1. STARTING OUT WITH Visual Basic 2008
FOURTH EDITION
Tony Gaddis
Haywood Community College
Kip Irvine
Florida International University

2. Procedures And Functions
Chapter
Procedures And Functions 6
A procedure is a collection of statements that performs a task
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

3. Introduction
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 VB statement that executed it
Functions work like intrinsic functions, such as CInt and IsNumeric
A method can be either a procedure or a function

4. Procedures
6.1
You Can Write Your Own General Purpose Procedures That Perform Specific Tasks
General Purpose Procedures Are Not Triggered by Events but Called From Statements in Other Procedures
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

5. Procedure Uses Ann event handler is a type of procedure
Automatically executed when an event such as a mouse click occurs
General purpose procedures are triggered by statements in other procedures, not by events
Procedures help simplify & modularize code by:
Breaking it into small, manageable pieces
Performing a task that is needed repeatedly
Dividing a program into a set of logical tasks

6. Sample Procedure, Tutorial 6-1
Private Sub btnGo_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnGo.Click
' This procedure calls the DisplayMessage procedure.
lstOutput.Items.Add("Hello from btnGo_Click procedure.")
lstOutput.Items.Add("Calling the DisplayMessage " & _
"procedure.")
DisplayMessage()
lstOutput.Items.Add("Now I am back ” _ & “in the btnGo_Click procedure.")
End Sub
Calls DisplayMessage procedure
Returns to btnGo_Click
Sub DisplayMessage()
'A Sub procedure that displays a message.
lstOutput.Items.Add("")
lstOutput.Items.Add("Hello from DisplayMessage.")
End Sub

7. Declaring a Procedure
[AccessSpecifier] Sub ProcedureName ([ParameterList])
[Statements]
End Sub
AccessSpecifier is optional and establishes accessibility to the program
Sub and End are keywords
ProcedureName used to refer to procedure
Use Pascal casing, capitalize 1st character of the name and each new word in the name
ParameterList is a list of variables or values being passed to the sub procedure

8. More on the Access Specifier
Private allows use only from that form
Public allows use from other forms
If not specified, default is Public
There are other access specifiers such as:
Protected
Friend
Protected Friend
These will be discussed in later chapters
Access specifiers won’t be used for now
Practice writing procedures in Tutorial 6-2

9. Procedures and Static Variables
Variables needed only in a procedure, should be declared within that procedure
Creates a local variable with scope only within the procedure where declared
Local variable values are not saved from one procedure call to the next
To save value between procedure calls, use Static keyword to create a static local variable
Static VariableName As DataType
Scope is still only within the procedure
But variable exists for lifetime of application

10. Passing Arguments to a Procedure
6.2
When calling a procedure, you can pass it values known as arguments
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

11. Arguments Argument – a value passed to a procedure
We’ve already done this with functions
Value = CInt(txtInput.Text)
Calls the CInt function and passes txtInput.Text as an argument
A procedure must be declared with a parameter list in order to accept an argument

12. Passing Arguments By Value
DisplayValue(5) ‘calls DisplayValue procedure
Sub DisplayValue(ByVal intNumber As Integer)
' This procedure displays a value in a message box.
MessageBox.Show(intNumber.ToString)
End Sub
intNumber declared as an integer argument
Storage location intNumber created by procedure
A value, 5 in this case, must be supplied and is copied into the storage location for intNumber
The DisplayValue procedure then executes
Tutorial 6-3 demonstrates passing arguments

13. Passing Multiple Arguments
ShowSum(intValue1, intValue2) ‘calls ShowSum procedure
Sub ShowSum(ByVal intNum1 As Integer, _
ByVal intNum2 As Integer)
' This procedure accepts two arguments, and prints
' their sum on the form.
Dim intSum As Integer
intSum = intNum1 + intNum2
MessageBox.Show("The sum is " & intSum.ToString)
End Sub
Multiple arguments separated by commas
Value of first argument is copied to first
Second to second, etc.

14. Passing Arguments ByVal or ByRef
Arguments are usually passed ByVal
New storage location created for procedure
Storage location gets a copy of the value
Any changes in value are made to the copy
Calling procedure won’t “see” the changes
Arguments can also be passed ByRef
Procedure points to (references) argument’s original storage location
Any changes are made to the original value
Calling procedure “sees” the changes

15. ByVal or ByRef Argument Example
Tutorial 6-4 demonstrates the difference between parameters passed ByVal & ByRef
Passed ByVal
Calling procedure does not “see” changes made to the value of an argument
Passed ByRef
Calling procedure “sees” changes made to the value of an argument

16. 6.3
Functions
A Function Returns a Value to the Part of the Program That Called the Function
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

17. Declaring a Function New keyword Function
[AccessSpecifier] Function FunctionName ([ParameterList]) _
As DataType
[Statements]
End Function
New keyword Function
Also new is As DataType which states the data type of the value to be returned
Return value is specified in a Return expression

18. Function Call Example sngValue1 & sngValue2 must be data type Single
sngTotal = Sum(sngValue1, sngValue2)
Function Sum(ByVal sngNum1 As Single, _
ByVal sngNum2 As Single) As Single
Dim sngResult As Single
sngResult = sngNum1 + sngNum2
Return sngResult
End Function
sngValue1 & sngValue2 must be data type Single
Data types must agree with parameter list
sngTotal must be Single, agrees with return value
Tutorial 6-5 demonstrates function use

19. Returning Nonnumeric Values
Function FullName(ByVal strFirst As String, _
ByVal strLast As String) As String
Dim strName As String
strName = strLast & ", " & strFirst
Return strName
End Function
Function IsValid(intNum As Integer) As Boolean
Dim blnStatus As Boolean
If intNum >= 0 And intNum <= 100 Then
blnStatus = True
Else
blnStatus = False
End If
Return blnStatus
End Function

20. 6.4 More About Debugging Step Into Step Over Step Out
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21. Debugging Involving Procedures
Step Into - continue to debug by single-stepping through a procedure
Step Over - run procedure without single-stepping, continue single-step after the call
Step Out - end single-stepping in procedure, continue single-step after the call
Tutorial 6-6 provides examples

22. Building the Bagel and Coffee Price Calculator Application
6.5
Building the Bagel and Coffee Price Calculator Application
Use procedures and functions to calculate the total of a customer order.
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

23. Bagel and Coffee Price Calculator

24. Button Click Event Flowcharts
Calculate Button
Reset Button

25. Cost Calculation Functions
Topping Cost Function
Bagel Cost Function

26. Cost Calculations Functions
Coffee Cost Function
Calc Tax Function