1. Microsoft Visual Basic 2010: Reloaded Fourth Edition Chapter Eight Sub and Function Procedures

2. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Objectives After studying this chapter, you should be able to: Explain the difference between Sub and Function procedures Create a Sub procedure Pass information to a procedure Explain the difference between passing data by value and passing data by reference Explain the purpose of the sender and e parameters

3. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Objectives (cont'd.) Associate a procedure with more than one object and event Create a Function procedure Convert an Object variable to a different type using the TryCast operator Utilize a timer control

4. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Procedures Procedure: a block of program code that performs a specific task Two types of procedures: –Function procedure: returns a value after performing its task –Sub procedure: does not return a value

5. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Sub Procedures Two types of Sub procedures: event procedures and independent Sub procedures Event procedure: –Sub procedure that is associated with a specific object and event –Automatically processed when the associated event occurs Independent Sub procedure: –Collection of code that can be invoked from one or more places in an application –Not associated with an event –Processed only when called (invoked)

6. Sub Procedures (cont'd.) Independent Sub procedures are used to: –Eliminate the duplication of code in different parts of a program –Allow a large, complex application to be broken into small and manageable tasks –Allow multiple programmers to work on an application simultaneously Microsoft Visual Basic 2010: Reloaded, Fourth Edition

7. Figure 8-1: How to create an independent Sub procedure

8. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Sub Procedures (cont'd.) Independent Sub procedures: –Have a procedure header and procedure footer –Use Pascal case for names –Optionally contain a parameter list Parameters: –Declared in the procedure header –Store the information passed into the procedure when it is invoked Call statement: invokes an independent Sub procedure

9. Sub Procedures (cont'd.) Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-2: How to call an independent Sub procedure

10. Microsoft Visual Basic 2010: Reloaded, Fourth Edition The Gadis Antiques Application Figure 8-3: Sample run of the Gadis Antiques application

11. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-4: Partial code for the Gadis Antiques application

12. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Including Parameters in an Independent Sub Procedure Parameter: stores data that is passed to the procedure when the procedure is invoked When calling a procedure with parameters, you must pass: –The same number of arguments –The same type of arguments –The arguments in the same order as declared in the procedure Can pass a variable, named constant, literal constant, or keyword as parameter

13. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Passing Variables Each variable has a value and a unique memory address Variable can be passed to a procedure in two ways: –By value: you pass the variable’s value –By reference: you pass the variable’s address Passing by value: the procedure receives only the value and cannot change the actual variable’s value Passing by reference: the procedure receives the address and can make changes to the variable’s value

14. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Passing Variables by Value Use the keyword ByVal before the parameter in the procedure declaration ByVal is the default method of passing variables Procedure cannot change the actual variable’s value Figure 8-5: Sample run of the Pet Information application

15. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-6: Partial code for the Pet Information application

16. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Passing Variables by Reference Use the keyword ByRef before the parameter in the procedure declaration Procedure receives the address of the variable and is able to change the variable’s value Figure 8-7: Sample run of the Gross Pay application

17. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-8: CalcGrossPay procedure and calcButton_click event procedure

18. Passing Variables by Reference (cont’d.) Desk-checking: the process of reviewing the program instructions while seated at your desk instead of in front of a computer –Also called hand-tracing Choose a set of sample data for input values Manually compute the expected output values Microsoft Visual Basic 2010: Reloaded, Fourth Edition

19. Figure 8-9: Gross pay calculation using sample input values Figure 8-10: Desk-check table before the Call statement is processed Passing Variables by Reference (cont’d.)

20. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-11: Desk-check table after the Call statement and CalcGrossPay procedure header are processed Passing Variables by Reference (cont’d.)

21. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-12: Desk-check table after the first statement in the CalcGrossPay procedure is processed Passing Variables by Reference (cont’d.)

22. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-13: Desk-check table after the statement in the selection structure’s true path is processed Passing Variables by Reference (cont’d.)

23. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-14: Desk-check table after the CalcGrossPay procedure ends Passing Variables by Reference (cont’d.)

24. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Associating a Procedure with Different Objects and Events Handles keyword: –Appears in event procedure header –Indicates the object and event associated with the procedure –Controls when the procedure is invoked By default, the event procedure name matches the name of the associated object and event

25. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-15: Some of the Gadis Antiques application’s code from Figure 8-4

26. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Associating a Procedure with Different Objects and Events (cont'd.) Event procedure: –Name of event procedure can be changed –Can be associated with more than one object and event as long as each event has the same parameters Add the additional object/events to the Handles clause Sender parameter: contains the memory address of the object that raised the event e parameter: contains additional information about the object that raised the event

27. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-16: ClearLabels procedure Associating a Procedure with Different Objects and Events (cont'd.)

28. Function Procedures Function procedure (or Function): –Block of code that performs a specific task –Returns a value after completing its task Visual Basic contains many built-in functions You can create your own functions with or without parameters A function is invoked by including its name with any arguments in a statement Microsoft Visual Basic 2010: Reloaded, Fourth Edition

29. Function Procedures (cont'd.) Function procedure header: –As datatype clause indicates the type of the return value Function procedure footer statement: –End Function Return keyword: –Sets the value to be returned by the function –Ends the function

30. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-17: How to create a Function procedure

31. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-18: How to invoke a Function procedure Function Procedures (cont'd.)

32. Microsoft Visual Basic 2010: Reloaded, Fourth Edition The Circle Area Calculator Application Figure 8-19: Sample run of the Circle Area Calculator application

33. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-20: Partial code for the Circle Area Calculator application The Circle Area Calculator Application (cont’d.)

34. Converting Object Variables sender parameter in an event procedure is declared as type Object –Contains the address of the object that raised the event To access the properties of the object, you must convert it to the appropriate data type Type casting (or casting): the process of converting a variable from one data type to another TryCast operator: used to cast a variable to a different data type Microsoft Visual Basic 2010: Reloaded, Fourth Edition

35. Figure 8-21: How to use the TryCast operator

36. The Full Name Application User enters a first and last name Application concatenates both names and displays results Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-22: Sample run of the Full Name application

37. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 8-23: Two ways of writing the Enter event procedures for both text boxes

38. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Using a Timer Control Timer control: processes code at one or more regular intervals Timer control does not appear on the form, but in the component tray Component tray: stores controls that do not appear in the user interface at run time Interval property: specifies the length of time (in milliseconds) until the next Tick event occurs Tick event: occurs after each interval has elapsed Enabled property: must be True for timer to run

39. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Figure 24: Interface for the Timer Example application Using a Timer Control (cont’d.)

40. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Using a Timer Control (cont'd.) Figure 8-25: Code entered in two of the application’s procedures

41. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Programming Tutorial 1 The Tri-County Electricity Application Figure 8-27: MainForm in the Tri-County Electricity application

42. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Programming Tutorial 2 The Concentration Game Figure 8-35: MainForm for the Concentration Game application

43. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Programming Example Figure 8-47: MainForm in the Rainfall Calculator application Rainfall Calculator Application

44. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Summary Function procedures return a value; Sub procedures do not return a value Event procedure: a Sub procedure associated with one or more objects and events Independent Sub and Function procedures: not associated with any specific object or event Call statement: used to invoke a procedure When calling a procedure, you must pass the same number, type, and order of parameter values as those declared in the procedure

45. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Summary (cont'd.) Values can be passed to a procedure by value or by reference By Value: –Provides only the value of the variable to the procedure –Use the ByVal keyword By Reference: –Provides the address of the variable to the procedure, allowing the procedure to change the variable’s value –Use the ByRef keyword

46. Microsoft Visual Basic 2010: Reloaded, Fourth Edition Summary (cont'd.) Variables in the parameter list in a procedure header have procedure scope Can modify the Handles clause of an event procedure to associate it with more than one object and event TryCast keyword: allows you to convert an object from one data type to another Timer control: used to process code at one or more specified intervals