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Microsoft Visual Basic 2010: Reloaded Fourth Edition

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Title: Microsoft Visual Basic 2010: Reloaded Fourth Edition


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

2
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
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
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
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

7
Figure 8-1 How to create an independent Sub
procedure
8
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.)
Figure 8-2 How to call an independent Sub
procedure
10
The Gadis Antiques Application
Figure 8-3 Sample run of the Gadis Antiques
application
11
Figure 8-4 Partial code for the Gadis Antiques
application
12
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
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 variables value
  • By reference you pass the variables address
  • Passing by value the procedure receives only the
    value and cannot change the actual variables
    value
  • Passing by reference the procedure receives the
    address and can make changes to the variables
    value

14
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 variables
    value

Figure 8-5 Sample run of the Pet Information
application
15
Figure 8-6 Partial code for the Pet Information
application
16
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 variables value

Figure 8-7 Sample run of the Gross Pay
application
17
Figure 8-8 CalcGrossPay procedure and
calcButton_click event procedure
18
Passing Variables by Reference (contd.)
  • 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

19
Passing Variables by Reference (contd.)
Figure 8-9 Gross pay calculation using sample
input values
Figure 8-10 Desk-check table before the Call
statement is processed
20
Passing Variables by Reference (contd.)
Figure 8-11 Desk-check table after the Call
statement and CalcGrossPay procedure header are
processed
21
Passing Variables by Reference (contd.)
Figure 8-12 Desk-check table after the first
statement in the CalcGrossPay procedure is
processed
22
Passing Variables by Reference (contd.)
Figure 8-13 Desk-check table after the statement
in the selection structures true path is
processed
23
Passing Variables by Reference (contd.)
Figure 8-14 Desk-check table after the
CalcGrossPay procedure ends
24
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
Figure 8-15 Some of the Gadis Antiques
applications code from Figure 8-4
26
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
Associating a Procedure with Different Objects
and Events (cont'd.)
Figure 8-16 ClearLabels procedure
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

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
Figure 8-17 How to create a Function procedure
31
Function Procedures (cont'd.)
Figure 8-18 How to invoke a Function procedure
32
The Circle Area Calculator Application
Figure 8-19 Sample run of the Circle Area
Calculator application
33
The Circle Area Calculator Application (contd.)
Figure 8-20 Partial code for the Circle Area
Calculator application
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

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

Figure 8-22 Sample run of the Full Name
application
37
Figure 8-23 Two ways of writing the Enter event
procedures for both text boxes
38
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
Using a Timer Control (contd.)
Figure 24 Interface for the Timer Example
application
40
Using a Timer Control (cont'd.)
Figure 8-25 Code entered in two of the
applications procedures
41
Programming Tutorial 1
  • The Tri-County Electricity Application

Figure 8-27 MainForm in the Tri-County
Electricity application
42
Programming Tutorial 2
  • The Concentration Game

Figure 8-35 MainForm for the Concentration Game
application
43
Programming Example
  • Rainfall Calculator Application

Figure 8-47 MainForm in the Rainfall Calculator
application
44
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
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
    variables value
  • Use the ByRef keyword

46
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
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