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Chapter 7: Sub and Function Procedures

Chapter 7: Sub and Function Procedures. Programming with Microsoft Visual Basic 2005, Third Edition. Creating Sub and Function Procedures Lesson A Objectives. Explain the difference between a Sub procedure and a Function procedure Create a procedure that receives information passed to it

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Chapter 7: Sub and Function Procedures

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  1. Chapter 7: Sub and Function Procedures Programming with Microsoft Visual Basic 2005, Third Edition

  2. Creating Sub and Function ProceduresLesson A Objectives • Explain the difference between a Sub procedure and a Function procedure • Create a procedure that receives information passed to it • Explain the difference between passing data byvalue and passing data by reference • Create a Function procedure Programming with Microsoft Visual Basic 2005, Third Edition

  3. Previewing the Completed Application • Go to Run command on Windows Start menu • Browse to the VB2005\Chap07 folder • Open the Harvey Industries.exe file • The Harvey Industries user interface appears Programming with Microsoft Visual Basic 2005, Third Edition

  4. Previewing the Completed Application (continued) Figure 7-1: Payroll amounts shown in the user interface Programming with Microsoft Visual Basic 2005, Third Edition

  5. Procedures • Procedure • Block of program code that performs a specific task • Two types of procedures in Visual Basic • Sub procedure: does not return a value • Function procedure: does return a value Programming with Microsoft Visual Basic 2005, Third Edition

  6. Sub Procedures • Event procedure • Associated with a specific object and event • Called by Visual Basic in response to an event • Independent procedure • Independent of any object and event • Invoked from code using a Call statement • Parameter: data passed to procedure at call time Programming with Microsoft Visual Basic 2005, Third Edition

  7. Passing Information to an Independent Sub Procedure • Call syntax: Call procedurename([argumentlist]) • argumentlist: used to pass information (optional) • Argument: data item in an argumentlist • Parameter: data item in a parameterlist • Relationship between arguments and parameters • Should agree in number, position, and data type • Types of arguments passed to a procedure • Variable, literal constant, named constant, keyword Programming with Microsoft Visual Basic 2005, Third Edition

  8. Passing Variables • Variables specified by name, address, and value • Passing by value • Passes a copy of the data stored in a variable • Passing by reference • Passes the memory address of a variable • Allows a procedure to change contents of variable Programming with Microsoft Visual Basic 2005, Third Edition

  9. Passing by Value • Copy of data, not variable address, is passed • How to pass by value • Include the keyword ByVal before a parameter • Reasons to pass by value • Procedure needs to know contents of variable • Procedure does not need to change original value • By default, Visual Basic passes by value Programming with Microsoft Visual Basic 2005, Third Edition

  10. Passing By Value (continued) Figure 7-4: Code for the Pet Information application (continued) Programming with Microsoft Visual Basic 2005, Third Edition

  11. Passing By Value (continued) Figure 7-4: Code for the Pet Information application Programming with Microsoft Visual Basic 2005, Third Edition

  12. Passing By Reference • Variable address (memory location) is passed • Receiving procedure can access variable • Reason to pass by reference • Procedure needs to change a variable’s contents • How to pass by reference • Include keyword ByRef before a parameter Programming with Microsoft Visual Basic 2005, Third Edition

  13. Passing By Reference (continued) Figure 7-8: Code for the Gross Pay application (continued) Programming with Microsoft Visual Basic 2005, Third Edition

  14. Passing By Reference (continued) Figure 7-8: Code for the Gross Pay application (continued) Programming with Microsoft Visual Basic 2005, Third Edition

  15. Passing By Reference (continued) Figure 7-8: Code for the Gross Pay application Programming with Microsoft Visual Basic 2005, Third Edition

  16. Function Procedures • Function procedure • A block of code that performs a specific task • Returns a value after completing task • Examples of built-in functions: Val and InputBox • Create your own functions using syntax template • As datatype in header indicates return type of data • Return expression type must agree with Asdatatype Programming with Microsoft Visual Basic 2005, Third Edition

  17. Function Procedures (continued) Figure 7-11: Syntax and an example of a Function procedure Programming with Microsoft Visual Basic 2005, Third Edition

  18. The Pine Lodge Application • Objective: calculate employee’s new pay • Application uses a function to calculate new pay • Function requirements • Input: employee’s current hourly pay and raise rate • Process: calculate raise and then add to current pay • Output: return new pay to calling procedure Programming with Microsoft Visual Basic 2005, Third Edition

  19. The Pine Lodge Application (continued) Figure 7-12: Sample run of the Pine Lodge application Programming with Microsoft Visual Basic 2005, Third Edition

  20. Summary – Lesson A • Two types of procedures: event and independent • Function: performs a task and returns a value • Independent procedures and functions are called from the application’s code • Pass by value: send a copy of variable contents to a procedure or function • Pass by reference: send a variable address to a procedure or function Programming with Microsoft Visual Basic 2005, Third Edition

  21. Coding The Harvey Industries PayrollApplication Lesson B Objectives • Add a combo box to a form • Add items to a combo box • Sort the contents of a combo box • Select a combo box item from code Programming with Microsoft Visual Basic 2005, Third Edition

  22. Coding The Harvey Industries PayrollApplication Lesson B Objectives (continued) • Determine the current item in a combo box • Round a number • Code a combo box’s TextChanged event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  23. Harvey Industries • Objective: calculate and display pay information • Components of employee pay • Employee’s weekly gross pay • Social Security tax • Medicare (FICA) tax • Federal withholding tax (FWT) • Net pay • Input: name, marital status, hours, rate, allowances Programming with Microsoft Visual Basic 2005, Third Edition

  24. Harvey Industries (continued) Figure 7-15: Partially completed user interface for the Harvey Industries application Programming with Microsoft Visual Basic 2005, Third Edition

  25. Including a Combo Box in an Interface • Combo box • Allows user to select from a number of choices • Allows user to type an entry not on list • Can save a space on a form • List box does not share features two and three • DropDownStyle property • Values: Simple, DropDown (default), DropDownList Programming with Microsoft Visual Basic 2005, Third Edition

  26. Including a Combo Box in an Interface (continued) Figure 7-16: Examples of the combo box styles Programming with Microsoft Visual Basic 2005, Third Edition

  27. Including a Combo Box in an Interface (continued) • Change in item value causes TextChanged event • Use Item collection’s Add method to add an item • Other properties of a combo box • Sorted: sorts items in dictionary order • SelectedIndex: used to select an item in list portion • SelectedItem: determines which item is selected • Text: used to get or set a value in text portion Programming with Microsoft Visual Basic 2005, Third Edition

  28. Including a Combo Box in an Interface (continued) Figure 7-17: Code corresponding to the combo boxes shown in Figure 7-16 Programming with Microsoft Visual Basic 2005, Third Edition

  29. Coding the xCalcButton’s Click Event Procedure • Review pay components to calculate and display: • Gross pay • FWT (federal withholding tax) • FICA tax • Net pay • Procedure will call the GetFwt function Programming with Microsoft Visual Basic 2005, Third Edition

  30. Coding the xCalcButton’s Click Event Procedure (continued) Figure 7-22: Pseudocode for the xCalcButton’s Click event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  31. Coding the GetFwt Function • How to calculate weekly taxable wages • Multiply number of withholding allowances by $63.46 • Subtract result in first step from weekly wages • Determining federal withholding tax (FWT) • Evaluate weekly taxable wages and filing status • Use data to look up FWT in special FWT tables • GetFwt function emulates FWT table lookup Programming with Microsoft Visual Basic 2005, Third Edition

  32. Coding the GetFwt Function (continued) Figure 7-25: Weekly FWT tables (Single person) Programming with Microsoft Visual Basic 2005, Third Edition

  33. Coding the GetFwt Function (continued) Figure 7-30: FWT calculations for Single taxpayers Programming with Microsoft Visual Basic 2005, Third Edition

  34. Completing the xCalcButton’s Click Event Procedure • Call GetFwt function from Click event procedure • Three values passed to GetFwt • status variable contents • allowances variable contents • gross variable contents • Value returned by GetFwt assigned to fwt variable • Use Math.Round function to round three values • Gross pay, FWT, and FICA tax amounts Programming with Microsoft Visual Basic 2005, Third Edition

  35. Completing the xCalcButton’s Click Event Procedure (continued) Figure 7-34: Payroll calculations displayed in the interface Programming with Microsoft Visual Basic 2005, Third Edition

  36. Summary – Lesson B • Combo box displays a list of items for selection • Combo box allows user to type entry not on list • Specify style of combo box using DropDownStyle property • Use Items collection’s Add method to add items to a Combo box • Math.Round function rounds off values to arbitrary number of decimal places Programming with Microsoft Visual Basic 2005, Third Edition

  37. Completing the Harvey Industries Payroll ApplicationLesson C Objectives • Prevent a form from closing Programming with Microsoft Visual Basic 2005, Third Edition

  38. Coding The MainForm’sFormClosing Event Procedure • FormClosing event • Occurs when a form is about to be closed • Two ways to cause a FormClosing event • Computer processes the Me.Close() statement • User clicks the Close button on the form’s title bar • Requirement for FormClosing event procedure • Verifying that user wants to close the application • Taking appropriate action based on user’s response Programming with Microsoft Visual Basic 2005, Third Edition

  39. Coding The MainForm’sFormClosing Event Procedure (continued) Figure 7-35: Pseudocode for the MainForm’s FormClosing event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  40. Coding The MainForm’sFormClosing Event Procedure (continued) Figure 7-36: Message box displayed by the FormClosing event procedure Programming with Microsoft Visual Basic 2005, Third Edition

  41. Summary – Lesson C • FormClosing event occurs when Me.Close () is called • FormClosing event also occurs when Close button is clicked • Form’s FormClosing event procedure is processed before a form is about to close Programming with Microsoft Visual Basic 2005, Third Edition

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