Programming Language 2 (PL2)

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Cori Hart
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1 Programming Language 2 (PL2)

parameters 2 Identify the scope of local and

2 Explain the concept of arguments and parameters Identify the scope of local and global variables Apply global and local variables to sub procedures and functions

to the part of the program that executed it You have already worked with Visual Basic s

3 A procedure is a collection of statements that performs a task Event handlers are a type of procedure A function is a collection of statements that performs a task and returns a value to the part of the program that executed it You have already worked with Visual Basic s built-in functions, such as CInt and IsNumeric A method can be either a procedure or a function

4 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 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 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

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

8 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 Figure 8-2: How to call an independent Sub procedure

10 Figure 8-3: Sample run of the Gadis Antiques application

11 Figure 8-4: Partial code for the Gadis Antiques application

12 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

text as an argument Two ways to pass arguments by value you

13 An Argument is value passed to a procedure For example: CInt(txtInput.Text) Calls the CInt function Passes txtinput.text as an argument Two ways to pass arguments by value you pass the variable s value by reference you pass the variable s address

14 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

15 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

16 Figure 8-6: Partial code for the Pet Information application

17 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

18 Figure 8-8: CalcGrossPay procedure and calcbutton_click event procedure

19 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

20 Figure 8-9: Gross pay calculation using sample input values Figure 8-10: Desk-check table before the Call statement is processed

21 Figure 8-11: Desk-check table after the Call statement and CalcGrossPay procedure header are processed

22 Figure 8-12: Desk-check table after the first statement in the CalcGrossPay procedure is processed

23 Figure 8-13: Desk-check table after the statement in the selection structure s true path is processed

24 Figure 8-14: Desk-check table after the CalcGrossPay procedure ends

25 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

26 Figure 8-15: Some of the Gadis Antiques application s code from Figure 8-4

27 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

28 Figure 8-16: ClearLabels procedure

29 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

30 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

31 Figure 8-17: How to create a Function procedure

32 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

33 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

34 Figure 8-13: How to create a Function procedure

35 Figure 8-14: Sample run of the Pine Lodge application

36 Figure 8-15: Partial code for the Pine Lodge application

37 Figure 8-15: Partial code for the Pine Lodge application (continued)

38 Timer control: processes code at one or more regular intervals Interval property: specifies the length of time (in milliseconds) until the next Tick event occurs Tick event: occurs after each interval has elapsed Timer control does not appear on the form, but in the component tray Component tray: area in the IDE that stores all controls that do not appear in the user interface at runtime Enabled property: must be True for timer to run

39 Figure 8-16: Timer shown in the Component tray

40 Figure 8-17: The cartimer s Tick event procedure

41 The Concentration Game Figure 8-19: User interface

42 Rainfall application Figure 8-35: User interface

Part 4 - Procedures and Functions

Part 4 - Procedures and Functions Problem Solving Methodology... 2 Top Down Design... 2 Procedures and Functions... 5 Sub Procedures... 6 Sending Parameters to a Sub Procedure... 7 Pass by Value... 8 Pass