Programming with Microsoft Visual Basic 2008 Fourth Edition

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 • 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 • Flowchart: • 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: Usedto 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. 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 • 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