Sub Procedures

1
Chapter 6

• Sub Procedures
• And Functions

2
Chapter 6Introduction

• A procedure is a collection of statements that
  performs a task.

3
Chapter 6 Topics

• A Sub procedure is a collection of statements
  that performs a task
• An abbreviation of the older term subroutine
• Event procedures are Sub procedures
• A Function procedure is a collection of
  statements that performs a task and returns a
  value to the VB statement that executed it
• Function procedures work like intrinsic
  functions, such as Val and IsNumeric
• A method is a procedure declared in a class

4
Section 6.1Sub Procedures

• You Can Write Your Own General Purpose Sub
  Procedures That Perform Specific Tasks
• General Purpose Sub Procedures Are Not Triggered
  by Events but Called From Statements in Other
  Procedures

5
Sub Procedure Uses

• May handle events such as a click event
• Also used to simplify a program by
• Breaking it into small, manageable pieces or
• Performing a task that is needed repeatedly
• Sub procedures help to modularize code
• Divides a program into a set of logical tasks

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Sample Sub 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("Calling the DisplayMessage "
_ "procedure.") DisplayMessage() lstOutput.Ite
ms.Add("Now I am back in the btnGo_Click procedu
re.") End Sub
Returns to btnGo_Click
Calls DisplayMessage procedure
Sub DisplayMessage() 'A Sub procedure that
displays a message. lstOutput.Items.Add("") lstO
utput.Items.Add("Hello from DisplayMessage.") lst
Output.Items.Add("") End Sub
7
Declaring a Sub Procedure
AccessSpecifier Sub ProcedureName
(ParameterList) Statement(s) 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

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More on Access Specifier

• Private allows use only from that form or class
• Public allows use from other forms or classes
• If not specified, default is Public
• Additional access specifiers
• Protected
• Friend
• Protected Friend
• These will be discussed in later chapters
• Access specifiers wont be used for now
• Practice writing procedures in Tutorial 6-2

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Procedures and Static Variables

• Variables needed only in a Sub procedure, should
  be declared within the Sub procedure
• Creates a local variable with scope only within
  the sub procedure where declared
• Local variable values are not saved from one sub
  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 only within the procedure
• But variable exists for lifetime of procedure

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Section 6.2Passing Arguments to a Sub Procedure

• When calling a procedure, you can pass it values
  known as arguments

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Arguments

• Argument a value passed to a procedure
• Weve already done this with functions
• Value CInt(txtInput.Text)
• Calls CInt function and passes txtInput.Text
• A Sub must be declared so it accepts an argument

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Passing Arguments By Value
DisplayValue(5) calls DisplayValue
procedure Sub DisplayValue(ByVal number As
Integer) ' This procedure displays a value in a
message box. MessageBox.Show(number.ToString) End
Sub

• Number declared as an integer argument
• Storage location number created
• A value, 5 in this case, must be supplied and is
  copied into the storage location for number
• DisplayValue then executes
• Tutorial 6-3 demonstrates passing arguments

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Passing Multiple Arguments
ShowSum(5, 10) calls ShowSum procedure Sub
ShowSum(ByVal num1 As Integer, ByVal num2 As
Integer) ' This procedure accepts two arguments,
and prints ' their sum on the form. Dim sum As
Integer sum num1 num2 MessageBox.Show("The
sum is " sum.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 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 wont see the changes
• Arguments can also be passed ByRef
• Procedure points to (references) arguments
  original storage location
• Any changes are made to the original value
• Calling procedure sees the changes
• Tutorial 6-4 demonstrates this difference

15
Section 6.3Function Procedures

• A Function Procedure Returns a Value to the Part
  of the Program That Called the Function Procedure

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Declaring a Function Procedure
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
total Sum(value1, value2) Function Sum(ByVal
num1 As Single, ByVal num2 As Single) _ As
Single Dim result As Single result num1
num2 Return result End Function

• value1 and value2 must be data type Single
• Total must be declared as data type Single
• Tutorial 6-5 demonstrates function use

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Returning Nonnumeric Values
Function FullName(ByVal first As String, ByVal
last As String)_ As String Dim name As
String name last ", " first Return
name End Function
Function IsValid(num As Integer) As Boolean Dim
status As Boolean If num gt 0 And num lt 100
Then status True Else status False End
If Return status End Function
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Section 6.4More About Debugging

• Step Into
• Step Over
• Step Out

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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

21
Section 6.5Building the Bagel and Coffee Price
Calculator Application

• Use Sub procedures and functions to calculate the
  total of a customer order.