1. Programming with Microsoft Visual Basic 2010 5th Edition
Chapter Four
The Selection Structure

2. Previewing the Monthly Payment Calculator Application
The Monthly Payment Calculator application uses the selection structure

3. Monthly payment amount shown in the interface
Figure 4-1
Message box
Figure 4-2
Monthly payment amount shown in the interface

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
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

5. 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

6. 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

7. 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

8. Making Decisions in a Program (cont’d.)
Figure 4-3 Selection structures you might use today

9. Making Decisions in a Program (cont’d.)
Example: Kanton Boutique
Figure 4-4 Problem specification for Kanton Boutique

10. Figure 4-5 Interface for the Kanton Boutique application
Figure 4-6 Pseudocode containing only the sequence structure

11. Figure 4-7 Modified problem specification and pseudocode containing a single-alternative selection structure

12. 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

13. Figure 4-8 Single-alternative selection structure shown in a flowchart

14. Figure 4-9 Modified problem specification and pseudocode containing a dual-alternative selection structure

15. Figure 4-10 Dual-alternative selection structure shown in a flowchart

16. Coding Single-Alternative and Dual-Alternative 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

17. Figure 4-11 Syntax and examples of the If…Then…Else statement (continues)

18. Figure 4-11 Syntax and examples of the If…Then…Else statement (cont’d

19. 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

20. Figure 4-12 Listing and examples of commonly used comparison operators

21. 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

22. Comparison Operators (cont’d.)
Figure 4-15 Pseudocode containing a single-alternative selection structure

23. Figure 4-16 Flowchart containing a single-alternative selection structure

24. Figure 4-17 Display button’s Click event procedure

25. 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

26. Comparison Operators (cont’d.)
Figure 4-18 Illustration of the swapping concept

27. 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

28. Comparison Operators (cont’d.)
Figure 4-20 Pseudocode containing a dual-alternative selection structure

29. Figure 4-21 Pseudocode containing a dual-alternative selection structure

30. Figure 4-22 Calculate button’s Click event procedure

31. 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

32. Figure 4-23 Listing and examples of logical operators (continues)

33. Figure 4-23 Listing and examples of logical operators (cont’d.)

34. 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

35. Truth tables for the logical operators
Figure 4-24
Truth tables for the logical operators

36. 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

37. 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

38. 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

39. Examples of using string comparisons in a procedure
Figure 4-28
Examples of using string comparisons in a procedure

40. 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

41. Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (continues)

42. Figure 4-29 Syntax and examples of the ToUpper and ToLower methods (cont’d.)

43. Converting a String 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

44. Examples of using the ToUpper and ToLower methods in a procedure
Figure 4-29
Examples of using the ToUpper and ToLower methods in a procedure

45. 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

46. Comparing Boolean Values (cont’d.)
Figure 4-32 Examples of using Boolean values in a condition

47. 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

48. Figure 4-33 Syntax and example of using the Boolean value returned by the TryParse method

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. Figure 4-42 TOE chart for the Monthly Payment Calculator application

57. 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

58. Coding the Monthly Payment Calculator Application (cont’d.)
Figure 4-43 Pseudocode for the btnCalc control’s Click event procedure

59. Figure 4-44 Partially completed Click event procedure

60. 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 annual interest rate to monthly rate by dividing by 12
Convert annual term to monthly term by multiplying by 12

61. Figure 4-45 Basic syntax and examples of the Financial.Pmt method

62. Using the Financial.Pmt Method (cont’d.)
Figure 4-46 Selection structure’s true path coded in the procedure

63. The 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

64. The MessageBox.Show Method (cont’d.)
Figure 4-50 Values returned by the MessageBox.Show method (continues)

65. The MessageBox.Show Method (cont’d.)
Figure 4-50 Values returned by the MessageBox.Show method (cont’d.)

66. 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

67. 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

68. 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

69. Figure 4-57 Examples of using the KeyChar and Handled properties in the KeyPress event procedure

70. Coding the KeyPress Event Procedures (cont’d.)
Figure 4-58 CancelKeys procedure

71. Coding the Enter Event Procedures
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

72. Coding the Enter Event Procedures (cont’d.)
Figure 4-59 Syntax and an example of the SelectAll method

73. 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