12
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Figure 4-45 Basic syntax and examples of the Financial.Pmt method
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Using the Financial.Pmt Method
(cont’d.)
Figure 4-46 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
(cont’d.)
Figure 4-50 Values returned by the MessageBox.Show method (continues)
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The MessageBox.Show Method
(cont’d.)
Figure 4-50 Values returned by the MessageBox.Show method (cont’d.)
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Lesson B Summary
 Group box: A container control that treats its contents as one
unit
 Use GroupBox tool to add a group box
 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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Figure 4-57 Examples of using the KeyChar and Handled
properties in the KeyPress event procedure
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Coding the KeyPress Event
Procedures (cont’d.)
Figure 4-58 CancelKeys procedure
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Coding the Enter Event Procedures
 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 Procedures
(cont’d.)
Figure 4-59 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 pressed by user
 Use SelectAll method to select all contents of text box
 Enter event occurs when text box receives focus
 Use Enter event to process code when control receives focus
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