Programming with Microsoft Visual Basic 2008 Fourth Edition

1. Programming with Microsoft Visual Basic 2008 Fourth 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

3. Previewing the Math Practice Application (continued) Figure 5-1: Math Practice application’s user interface Programming with Microsoft Visual Basic 2008, Fourth Edition

4. Lesson A Objectives After studying Lesson A, you should be able to: • Include a nested selection structure in both pseudocode and a flowchart • Code a nested selection structure • Desk-check an algorithm • Recognize common logic errors in selection structures Programming with Microsoft Visual Basic 2008, Fourth Edition

5. Lesson A Objectives (continued) • Include a multiple-path selection structure in both pseudocode and a flowchart • Code a multiple-path selection structure using the If/ElseIf/Else and Case forms of the selection structure Programming with Microsoft Visual Basic 2008, Fourth Edition

6. Nested Selection Structures • Selection structure: • Chooses true or false path based on comparison or decision • Nested selection structure: • Selection structure that lies on true or false path of an outer selection structure • Also called an inner selection structure • Two decisions are involved: • Primary decision: Made by outer structure • Secondary decision: Made by inner structure Programming with Microsoft Visual Basic 2008, Fourth Edition

7. Nested Selection Structures (continued) Figure 5-5: Display Message button’s pseudocode with the nested selection structure in the true path Programming with Microsoft Visual Basic 2008, Fourth Edition

8. Nested Selection Structures (continued) Figure 5-6: Display Message button’s flowchart with the nested selection structure in the true path Programming with Microsoft Visual Basic 2008, Fourth Edition

9. Nested Selection Structures (continued) Figure 5-7: Display Message button’s Click event with the nested selection structure in the true path Programming with Microsoft Visual Basic 2008, Fourth Edition

10. Nested Selection Structures (continued) Figure 5-7: Display Message button’s Click event with the nested selection structure in the true path (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition

11. Nested Selection Structures (continued) Figure 5-8: Display Message button’s pseudocode with the nested selection structure in the false path Programming with Microsoft Visual Basic 2008, Fourth Edition

12. Nested Selection Structures (continued) Figure 5-9: Display Message button’s flowchart with the nested selection structure in the true path Programming with Microsoft Visual Basic 2008, Fourth Edition

13. Nested Selection Structures (continued) Figure 5-10: Display Message button’s Click event with the nested selection structure in the false path Programming with Microsoft Visual Basic 2008, Fourth Edition

14. Logic Errors in Selection Structures • Common logic errors include: • 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 task • 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

15. Logic Errors in Selection Structures (continued) Figure 5-11: A correct algorithm for the bonus procedure Programming with Microsoft Visual Basic 2008, Fourth Edition

16. Logic Errors in Selection Structures (continued) Figure 5-12: Results of desk-checking the correct algorithm Programming with Microsoft Visual Basic 2008, Fourth Edition

17. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

18. Using a Compound Condition Rather Than a Nested Selection Structure (continued) Figure 5-13: Correct algorithm and an incorrect algorithm containing the first logic error Programming with Microsoft Visual Basic 2008, Fourth Edition

19. Using a Compound Condition Rather Than a Nested Selection Structure (continued) Figure 5-14: Results of desk-checking the incorrect algorithm shown in Figure 5-13 Programming with Microsoft Visual Basic 2008, Fourth Edition

20. Reversing the Primary and Secondary Decisions • Incorrect algorithm reverses primary and secondary decisions • All salespeople with sales >= 10000 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

21. Reversing the Primary and Secondary Decisions (continued) Figure 5-15: Correct algorithm and an incorrect algorithm containing the second logic error Programming with Microsoft Visual Basic 2008, Fourth Edition

22. Reversing the Primary and Secondary Decisions (continued) Figure 5-16: Results of desk-checking the incorrect algorithm shown in Figure 5-15 Programming with Microsoft Visual Basic 2008, Fourth Edition

23. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

24. Using an Unnecessary Nested Selection Structure (continued) Figure 5-17: Correct algorithm and an incorrect algorithm containing the third logic error Programming with Microsoft Visual Basic 2008, Fourth Edition

25. Using an Unnecessary Nested Selection Structure (continued) Figure 5-18: Results of desk-checking the inefficient algorithm shown in Figure 5-17 Programming with Microsoft Visual Basic 2008, Fourth Edition

26. Multiple-Path Selection Structures • Multiple-path (or extended) selection structures: • Designed to choose from several alternatives • Condition in multiple-path 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

27. Multiple-Path Selection Structures (continued) Figure 5-19: Letter grades and messages Programming with Microsoft Visual Basic 2008, Fourth Edition

28. Multiple-Path Selection Structures (continued) Figure 5-20: Pseudocode and flowchart showing a multiple-path selection structure Programming with Microsoft Visual Basic 2008, Fourth Edition

29. The If/ElseIf/Else Form of the Selection Structure • Allows for selection among more than two paths • Provides more convenient way of writing multiple-path selection structure than using nested If/Else structures Programming with Microsoft Visual Basic 2008, Fourth Edition

30. The If/ElseIf/Else Form of the Selection Structure (continued) Figure 5-21: Two versions of the code for the multiple-path selection structure Programming with Microsoft Visual Basic 2008, Fourth Edition

31. The Case Selection Structure • 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

32. The Case Selection Structure (continued) Figure 5-23: Syntax and an example of the Select Case statement Programming with Microsoft Visual Basic 2008, Fourth Edition

33. The Case Selection Structure (continued) Figure 5-23: Syntax and an example of the Select Case statement (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition

34. Specifying a Range of Values in an ExpressionList • To specify range of minimum and maximum values, use: • 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 • Relational operators used with Is: =, <, <=, >, >=, <> Programming with Microsoft Visual Basic 2008, Fourth Edition

35. Specifying a Range of Values in an ExpressionList (continued) Figure 5-24: Example of using the To and Is keywords in a Select Case statement Programming with Microsoft Visual Basic 2008, Fourth Edition

36. Lesson A Summary • Nested selection structures lies on true or false path of outer selection structure • Desk-check: Validate algorithm by hand with sample data • Multiple-path 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

37. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

38. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

39. Adding a Radio Button to the Interface • Radio Button control: • 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 • Default radio button: Button initially selected • Each group of radio buttons must be placed within container such as group box • Checked property: Set to True to designate default radio button Programming with Microsoft Visual Basic 2008, Fourth Edition

40. Adding a Radio Button to the Interface (continued) Figure 5-27 Subtraction radio button added to the Operation group box Programming with Microsoft Visual Basic 2008, Fourth Edition

41. Adding a Check Box Control to the Interface • Check box control: • 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

42. Adding a Check Box Control to the Interface (continued) Figure 5-28: Display summary check box added to the interface Programming with Microsoft Visual Basic 2008, Fourth Edition

43. Coding the Math Practice Application • Procedures to code based on TOE chart: • Click event procedures for seven controls • Load event for form • One task will be coded with an independent Sub procedure • Generating and displaying two random numbers Programming with Microsoft Visual Basic 2008, Fourth Edition

44. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

45. Creating an Independent Sub Procedure (continued) Figure 5-31: Syntax and steps for creating an independent Sub procedure Programming with Microsoft Visual Basic 2008, Fourth Edition

46. Creating an Independent Sub Procedure (continued) Figure 5-32: Pseudocode for the GenerateAndDisplayIntegers procedure Programming with Microsoft Visual Basic 2008, Fourth Edition

47. 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 Programming with Microsoft Visual Basic 2008, Fourth Edition

48. Generating Random Integers (continued) Figure 5-33: Syntax and examples of generating random integers Programming with Microsoft Visual Basic 2008, Fourth Edition

49. Generating Random Integers (continued) Figure 5-33: Syntax and examples of generating random integers (continued) Programming with Microsoft Visual Basic 2008, Fourth Edition

50. Generating Random Integers (continued) Figure 5-34: Random number generation code entered in the procedure Programming with Microsoft Visual Basic 2008, Fourth Edition