Sub Procedures and Functions

1
Chapter 6

• Sub Procedures and Functions

2
Agenda

• Sub Procedures
• Passing Arguments
• Function Procedures
• Run through tutorials

3
Definitions

• A Sub procedure is a collection of statements
  that performs a task
• Example Event procedures
• A Function procedure is a collection of
  statements that performs a task and returns a
  value to the VB statement that executed it
• Example Val and IsNumeric
• A method is a procedure declared in a class
• Subs and Functions are methods

4
Sub Procedures

• Help to modularize the code
• Divide it up into a set of logical tasks
• Simplify the program
• Break up the code into pieces
• Use for repeated tasks
• Click events

5
Sample

• 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

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
6
Declaring a Sub Procedure

• AccessSpecifier Sub ProcedureName
  (ParameterList)
• Statement(s)
• End Sub
• Access Specifier Optional, establishes the
  accessibility of the sub
• Public vs. Private
• Procedure Name used to call the sub
• Parameter List a list of variables or values
  that are being passed into the sub

7
Issues with Variables

• Local Variable within the scope of the
  procedure in which it is declared
• Not saved from one procedure call to the next
• Values are reset
• Static Local Variable saves value of variable
  between procedure calls
• Static VariableName As DataType

8
Arguments

• Argument a value passed to a procedure
• Value CInt(txtInput.Text)
• The sub must be declared to accept an argument
• Example

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
9
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

10
ByVal vs. ByRef

• ByVal
• New variable is created and assigned the value of
  the passing argument
• Changes are made to the copy
• ByRef
• Procedure points to the original variable
• Changes are made to the orginal

11
Functions
AccessSpecifier Function FunctionName
(ParameterList) _ As DataType Statements E
nd Function
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
12
Functions
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(ByVal 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