1. Programming with Microsoft Visual Basic 2010 5th Edition
Chapter Five
More on the Selection Structure

2. Previewing the Math Practice Application
Open Math.exe file
The Math Practice application will be used by 1st and 2nd grade students

3. Previewing the Math Practice Application (cont’d.)
Figure 5-1 Math Practice application’s user interface

4. Lesson A Objectives After studying Lesson A, you should be able to:
Include a nested selection structure in pseudocode and in a flowchart
Code a nested selection structure
Desk-check an algorithm
Recognize common logic errors in selection structures

5. Lesson A Objectives (cont’d.)
Include a multiple-alternative selection structure in pseudocode and in a flowchart
Code a multiple-alternative selection structure

6. Nested Selection Structures
Used to make a decision and select the appropriate path
Nested selection structure
Selection structure that lies on true or false path of an outer selection structure
Two decisions are involved
Primary decision: Made by outer structure
Secondary decision: Made by inner structure

7. Figure 5-3 Selection structures you might use today

8. The Voter Eligibility Application
Figure 5-4 Problem specification for the Voter Eligibility application

9. The Voter Eligibility Application (cont’d.)
Figure 5-5 Sample run of the Voter Eligibility application

10. Figure 5-6 Flowchart showing the nested selection structure in the true path

11. Figure 5-7 Flowchart showing the nested selection structure in the false path

12. Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7 (continues)

13. Figure 5-8 Code for the flowcharts in Figures 5-6 and 5-7 (cont’d.)

14. Logic Errors in Selection Structures
Common logic errors
Using compound condition instead of nested structure
Reversing primary and secondary decisions
Using unnecessary nested selection structure
Algorithm
Set of step-by-step instructions for accomplishing a task

15. Logic Errors in Selection Structures (cont’d.)
Desk-checking (or hand-tracing)
Using pencil and paper to follow algorithm’s steps
Using sample data
Goal is to verify that algorithm is correct

16. Logic Errors in Selection Structures (cont’d.)
Figure 5-9 A correct algorithm for the bonus procedure

17. Logic Errors in Selection Structures (cont’d.)
Figure 5-10 Results of desk-checking the correct algorithm

18. Using a Compound Condition Rather Than a Nested Selection Structure
Bonus scenario
Calculate bonus amount for all employee types
Give extra bonus to X contingent on sales amount
Display the bonus amount
Problem when using compound condition
It does not indicate hierarchy in decision process
Nested selection structure does enforce hierarchy of decisions
Extra bonus decision depends on sales type decision

19. Figure 5-11 Correct algorithm and an incorrect algorithm containing the first logic error

20. Figure 5-12 Results of desk-checking the incorrect algorithm shown in Figure 5-11

21. Reversing the Primary and Secondary Decisions
Incorrect algorithm reverses primary and secondary decisions
All salespeople with sales >= get extra bonus
Amount of bonus depends on sales type
Extra bonus decision should depend on sales type decision first
Reminder: Desk-check your algorithm with sample data to verify its correctness

22. Figure 5-13 Correct algorithm and an incorrect algorithm containing the second logic error

23. Reversing the Primary and Secondary Decisions (cont’d.)
Figure 5-14 Results of desk-checking the incorrect algorithm shown in Figure 5-13

24. Using an Unnecessary Nested Selection Structure
Problem with incorrect algorithm
Adds extra, redundant selection structure on inner false path
Extra code reduces efficiency and readability
The redundant selection structure:
Makes decision that was already decided in prior selection structure
Solution: Eliminate second nested structure

25. Using an Unnecessary Nested Selection Structure (cont’d.)
Figure 5-15 Correct algorithm and an incorrect algorithm containing the third logic error

26. Using an Unnecessary Nested Selection Structure (cont’d.)
Figure 5-16 Results of desk-checking the inefficient algorithm shown in Figure 5-15

27. Multiple-Alternative Selection Structures
Designed to choose from several alternatives
Condition in multiple-alternative selection structure does not require true/false answer
Requires expression whose value determines which path is chosen
Example: Display message based on grades ranging from A to F

28. Multiple-Alternative Selection Structures (cont’d.)
Figure 5-17 Letter grades and messages

29. Figure 5-18 Pseudocode containing a multiple-alternative selection structure

30. Figure 5-19 Flowchart containing a multiple-alternative selection structure

31. Multiple-Alternative Selection Structures (cont’d.)
Figure 5-20
Shows two versions of code for btnDisplay control’s Click event procedure
Both versions use If…Then…Else statements
Second version contains more convenient way of writing a multiple- alternative selection structure

32. Figure 5-20 Two versions of the code containing a multiple-alternative selection structure (continues)

33. Figure 5-20 Two versions of the code containing a multiple-alternative selection structure (cont’d.)

34. The Select Case Statement
Simpler and clearer when there are many paths
Select Case statement
Begins with Select Case, ends with End Select
Each case represents a different instruction path
Optional Case Else clause handles all values not covered by other Case paths
selectorExpression is evaluated to determine path
Each case, except Case Else, has expressionList that is compared to selectorExpression

35. Figure 5-22 Syntax and an example of the Select Case statement (continues)

36. Figure 5-22 Syntax and an example of the Select Case statement (cont’d

37. Specifying a Range of Values in a Case Clause
To specify range of minimum and maximum values
To keyword: Use if both upper and lower bounds are known
Is keyword: Use if only upper or lower bound is known
Example with To: Case 1 To 5
Example with Is: Case Is > 10
Comparison operators used with Is
= , < , <= , > , >= , <>

38. Figure 5-23 Syntax and an example of specifying a range of values

39. Lesson A Summary
Nested selection structure lies on true or false path of outer selection structure
Desk-check
Validate algorithm by hand with sample data
Multiple-alternative selection structures
Use If/ElseIf/Else statement or Select Case statement
Use To and Is keywords to specify range of values in Case clause of Select Case statement

40. Lesson B Objectives After studying Lesson B, you should be able to:
Include a group of radio buttons in an interface
Designate a default radio button
Include a check box in an interface
Create and call an independent Sub procedure
Generate random numbers

41. Creating the Math Practice Application
Objective: Practice addition and subtraction
Specifications
1st grade students use numbers 1 through 10
2nd grade students use numbers 10 through 99
Should not allow problems whose answers will be negative numbers
Students should be able to check their answers
Extra attempts allowed when answer is incorrect
Application should track and display number of correct and incorrect responses

42. Adding a Radio Button to the Form
Limits user to one choice in group of options
Should be labeled so its purpose is understood
Should have unique keyboard access key
RadioButton tool: Used to add radio button
Each group of radio buttons must be placed within container such as group box
Default radio button: Button initially selected
Set Checked property to True

43. Adding a Radio Button to the Form (cont’d.)
Figure 5-29 Subtraction radio button added to the Operation group box

44. Adding a Check Box Control to the Form
Allows multiple selections among group of choices
Any number of check boxes can be selected at once
Does not limit choices like radio button control
Should be labeled to indicate its purpose
Checkbox tool: Used to add check box control
Lock controls, then set TabIndex after interface design is complete

45. Adding a Check Box Control to the Form (cont’d.)
Figure 5-30 Display summary check box added to the form

46. Coding the Math Practice Application
Procedures to code based on TOE chart
Form’s load event procedure
Click event procedures for seven controls
One task will be coded with an independent Sub procedure
Generating and displaying two random numbers

47. Creating an Independent Sub Procedure
Event procedure
Sub procedure associated with specific object and event
Processed when specific event occurs
Independent Sub procedure
Processed only when called (invoked) from code
Can be invoked from one or more places in an application
Not associated with specific control
Reduces amount of code; promotes modularity

48. Figure 5-33 Independent Sub procedure syntax, example, and steps

49. Figure 5-34 Pseudocode for the GenerateAndDisplayIntegers procedure

50. Generating Random Integers
Pseudo-random number generator
Device that produces sequence of pseudo-random numbers
Pseudo-random: Values statistically resemble randomness
To use pseudo-random number generator:
Create Random object within procedure
Generate random integers with Random.Next method

51. Figure 5-35 Syntax and examples of generating random integers

52. Figure 5-36 Random number generation code entered in the procedure

53. Figure 5-37 Additional comments and code entered in the procedure

54. Coding the Grade Radio Buttons’ Click Event Procedures
Radio button controls should invoke GenerateAndDisplayNumbers procedure
Call statement: Invokes independent procedure
Syntax
Call procedureName([argumentList])
procedureName: Name of procedure to be invoked
argumentList: Optional list of arguments to pass
Keyword Call is optional
Can simply use procedure name

55. Figure 5-38 Syntax and examples of the Call statement

56. Coding the Operation Radio Buttons’ Click Event Procedures
Requirements for Click event procedures
Display appropriate mathematical operator (+ or -)
Generate and display two random integers
Show addition problem by default when program starts

57. Coding the Form’s Load Event Procedure
Processed when application is started
Form is not displayed until its instructions are processed
Two ways to initially display addition problem
Call GenerateAndDisplayIntegers()
radAddition.PerformClick()
Enter either statement in Load event procedure

58. Lesson B Summary Radio button control Check box control
Limits user to only one choice among group of choices
Check box control
Allows user to select multiple choices in a group
Independent Sub procedures
Not linked to controls and can be invoked anywhere in program
Use Random object to generate pseudo-random numbers

59. Lesson B Summary (cont’d.)
Call statement
Used to call (invoke) independent Sub procedure
Use object.PerformClick() to invoke object’s Click event

60. Lesson C Objectives After studying Lesson C, you should be able to:
Code a check box’s Click event procedure
Show and hide a control

61. Coding the Check Answer Button’s Click Event Procedure
Study procedure’s pseudocode to determine what variables and named constants are needed
Six integer variables and one named constant are needed
Two variables have static storage type

62. Figure 5-44 Pseudocode for the btnCheckAnswer control’s Click event procedure

63. Figure 5-45 Memory locations used by the btnCheckAnswer control’s Click event procedure

64. Coding the Display Summary Check Box’s Click Event Procedure
chkSummary control’s Click event procedure
Displays and hides grpSummary control
When user selects check box, event is triggered, showing group box and its contents
When user deselects check box, event is triggered, hiding group box control
Checked property indicates whether check box is selected
Visible property indicates whether control is shown

65. Lesson C Summary
To show or hide control, set control’s Visible property
Use selection structure to determine if check box was either selected or deselected by user