Microsoft Visual Basic 2010:
Reloaded
Fourth Edition
Chapter Eight
Sub and Function Procedures
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
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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
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
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)
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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
Figure 8-1: How to create an independent Sub procedure
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
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Sub Procedures (cont'd.)
Figure 8-2: How to call an independent Sub procedure
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The Gadis Antiques Application
Figure 8-3: Sample run of the Gadis Antiques application
Microsoft Visual Basic 2010: Reloaded, Fourth Edition
Figure 8-4: Partial
code for the Gadis
Antiques application
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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
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
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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
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Figure 8-6: Partial
code for the Pet
Information
application
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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
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Figure 8-8:
CalcGrossPay procedure
and calcButton_click
event procedure
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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
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Passing Variables by Reference
(cont’d.)
Figure 8-9: Gross pay calculation using sample input values
Figure 8-10: Desk-check table before the Call statement is processed
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Passing Variables by Reference
(cont’d.)
Figure 8-11: Desk-check table after the Call statement and
CalcGrossPay procedure header are processed
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Passing Variables by Reference
(cont’d.)
Figure 8-12: Desk-check table after the first statement in the
CalcGrossPay procedure is processed
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Passing Variables by Reference
(cont’d.)
Figure 8-13: Desk-check table after the statement in the selection
structure’s true path is processed
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Passing Variables by Reference
(cont’d.)
Figure 8-14: Desk-check table after the CalcGrossPay procedure
ends
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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
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Figure 8-15: Some of the Gadis Antiques application’s code from Figure 8-4
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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
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Associating a Procedure with Different
Objects and Events (cont'd.)
Figure 8-16: ClearLabels procedure
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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
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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
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Figure 8-17: How to create a Function procedure
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Function Procedures (cont'd.)
Figure 8-18: How to invoke a Function procedure
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The Circle Area Calculator Application
Figure 8-19: Sample run of the Circle Area Calculator application
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The Circle Area Calculator Application
(cont’d.)
Figure 8-20: Partial code for the Circle Area Calculator application
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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
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Figure 8-21: How to use the TryCast operator
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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
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Figure 8-23: Two ways of writing the Enter event procedures for both text boxes
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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
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Using a Timer Control (cont’d.)
Figure 24: Interface for the Timer Example application
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Using a Timer Control (cont'd.)
Figure 8-25: Code entered in two of the application’s procedures
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Programming Tutorial 1
• The Tri-County Electricity Application
Figure 8-27: MainForm in the Tri-County Electricity application
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Programming Tutorial 2
• The Concentration Game
Figure 8-35: MainForm for the Concentration Game application
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Programming Example
• Rainfall Calculator Application
Figure 8-47: MainForm in the Rainfall Calculator application
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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
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
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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
Microsoft Visual Basic 2010: Reloaded, Fourth Edition