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Chapter 4: The Selection Structure

Chapter 4: The Selection Structure. Programming with Microsoft Visual Basic 2005, Third Edition. The If…Then…Else Statement Lesson A Objectives. Write pseudocode for the selection structure Create a flowchart to help you plan an application’s code Write an If...Then...Else statement

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Chapter 4: The Selection Structure

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  1. Chapter 4: The Selection Structure Programming with Microsoft Visual Basic 2005, Third Edition

  2. The If…Then…Else Statement Lesson A Objectives • 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 Programming with Microsoft Visual Basic 2005, Third Edition

  3. The If…Then…Else Statement Lesson A Objectives (continued) • Change the case of a string • Determine whether a text box contains data • Determine the success of the TryParse method Programming with Microsoft Visual Basic 2005, Third Edition

  4. The Selection Structure • Condition: expression evaluating to true or false • Selection (decision) structure • Chooses one of two paths based on a condition • Example • If employee works over 40 hours, add overtime pay • Four selection structures in Visual Basic • If, If/Else, If/ElseIf/Else, and Case Programming with Microsoft Visual Basic 2005, Third Edition

  5. 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 • Contains two sets of instructions • True path: instruction set following true condition • False path: instruction set following false condition Programming with Microsoft Visual Basic 2005, Third Edition

  6. Writing Pseudocode for If and If/Else Selection Structures Figure 4-4: Examples of the If and If/Else selection structures written in pseudocode Programming with Microsoft Visual Basic 2005, Third Edition

  7. Flowcharting the If and If/Else Selection Structures • Flowchart • Set of standardized symbols showing program flow • Oval: the start/stop symbol • Rectangle: the process symbol • Parallelogram: the input/output symbol • Diamond: selection/repetition symbol Programming with Microsoft Visual Basic 2005, Third Edition

  8. 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 2005, Third Edition

  9. Coding the If and If/Else Selection Structures • Syntax • If conditionThen statement block for true path [Else statement block for false path] End If • condition must be a Boolean expression • The Else clause is optional Programming with Microsoft Visual Basic 2005, Third Edition

  10. Comparison Operators • Comparison (relational) operators: • Test two items for equality or types of non-equality • Rules for comparison operators • Cause an expression to evaluate to true or false • Have lower precedence than arithmetic operators • Are evaluated from left to right • Example: 5 -2 > 1 + 2  3 > 3  False Programming with Microsoft Visual Basic 2005, Third Edition

  11. Comparison Operators (continued) Figure 4-7: Listing and examples of commonly used comparison operators (continued) Programming with Microsoft Visual Basic 2005, Third Edition

  12. Comparison Operators (continued) Figure 4-7: Listing and examples of commonly used comparison operators Programming with Microsoft Visual Basic 2005, Third Edition

  13. Using Comparison Operators–Swapping Numeric Values • xDisplayButton Click event procedure – pseudocode • Store text box values in the num1 and num2 variables • If the value of num1 is greater than the value of num2 • Swap numbers so num1 contains the smaller number • End if • Display message stating lowest and highest numbers • Block scope: restricts variable to a statement block • Swap variable in if clause will have block scope Programming with Microsoft Visual Basic 2005, Third Edition

  14. Using Comparison Operator–Swapping Numeric Values (continued) Figure 4-11: The If selection structure shown in the xDisplayButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  15. Using Comparison Operators—Displaying the Sum or Difference • xCalcButton Click event procedure – pseudocode • Store values in operation, number1, and number2 • If the operation variable contains “1” • Calculate the sum of number1 and number2 • Display the “Sum:” message along with the sum • Else • Subtract number2 from number1 • Display “Difference:” message along with difference • End if Programming with Microsoft Visual Basic 2005, Third Edition

  16. Using Comparison Operators—Displaying the Sum or Difference (continued) Figure 4-16: The If/Else selection structure shown in the xCalcButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  17. Logical Operators • Logical (Boolean) operators • Operators that create compound conditions • Types: And, Or, Not, AndAlso, OrElse, Xor • Example: If hours > 0 And hours <= 40 Then • Truth tables: show how operators are evaluated • Short circuit evaluation: bypasses a condition • Operators using technique: AndAlso, OrElse • Example: If state = "TN" AndAlso st > 50000D Then • If st is not TN, no need to evaluate sales > 50000D Programming with Microsoft Visual Basic 2005, Third Edition

  18. Logical Operators (continued) Figure 4-18: Listing and examples of logical operators (partial) Programming with Microsoft Visual Basic 2005, Third Edition

  19. Logical Operators (continued) • Truth table for Not operator

  20. Logical Operators (continued) • Truth table for And operator

  21. Logical Operators (continued) • Truth table for AndAlso operator

  22. Logical Operators (continued) • Truth table for Or operator

  23. Logical Operators (continued) • Truth table for OrElse operator

  24. Logical Operators (continued) • Truth table for Xor operator

  25. 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 • Lower than that of arithmetic or comparison operators Programming with Microsoft Visual Basic 2005, Third Edition

  26. Using Truth Tables (continued) Figure 4-20: Order of precedence for arithmetic, comparison, and logical operators Programming with Microsoft Visual Basic 2005, Third Edition

  27. Using Logical Operators: Calculating Gross Pay • Data validation: verifying input data is within range • Scenario: calculate and display employee gross pay • Requirements for application implementing scenario • 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 • Compound condition can use AndAlso or OrElse Programming with Microsoft Visual Basic 2005, Third Edition

  28. Using Logical Operators: Calculating Gross Pay (continued) Figure 4-22: AndAlso and OrElse logical operators in the If...Then...Else statement (continued) Programming with Microsoft Visual Basic 2005, Third Edition

  29. Using Logical Operators: Calculating Gross Pay (continued) Figure 4-22: AndAlso and OrElse logical operators in the If...Then...Else statement Programming with Microsoft Visual Basic 2005, Third Edition

  30. Comparing Strings Containing Letters • Scenario • Display “Pass” if ‘P’ is entered in xLetterTextBox • Display “Fail” if ‘F’ is entered in xLetterTextBox • One of the possible implementations • Dim letter As String letter = Me.xLetterTextBox.Text If letter = "P" OrElse letter = "p“ Then Me.xResultLabel.Text = “Pass“ Else Me.xResultLabel.Text = "Fail“ End if Programming with Microsoft Visual Basic 2005, Third Edition

  31. 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 letter.ToUpper = "P" Then Programming with Microsoft Visual Basic 2005, Third Edition

  32. 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 • Account for case variations in state text entered • Choices for controlling case: ToLower or ToUpper • One way to enforce case for input • Dim state As String state = Me.xStateTextBox.Text.ToUpper • Use If/Else to test state value and display message Programming with Microsoft Visual Basic 2005, Third Edition

  33. Comparing Boolean Values • Boolean variable: contains either true or false • Naming convention: “is” denotes Boolean type • Example: isInsured • Determining whether a text box contains data • Compare Text property to String.empty value or “” • Alternative: use String.IsNullorEmpty method • Determining whether a string can be converted to a number • Use Boolean value returned by TryParse method Programming with Microsoft Visual Basic 2005, Third Edition

  34. Comparing Boolean Values (continued) Figure 4-29: Syntax and examples of the String.IsNullOrEmpty method Programming with Microsoft Visual Basic 2005, Third Edition

  35. Comparing Boolean Values (continued) Figure 4-31: Syntax and an example of using the Boolean value returned by the TryParse method Programming with Microsoft Visual Basic 2005, Third Edition

  36. Summary – Lesson A • A Boolean condition evaluates to true or false • Selection structures choose an instruction path based on a condition • If...Then...Else statement: selection structure with a true path and a false path • Operator precedence: arithmetic, comparison, logical • ToUpper and ToLower modify case of input text Programming with Microsoft Visual Basic 2005, Third Edition

  37. The Monthly Payment Calculator ApplicationLesson B Objectives • 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 2005, Third Edition

  38. Completing the User Interface • Need: calculate monthly payment on a car loan • To make this calculation, the 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 2005, Third Edition

  39. Completing the User Interface (continued) Figure 4-34: Sketch of the Monthly Payment Calculator user interface Programming with Microsoft Visual Basic 2005, Third Edition

  40. Adding a Group Box to the Form • Group box: container for other controls • GroupBox tool • Located in the Toolbox window • Used to add a group box control to the interface • Purpose of a group box control • Visually separate related controls from other controls • Lock controls and set TabIndex after placement Programming with Microsoft Visual Basic 2005, Third Edition

  41. Coding the Monthly Payment Calculator Application • Procedures required according to TOE chart • Click event code for the two buttons • TextChanged, KeyPress, Enter code for text boxes • Procedures that are already coded • xExitButton’s Click event and TextChanged events • Procedure to code in Lesson B • xCalcButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  42. Coding the xCalcButton’s Click Event Procedure • Tasks for xCalcPayButton’s Click event procedure • Calculating the monthly payment amount • Displaying the result in the xPaymentLabel control • Two selection structures needed: If and If/Else • Determining named constants and variables • Constants: items that do not change with each call • Variables: items will likely change with each call Programming with Microsoft Visual Basic 2005, Third Edition

  43. Coding the xCalcPayButton Click Event Procedure (continued) Figure 4-39: Pseudocode for the xCalcButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  44. Using the Financial.Pmt Method • Calculates periodic payment on loan or investment • Syntax: Financial.Pmt(Rate, NPer, PV[, FV, Due]) • Rate: interest rate per period • NPer: total number of payment periods (the term) • PV: present value of the loan or investment • FV: future value of the loan or investment • Due: due date of payments Programming with Microsoft Visual Basic 2005, Third Edition

  45. The MessageBox.Show Method • Displays message box with text, button(s), icon • Syntax: MessageBox.Show(text, caption, buttons, icon[, defaultButton]) • text: text to display in the message box • caption: text to display in title bar of message box • buttons: buttons to display in the message box • icon: icon to display in the message box • defaultButton: automatically selected if Enter pressed Programming with Microsoft Visual Basic 2005, Third Edition

  46. The MessageBox.Show Method (continued) Figure 4-48: Completed xCalcButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  47. The MessageBox.Show Method (continued) Figure 4-50: Message box created by the MessageBox.Show method Programming with Microsoft Visual Basic 2005, Third Edition

  48. Summary – Lesson B • Group box control treats components as one unit • Add a group box using the GroupBox tool • Financial.Pmt method calculates loan or investment payments • MessageBox.Show method displays a message box with text, one or more buttons, and an icon Programming with Microsoft Visual Basic 2005, Third Edition

  49. Completing the Monthly Payment Calculator ApplicationLesson C Objectives • Specify the keys that a text box will accept • Select the existing text in a text box Programming with Microsoft Visual Basic 2005, Third Edition

  50. Coding the KeyPress Event Procedures • KeyPress event • Occurs when key pressed while a control has focus • Character corresponding to key is stored • Stored value sent to KeyPress event’s e parameter • One popular use of the KeyPress event • Prevents users from entering inappropriate characters • Selection structure used to test entered character Programming with Microsoft Visual Basic 2005, Third Edition

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