1. Programming with Microsoft Visual Basic 2008 Fourth Edition
Chapter Seven
Sub and Function Procedures

2. Previewing the Harvey Industries Application
Open the Harvey Industries.exe file
The Harvey Industries application calculates payroll for an employee
Programming with Microsoft Visual Basic 2008, Fourth Edition

3. Previewing the Harvey Industries Application (continued)
Figure 7-1: Payroll calculations 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:
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 by value and passing data by reference
Create a Function procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

5. Procedures Procedure: Two types of procedures in Visual Basic:
Block of program code that performs specific task
Two types of procedures in Visual Basic:
Sub procedure: Does not return value
Function procedure: Does return value
Programming with Microsoft Visual Basic 2008, Fourth Edition

6. Sub Procedures Two types of Sub procedures in Visual Basic:
Event procedures
Independent Sub procedures
Event procedure: Associated with specific object and event
Processed automatically in response to event
Independent Sub procedure: Independent of any object and event
Invoked from code using Call statement
Parameter: Data passed to procedure when invoked
Programming with Microsoft Visual Basic 2008, Fourth Edition

7. Sub Procedures (continued)
Call statement: Used to invoke procedure
Syntax: Call procedurename([argumentlist])
argumentlist: Used to pass information (optional)
Argument: Data item in argumentlist
Parameter: Data item in parameterlist
Relationship between arguments and parameters
Should agree in number, position, and data type
Types of data that can be passed as arguments to a procedure:
Variable, literal constant, named constant, keyword
Programming with Microsoft Visual Basic 2008, Fourth Edition

8. Sub Procedures (continued)
Figure 7-2: Syntax of an independent Sub procedure and the Call statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

9. Passing Variables A variable has value and unique address in memory
You can pass either variable’s value or its address to procedure
Passing by value:
Passes copy of value stored in variable
Passing by reference:
Passes memory address of variable
Allows procedure to change contents of variable
Programming with Microsoft Visual Basic 2008, Fourth Edition

10. Passing a Variable by Value
Passing by value: Provides only contents of variable to receiving procedure
How to pass by value:
Include keyword ByVal before 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 2008, Fourth Edition

11. Passing a Variable by Value (continued)
Figure 7-4: Additional lines of code entered in the ShowMsg procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

12. Passing a Variable by Value (continued)
Figure 7-5: ShowMsg procedure and btnDisplay Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

13. Passing a Variable by Reference
Passing by reference: Provides address (memory location) of variable to procedure
Receiving procedure can thus access variable
Reason to pass by reference:
Procedure needs to change variable’s contents
How to pass by reference:
Include keyword ByRef before parameter
Programming with Microsoft Visual Basic 2008, Fourth Edition

14. Passing a Variable by Reference (continued)
Figure 7-8: CalcGrossPay procedure and btnCalc contro’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

15. Passing a Variable by Reference (continued)
Figure 7-9: Desk-check table before the computer processes the Call statement
Programming with Microsoft Visual Basic 2008, Fourth Edition

16. Passing a Variable by Reference (continued)
Figure 7-10: Desk-check table after the computer processes the Call statement and the CalcGrossPay procedure header
Programming with Microsoft Visual Basic 2008, Fourth Edition

17. Passing a Variable by Reference (continued)
Figure 7-11: Desk-check table after the computer processes the first statement in the CalcGrossPay procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

18. Passing a Variable by Reference (continued)
Figure 7-12: Desk-check table after the computer processes the statement in the selection structure’s true path
Programming with Microsoft Visual Basic 2008, Fourth Edition

19. Passing a Variable by Reference (continued)
Figure 7-13: Desk-check table after the CalcGrossPay procedure ends
Programming with Microsoft Visual Basic 2008, Fourth Edition

20. Function Procedures Function procedure:
Block of code that performs specific task
Returns value after completing its task
Visual Basic provides built-in functions
Can also create your own functions
As datatype in header indicates return type of data
Return expression type must agree with As datatype
Programming with Microsoft Visual Basic 2008, Fourth Edition

21. Function Procedures (continued)
Figure 7-14: Syntax, example, and steps for creating a function
Programming with Microsoft Visual Basic 2008, Fourth Edition

22. Function Procedures (continued)
Figure 7-15: Examples of invoking the GetNewPrice function
Programming with Microsoft Visual Basic 2008, Fourth Edition

23. Function Procedures (continued)
Figure 7-16: CalcGrossPay function and btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

24. Lesson A Summary Two types of procedures: Event and independent
Function: Performs task and returns value
Independent procedures and functions are called from application’s code using Call statement
Pass by value: Send copy of variable’s contents to procedure or function
Pass by reference: Send variable’s address to procedure or function
Programming with Microsoft Visual Basic 2008, Fourth Edition

25. Lesson B Objectives After studying Lesson B, you should be able to:
Include a combo box in an interface
Add items to a combo box
Select a combo box item from code
Determine the item either selected or entered in a combo box
Code a combo box’s TextChanged event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

26. Including a Combo Box in an Interface
Allows user to select from number of choices
Allows user to type entry not on list
Can save space on form
List box does not share features two and three
DropDownStyle property:
Values: Simple, DropDown (default), DropDownList
Programming with Microsoft Visual Basic 2008, Fourth Edition

27. Including a Combo Box in an Interface (continued)
Figure 7-19: Examples of the combo box styles
Programming with Microsoft Visual Basic 2008, Fourth Edition

28. Including a Combo Box in an Interface (continued)
Change in item value causes TextChanged event
Use Item collection’s Add method to add item
Other properties of combo box
Sorted: Sorts items in dictionary order
SelectedIndex: Used to select item in list portion
SelectedItem: Determines which item is selected
Text: Used to get or set value in text portion
Programming with Microsoft Visual Basic 2008, Fourth Edition

29. Including a Combo Box in an Interface (continued)
Figure 7-20: Code used to add items to the combo boxes and also select a default item
Programming with Microsoft Visual Basic 2008, Fourth Edition

30. Including a Combo Box in an Interface (continued)
Figure 7-22: Interface showing the gross pay
Programming with Microsoft Visual Basic 2008, Fourth Edition

31. Lesson B Summary Combo box displays 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 Combo box
Use combo box’s Sorted property to sort items in combo’s list
Programming with Microsoft Visual Basic 2008, Fourth Edition

32. Lesson B Summary (continued)
Use SelectedIndex, SelectedItem, or Text property to select combo box item from code
Use SelectedIndex, SelectedItem, or Text property to determine item that was selected
Programming with Microsoft Visual Basic 2008, Fourth Edition

33. Lesson C Objectives After studying Lesson C, you should be able to:
Prevent a form from closing
Round a number
Programming with Microsoft Visual Basic 2008, Fourth Edition

34. Coding the Harvey Industries Application
Objective:
Calculate employee’s weekly gross pay, federal withholding tax (FWT), Social Security and Medicare (FICA) tax, and net pay
Review TOE chart for requirements
Programming with Microsoft Visual Basic 2008, Fourth Edition

35. Coding the Harvey Industries Application (continued)
Figure 7-25: User interface for the Harvey Industries application
Programming with Microsoft Visual Basic 2008, Fourth Edition

36. Coding the FormClosing Event Procedure
FormClosing event: Occurs when form is about to be closed because:
Computer processes Me.Close() statement
User clicks Close button on form’s title bar
Requirement for FormClosing event procedure:
Verifying that user wants to close application
Taking appropriate action based on user’s response
To prevent closing, set Cancel property of FormClosing procedure’s e parameter to true
Programming with Microsoft Visual Basic 2008, Fourth Edition

37. Coding the FormClosing Event Procedure (continued)
Figure 26: Pseudocode for the FormClosing event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

38. Coding the FormClosing Event Procedure (continued)
Figure 27: Message box displayed by the FormClosing event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

39. Coding the FormClosing Event Procedure (continued)
Figure 28: Pseudocode for the btnCalc control’s Click event procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

40. Coding the btnCalc Control’s Click Event Procedure
Figure 29: Selection structure entered in the procedure
Programming with Microsoft Visual Basic 2008, Fourth Edition

41. Coding the GetFwt Function
How to calculate weekly taxable wages:
Multiply number of withholding allowances by $67.31
Subtract this result from weekly gross pay
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 2008, Fourth Edition

42. Coding the GetFwt Function (continued)
Figure 7-31: FWT calculations for a married taxpayer with taxable wages of $288.46
Programming with Microsoft Visual Basic 2008, Fourth Edition

43. Coding the GetFwt Function (continued)
Figure 7-32: FWT calculations for a single taxpayer with taxable wages of $600.00
Programming with Microsoft Visual Basic 2008, Fourth Edition

44. Coding the GetFwt Function (continued)
Figure 7-33: Pseudocode for the GetFwt function
Programming with Microsoft Visual Basic 2008, Fourth Edition

45. Coding the GetFwt Function (continued)
Figure 7-34: GetFWT function header and footer
Programming with Microsoft Visual Basic 2008, Fourth Edition

46. Completing the btnCalc Control’s Click Event Procedure
Must call GetFwt function from btnCalc’s Click event procedure
Math.Round function: Used to round value to specific number of decimal places
Syntax: Math.Round (value[, digits])
value: Numeric value to work on
digits: Number of places to right of decimal point
Programming with Microsoft Visual Basic 2008, Fourth Edition

47. Completing the btnCalc Control’s Click Event Procedure (continued)
Figure 7-35: Payroll calculations displayed in the interface
Programming with Microsoft Visual Basic 2008, Fourth Edition

48. Lesson C Summary
Use form’s FormClosing event procedure to process code when form is about to be closed
Set Cancel property of FormClosing event procedure’s e parameter to true to prevent form from being closed
Use Math.Round function to round number to specific number of decimal places
Programming with Microsoft Visual Basic 2008, Fourth Edition