Chapter 6 Procedures and Functions

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Chapter 6 Procedures and Functions Instructor
Bindra Shrestha University of Houston Clear
Lake CSCI 3131.01
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Acknowledgement Dr. Xinhua Chen And
Starting Out with Visual Basic 2010 by Tony
Gaddis and Kip Irvine
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Topics

• Procedures
• Passing arguments to Procedures
• Functions
• Debugging
• Step into, over, and out of procedures and
  functions

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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

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Procedures A moderately complex program needs to
perform multiple tasks. A procedure is a block
of program code that performs a specific task.
A program can be considered as the assembly of
multiple procedures. Execution of a procedure is
commonly referred to as calling a procedure.
Most Visual Basic procedures are Sub procedures
and Function procedures.
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How Procedure Works A (called) procedure is
invoked by another procedure (caller). The called
procedure does the job and return the result to
the caller or simply transfer control to the
caller without returning anything. An analogy of
procedure calls
boss
worker2
worker3
worker1
worker4
worker5
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Sub Procedures vs. Function Procedures A Sub
procedure does not return a value after
performing its assigned task. A Function
procedure returns a value after performing its
assigned task. A Function procedure can only
return one value.
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• Sub Procedures
• Two types of Sub procedures
• Event procedures
• - Associated with a specific object and event
• General Purpose Sub procedures
• - Independent of any object and event can be
  called (or invoked) from one or more places in an
  application.

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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("He
llo from btnGo_Click procedure.") lstOutput.Items
.Add("Now I am calling the " _ "DisplayMessage
procedure." "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("") lstO
utput.Items.Add("Hello from DisplayMessage
procedure.") lstOutput.Items.Add("") End Sub
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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. It is optional.

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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

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Placing Sub Procedure in File General Purpose
Sub procedures can be placed anywhere in the
Form's code module. Likewise, the event
procedures can also be placed anywhere in the
Form's code module.
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Sample Procedure Private Sub DisplayGrade()
' Display the intAverage score.
lblAverage.Text intAverage.ToString() '
Determine and display the letter grade.
Select Case intAverage Case 90 To
100 lblLetterGrade.Text "A"
Case 80 To 90
lblLetterGrade.Text "B" Case 70 To
79 lblLetterGrade.Text "C"
Case 60 To 69
lblLetterGrade.Text "D" Case Else
lblLetterGrade.Text "E"
End Select End Sub
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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

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Arguments

• Argument a value passed to a procedure
• Weve 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

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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.Sh
ow("The sum is " intSum.ToString) End Sub

• Multiple arguments separated by commas
• Value of first argument is copied to first
• Second to second, etc.

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• Passing Variables
• A variable has both a value and a unique address
  in the computer's memory. Either the value or the
  address can be passed to a Sub procedure.
• Two ways of passing parameters
• Pass by value. The value of the variable is
  passed to the called procedure
• Pass by reference The address of the variable is
  passed to the called procedure.

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Two analogies Telling someone the balance of
your banking account passes the information about
your banking account. This is equivalent to "pass
by value". Giving someone the information on the
account number, PIN, etc. authorizes the person
to deposit or withdraw from the account, as well
as to query the balance. This equivalent to "pass
by reference".
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Pass Variables by Value Passing variables by
value is specified in the parameter list. Use
the keyword ByVal in the variable
declaration. When the procedure is invoked, only
the value of the variable is passed to the
procedure. Example Private Sub
DisplayMessage(ByVal pet As String, ByVal years
As String) messageLabel.Text "Your pet " pet
" is " _ years " years old." End Sub
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Pass Variables by Value (Cont'd) In the calling
procedure, you may have code like this Private
Sub getInfoButton_Click(ByVal sender As Object,
_ ByVal e As System.EventArgs) Handles
getInfoButton.Click Dim petName As String Dim
petAge As String petName InputBox("Pet's
name", "Name Entry") petAge InputBox("Pet's
age (years)", "Age Entry") DisplayMessage(petNa
me, petAge) End Sub
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• Passing Variables by Reference
• Passing variables by reference gives the
  receiving procedure access to the variable being
  passed.
• In most cases, you pass variables by reference
  when you want the receiving procedure to change
  the contents of the variables.
• Use the ByRef keyword in the parameter list.

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Passing Variables by Reference (Cont'd) Example
Private Sub calcButton_Click(ByVal sender As
Object, _ ByVal e As System.EventArgs) Handles
calcButton.Click Dim hoursWkd As Decimal Dim
rateOfPay As Decimal Dim grossPay As
Decimal hoursWkd Convert.ToDecimal(hoursListBo
x.SelectedItem) rateOfPay Convert.ToDecimal(rat
eListBox.SelectedItem) CalcGrossPay(hoursWkd,
rateOfPay, grossPay) grossLabel.Text
grossPay.ToString("C2") End Sub
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Look into Memory of variables The variables
used in exchange information between the
calcButton's click event procedure and the
CalcGrossPay procedures are allocated as follows
Variable Value Address
In the calling procedure
hoursWkd 41.0 40000 rateOfPay 8.0
40016
In the called procedure
hoursWkd 41.0 60000 rateOfPay 8.0
60016
In both procedures
GrossPay 332.0 40032
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Passing Variables by Reference (Cont'd) Example P
rivate Sub CalcGrossPay(ByVal hours As Decimal,
_ ByVal rate As Decimal, _ ByRef
gross As Decimal) ' calculates gross pay If
hours lt 40D Then gross hours
rate Else gross hours rate (hours - 40D)
rate / 2D End If End Sub
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Additional ByVal or ByRef 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 seeschanges made to the
  value of an argument

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Function Procedures Similar to a Sub procedure,
a function procedure is a block of code that
performs a specific task. Different from a Sub
procedure, a function procedure returns a value
upon completion of the task. The concepts of
passing by value and passing by reference apply
to both Sub procedures and Function procedures.
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Declaring a Function
AccessSpecifier Function FunctionName
(ParameterList) _ As DataType Statements E
nd 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

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Function Call Example
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

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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 gt 0
And intNum lt 100 Then blnStatus
True Else blnStatus False End If Return
blnStatus End Function
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Visual Basic 2010 addition

•  Nullable types were added to Visual Basic 2008. 
  In Visual Basic 2010 you can use them as
  parameters.  Here are two examples
• ' Assign Nothing as the default value for
  nullable optional parameter.
• Sub Calculate (ByVal x As Integer, ByVal y As
  Integer, Optional ByVal z As Integer? Nothing)
• ...
• End Sub
• ' Assign an integer value to a nullable optional
  paramter of type Double.
• Sub Process(ByVal x As Integer, ByVal y As
  Integer, Optional ByVal z As Double? 10)
• ...
• End Sub

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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