1. Programming with Microsoft Visual Basic 2008 Fourth Edition
Chapter Four
The Selection Structure

2. Previewing the Monthly Payment Calculator Application
The Monthly Payment Calculator application uses the selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

3. Previewing the Monthly Payment Calculator Application (continued)
Figure 4-2: Monthly payment amount shown in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

4. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

5. The 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

6. The Selection Structure (continued)
Figure 4-3: Selection structures you might use today
Programming with Microsoft Visual Basic 2008, Fourth Edition

7. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

8. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

9. Flowcharting the If and If/Else Selection Structures
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
Programming with Microsoft Visual Basic 2008, Fourth Edition

10. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

11. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

12. Comparison Operators Comparison (relational) operators:
Used to test two items for equality or types of non-equality
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
Programming with Microsoft Visual Basic 2008, Fourth Edition

13. Comparison Operators (continued)
Figure 4-8: Evaluation steps for an expression containing arithmetic and comparison operators
Programming with Microsoft Visual Basic 2008, Fourth Edition

14. Using Comparison Operators—Swapping Numeric Values
Sample application displays the lowest and highest of two numbers entered by the user
Programming with Microsoft Visual Basic 2008, Fourth Edition

15. Using Comparison Operators—Swapping Numeric Values (continued)
Figure 4-9: Sample run of the Lowest and Highest application
Programming with Microsoft Visual Basic 2008, Fourth Edition

16. Using Comparison Operators—Swapping Numeric Values (continued)
Figure 4-10: Display button’s pseudocode showing the If selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

17. Using Comparison Operators—Swapping Numeric Values (continued)
Figure 4-11: Display button’s flowchart showing the If selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

18. Using Comparison Operators—Swapping Numeric Values (continued)
Figure 4-12: Display button’s Click event procedure showing the If selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

19. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

20. Using Comparison Operators—Swapping Numeric Values (continued)
Figure 4-13: Illustration of the swapping concept
Programming with Microsoft Visual Basic 2008, Fourth Edition

21. Using Comparison Operators—Displaying the Sum or Difference
Sample application that displays the sum or difference of two numbers entered by the user
Programming with Microsoft Visual Basic 2008, Fourth Edition

22. Figure 4-14: Sample run of the Addition and Subtraction application
Using Comparison Operators—Displaying the Sum or Difference (continued)
Figure 4-14: Sample run of the Addition and Subtraction application
Programming with Microsoft Visual Basic 2008, Fourth Edition

23. Using Comparison Operators—Displaying the Sum or Difference (continued)
Figure 4-15: Calculate button’s pseudocode showing the If/Else selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

24. Using Comparison Operators—Displaying the Sum or Difference (continued)
Figure 4-16: Calculate button’s flowchart showing the If/Else selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

25. Using Comparison Operators—Displaying the Sum or Difference (continued)
Figure 4-17: Calculate button’s Click event procedure showing the If/Else selection structure
Programming with Microsoft Visual Basic 2008, Fourth Edition

26. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

27. Logical Operators (continued)
Figure 4-18: Listing and examples of logical operators
Programming with Microsoft Visual Basic 2008, Fourth Edition

28. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

29. Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators
Programming with Microsoft Visual Basic 2008, Fourth Edition

30. Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

31. Logical Operators (continued)
Figure 4-19: Truth tables for the logical operators (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

32. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

33. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

34. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

35. Comparing Strings Containing Letters (continued)
Figure 4-23: Visual Basic code showing string comparisons in the If...Then...Else statement’s condition
Programming with Microsoft Visual Basic 2008, Fourth Edition

36. Comparing Strings Containing Letters (continued)
Figure 4-23: Visual Basic code showing string comparisons in the If...Then...Else statement’s condition (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

37. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

38. Converting a String to Uppercase or Lowercase (continued)
Figure 4-24: Syntax and examples of the ToUpper and ToLower methods
Programming with Microsoft Visual Basic 2008, Fourth Edition

39. Converting a String to Uppercase or Lowercase (continued)
Figure 4-24: Syntax and examples of the ToUpper and ToLower methods (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

40. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

41. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

42. Comparing Boolean Values (continued)
Figure 4-27: Examples of comparing Boolean values in an If…Then…Else statement’s condition
Programming with Microsoft Visual Basic 2008, Fourth Edition

43. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

44. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

45. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

46. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

47. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

48. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

49. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

50. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

51. Coding the Monthly Payment Calculator Application (continued)
Figure 4-33: TOE chart for the Monthly Payment Calculator application
Programming with Microsoft Visual Basic 2008, Fourth Edition

52. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

53. Coding the btnCalc Control’s Click Event Procedure (continued)
Figure 4-34: Pseudocode for the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

54. Coding the btnCalc Control’s Click Event Procedure (continued)
Figure 4-35: Partially completed Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

55. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

56. Using the Financial.Pmt Method (continued)
Figure 4-36: Basic syntax and examples of the Financial.Pmt method
Programming with Microsoft Visual Basic 2008, Fourth Edition

57. Using the Financial.Pmt Method (continued)
Figure 4-37: Selection structure’s true path coded in the procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

58. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

59. The MessageBox.Show Method (continued)
Figure 4-41: Values returned by the MessageBox.Show method
Programming with Microsoft Visual Basic 2008, Fourth Edition

60. The MessageBox.Show Method (continued)
Figure 4-41: Values returned by the MessageBox.Show method (continued)
Programming with Microsoft Visual Basic 2008, Fourth Edition

61. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

62. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition

63. Coding the KeyPress Event Procedures
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
Programming with Microsoft Visual Basic 2008, Fourth Edition

64. Coding the KeyPress Event Procedures (continued)
Figure 4-47: Code template for the txtPrincipal’s KeyPress event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

65. Coding the KeyPress Event Procedures (continued)
Figure 4-48: Examples of using the KeyChar and Handled properties in the KeyPress event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

66. Coding the KeyPress Event Procedures (continued)
Figure 4-49: Completed CancelKeys procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

67. Coding the Enter Event Procedure
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
Programming with Microsoft Visual Basic 2008, Fourth Edition

68. Coding the Enter Event Procedure (continued)
Figure 4-50: Syntax and an example of the SelectAll method
Programming with Microsoft Visual Basic 2008, Fourth Edition

69. 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
Programming with Microsoft Visual Basic 2008, Fourth Edition