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Chapter 4: The Selection Structure

Chapter 4: The Selection Structure

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Chapter 4: The Selection Structure

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  1. Chapter 4: The Selection Structure Programming with Microsoft Visual Basic .NET, Second 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 Programming with Microsoft Visual Basic .NET, Second Edition

  3. The If…Then…Else Statement Lesson A Objectives (continued) • Write code that uses comparison operators and logical operators • Format numbers using the ToString method • Change the case of a string Programming with Microsoft Visual Basic .NET, Second Edition

  4. The Selection Structure • Use the selection structure to make a decision or comparison and select a particular set of tasks to perform • The selection structure is also called the decision structure • The condition must result in either a true (yes) or false (no) answer Programming with Microsoft Visual Basic .NET, Second Edition

  5. The Selection Structure (continued) • If the condition is true, the program performs one set of tasks • If the condition is false, there may or may not be a different set of tasks to perform • Visual Basic .NET provides four forms of the selection structure: If, If/Else, If/ElseIf/Else, and Case Programming with Microsoft Visual Basic .NET, Second Edition

  6. Writing Pseudocode for If and If/Else Selection Structures • An If selection structure contains only one set of instructions, which are processed when the condition is true • An If/Else selection structure contains two sets of instructions: • One set is processed when the condition is true • The other set is processed when the condition is false Programming with Microsoft Visual Basic .NET, Second Edition

  7. Flowcharting the If and If/Else Selection Structures Programming with Microsoft Visual Basic .NET, Second Edition

  8. Flowcharting the If and If/Else Selection Structures (continued) F T T F Programming with Microsoft Visual Basic .NET, Second Edition

  9. Coding the If and If/Else Selection Structures If conditionThen statement block containing one or more statements to be processed when the condition is true [Else statement block containing one or more statements to be processed when the condition is false] End If Programming with Microsoft Visual Basic .NET, Second Edition

  10. Coding the If and If/Else Selection Structures (continued) • The items in square brackets ([ ]) in the syntax are optional • You do not need to include the Else portion • Words in bold are essential components of the statement Programming with Microsoft Visual Basic .NET, Second Edition

  11. Coding the If and If/Else Selection Structures (continued) • Items in italic indicate where the programmer must supply information pertaining to the current application • The set of statements contained in the true path, as well as the statements in the false path, are referred to as a statement block Programming with Microsoft Visual Basic .NET, Second Edition

  12. Comparison Operators Programming with Microsoft Visual Basic .NET, Second Edition

  13. Comparison Operators (continued) • Comparison operators are also referred to as relational operators • All expressions containing a relational operator will result in either a true or false answer only • Comparison operators are evaluated from left to right, and are evaluated after any mathematical operators Programming with Microsoft Visual Basic .NET, Second Edition

  14. 10 + 3 < 5 * 2 5 * 2 is evaluated first, giving 10 10 + 3 is evaluated second, giving 13 13 < 10 is evaluated last, giving false 7 > 3 * 4 / 2 3 * 4 is evaluated first, giving 12 12 / 2 is evaluated second, giving 6 7 > 6 is evaluated last, giving true Comparison Operators (continued) Programming with Microsoft Visual Basic .NET, Second Edition

  15. Comparison Operators (continued) Programming with Microsoft Visual Basic .NET, Second Edition

  16. Logical Operators Programming with Microsoft Visual Basic .NET, Second Edition

  17. Logical Operators (continued) • Truth table for Not operator Programming with Microsoft Visual Basic .NET, Second Edition

  18. Logical Operators (continued) • Truth table for And operator Programming with Microsoft Visual Basic .NET, Second Edition

  19. Logical Operators (continued) • Truth table for AndAlso operator Programming with Microsoft Visual Basic .NET, Second Edition

  20. Logical Operators (continued) • Truth table for Or operator Programming with Microsoft Visual Basic .NET, Second Edition

  21. Logical Operators (continued) • Truth table for OrElse operator Programming with Microsoft Visual Basic .NET, Second Edition

  22. Logical Operators (continued) • Truth table for Xor operator Programming with Microsoft Visual Basic .NET, Second Edition

  23. Logical Operators (continued) Figure 4-19: Order of precedence for arithmetic, comparison, and logical operators Programming with Microsoft Visual Basic .NET, Second Edition

  24. Using the ToString Method to Format Numbers • Use the ToString method to format a number • Syntax: variablename.ToString(formatString) • variablename is the name of a numeric variable Programming with Microsoft Visual Basic .NET, Second Edition

  25. Using the ToString Method to Format Numbers (continued) • formatString is a string that specifies the format • Must be enclosed in double quotation marks • Takes the form Axx: • A is an alphabetic character called the format specifier • xx is a sequence of digits called the precision specifier Programming with Microsoft Visual Basic .NET, Second Edition

  26. Comparing Strings • Example 1: Using the OrElse operator Dim letter As String letter = Me.uiLetterTextBox.Text If letter = “P” OrElse letter = “p” Then Me.uiResultLabel.Text = “Pass” Else Me.uiResultLabel.Text = “Fail” End if Programming with Microsoft Visual Basic .NET, Second Edition

  27. Comparing Strings (continued) • Example 2: Using the AndAlso operator Dim letter As String letter = Me.uiLetterTextBox.Text If letter <> “P” AndAlso letter <> “p” Then Me.uiResultLabel.Text = “Fail” Else Me.uiResultLabel.Text = “Pass” End if Programming with Microsoft Visual Basic .NET, Second Edition

  28. Comparing Strings (continued) • Example 3: Correct, but less efficient, solution Dim letter As String letter = Me.uiLetterTextBox.Text If letter = “P” OrElse letter = “p” Then Me.uiResultLabel.Text = “Pass” End If If letter <> “P” AndAlso letter <> “p” Then Me.uiResultLabel.Text = “Fail” End if Programming with Microsoft Visual Basic .NET, Second Edition

  29. Comparing Strings (continued) • Example 4: Using the ToUpper method Dim letter As String letter = Me.uiLetterTextBox.Text If letter.ToUpper() = “P” Then Me.uiResultLabel.Text = “Pass” Else Me.uiResultLabel.Text = “Fail” End if Programming with Microsoft Visual Basic .NET, Second Edition

  30. 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 .NET, Second Edition

  31. Completing the User Interface • Herman Juarez has asked you to create an application that he can use to calculate the 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 .NET, Second Edition

  32. Completing the User Interface (continued) Figure 4-31: Sketch of the Monthly Payment Calculator user interface Programming with Microsoft Visual Basic .NET, Second Edition

  33. Adding a Group Box Control to the Form • Use the GroupBox tool in the Toolbox window to add a group box control to the interface • A group box control serves as a container for other controls • Use a group box control to visually separate related controls from other controls on the form Programming with Microsoft Visual Basic .NET, Second Edition

  34. Coding the uiCalcPayButton Click Event Procedure • The uiCalcPayButton’s Click event procedure is responsible for: • Calculating the monthly payment amount • Displaying the result in the uiPaymentLabel control • Figure 4-37 shows the pseudocode for the uiCalcPayButton’s Click event procedure Programming with Microsoft Visual Basic .NET, Second Edition

  35. Coding the uiCalcPayButton Click Event Procedure (continued) Figure 4-37: Pseudocode for the uiCalcPayButton Click event procedure Programming with Microsoft Visual Basic .NET, Second Edition

  36. Using the Financial.Pmt Method • Use the Visual Basic .NET Financial.Pmt method to calculate a periodic payment on either a loan or an investment • Syntax: Financial.Pmt(Rate, NPer, PV[, FV, Due]) • Rate: interest rate per period • NPer: total number of payment periods (the term) Programming with Microsoft Visual Basic .NET, Second Edition

  37. Using the Financial.Pmt Method (continued) • PV: present value of the loan or investment; the present value of a loan is the loan amount, whereas the present value of an investment is zero • FV: future value of the loan or investment; the future value of a loan is zero, whereas the future value of an investment is the amount you want to accumulate; if omitted, the number 0 is assumed Programming with Microsoft Visual Basic .NET, Second Edition

  38. Using the Financial.Pmt Method (continued) • Due: due date of payments; can be either the constant DueDate.EndOfPeriod or the constant DueDate.BegOfPeriod; if omitted, DueDate.EndOfPeriod is assumed Programming with Microsoft Visual Basic .NET, Second Edition

  39. The MessageBox.Show Method • Use the MessageBox.Show method to display a message box that contains text, one or more buttons, and an icon • Syntax: MessageBox.Show(text, caption, buttons, icon[, defaultButton]) • text: text to display in the message box • caption: text to display in the title bar of the message box Programming with Microsoft Visual Basic .NET, Second Edition

  40. The MessageBox.Show Method (continued) • buttons: buttons to display in the message box • icon: icon to display in the message box • defaultButton: button automatically selected when the user presses Enter Programming with Microsoft Visual Basic .NET, Second Edition

  41. Coding the TextChanged Event • A control’s TextChanged event occurs when the contents of a control’s Text property have changed as a result of: • The user entering data into the control, or • The application’s code assigning data to the control’s Text property Programming with Microsoft Visual Basic .NET, Second Edition

  42. Coding the TextChanged Event (continued) • When the user makes a change to the information entered in the three text box controls, the Monthly Payment Calculator application should delete the monthly payment displayed in the uiPaymentLabel control Programming with Microsoft Visual Basic .NET, Second Edition

  43. Completing the Monthly Payment Calculator ApplicationLesson C Objectives • Specify the keys that a text box will accept • Align the text in a label control • Handle exceptions using a Try/Catch block Programming with Microsoft Visual Basic .NET, Second Edition

  44. Coding the KeyPress Event • Template Private Sub uiPrincipalTextBox_KeyPress( _ ByVal sender As Object, _ ByVal e As System.Windows.Forms.KeyPressEventArgs) _ Handles uiPrincipalTextBox.KeyPress • Setting e.Handled = True will cancel the key Programming with Microsoft Visual Basic .NET, Second Edition

  45. Aligning the Text in a Label Control • The TextAlign property controls the placement of the text in a label control • The TextAlign property can be set to TopLeft (the default), TopCenter, TopRight, MiddleLeft, MiddleCenter, MiddleRight, BottomLeft, BottomCenter, or BottomRight Programming with Microsoft Visual Basic .NET, Second Edition

  46. Using a Try/Catch Block • An exception is an error that occurs while a program is running • Use the Try statement to catch (or trap) an exception when it occurs in a program • Use a Catch statement to take the appropriate action to resolve the problem • A block of code that uses both the Try and Catch statements is referred to as a Try/Catch block Programming with Microsoft Visual Basic .NET, Second Edition

  47. Using a Try/Catch Block (continued) Try one or more statements that might generate an exception Catch [variablename As exceptionType] one or more statements that will execute when an exceptionType exception occurs [Catch [variablename As exceptionType] one or more statements that will execute when an exceptionType exception occurs] End Try Programming with Microsoft Visual Basic .NET, Second Edition

  48. Summary • To evaluate an expression containing arithmetic, comparison, and logical operators, evaluate arithmetic operators first, then comparison operators, and then logical operators • To code a selection structure, use the If...Then...Else statement • To create a compound condition, use the logical operators Programming with Microsoft Visual Basic .NET, Second Edition

  49. Summary (continued) • Use the GroupBox tool to add a group box control to the form; drag controls from the form or the Toolbox window into the group box control • To calculate a periodic payment on either a loan or an investment, use the Financial.Pmt method • To display a message box that contains text, one or more buttons, and an icon, use the MessageBox.Show method Programming with Microsoft Visual Basic .NET, Second Edition

  50. Summary (continued) • To allow a text box to accept only certain keys, code the text box’s KeyPress event • To align the text in a control, set the control’s TextAlign property • To catch an exception, and then have the computer take the appropriate action, use a Try/Catch block Programming with Microsoft Visual Basic .NET, Second Edition