110-G1

Motivation:

Within a program, may have to performthe same computation over and overMany programs share the samecomputation (e.g. sorting)

To break a program into smaller pieces• easier to write• reusable for other problems

Procedure: a piece of code with a name

e.g.Private Sub btnValidate_Click ()

'the program is written hereEnd Sub

Procedures

110-G2

A Procedure can be a Function or a Sub

A Function gives back a valuee.g. IsNumeric is a function: gives back True

or False A Sub does not give any value back

e.g. btnClear_Click does not return any value

We can write our own Functions and Subs

Subs and Functions

110-G3

Functions versus Sub Procedures

• Sub Procedures– Can receive passed values (arguments)– Performs actions

• Functions– Can receive passed values (arguments)– Performs actions– Returns a value of a specific data type to the

procedure that called it originally

110-G4

Functions – Return Values

• To return a value to the calling procedure set up a return value

• The return value is placed by VB in a variable with the SAME name a as the Function's name

OR• Use the Return statement to return the value

110-G5

Examples

Public Sub DisplayTime() lblTime.Text = DateTime.Now.ToString()

End Sub

Public Function strGetTime() As String

Return DateTime.Now.ToString()

'strGetTime = DateTime.Now.ToString()

'is also OK

End Function

How are these two procedures different?

110-G6

Formal and actual parameters • Store in the formal parameters the input values

sent to the procedure, e.g. Public Sub PrintInfo(strName As String, intAge As Integer)

Console.WriteLine("My name is " & strName & _

". I am " & intAge & " years old.")

End Sub formal parameters

actual parameters actual parameters: what is sent to the procedure

PrintInfo("Sandra",26)

' prints My name is Sandra. I am 26 years old.

If no input, write ProcedureName()

110-G7

Rule for calling a method (1)• Actual and formal parameters must match– in number– in type– in order

'OK PrintInfo("Sandra",26)

'Oops! wrong orderPrintInfo(26,"Sandra")

'Boom! wrong number of argumentsPrintInfo(26)

'No way! 26.5 is not an integerPrintInfo("Sandra",26.5)

110-G8

Rule for calling a method (2)• Actual and formal parameters don't have to match by name,

e.g.

Public Sub PrintInfo(strName As String, _ intAge As Integer)

Console.WriteLine("My name is " & strName & _

". I am " & intAge & " years old.")

End Sub

' in some other procedure

Dim strSomeName As String = "David"Dim intSomeAge As Integer = 11PrintInfo(strSomeName, intSomeAge) 'OK

110-G9

Passing ByVal or ByRef

• ByVal (default)– Sends a copy, original cannot be altered

• ByRef– Sends a reference to the memory location

where the original is stored and therefore the original can be altered

110-G10

A Sub Example (1)Use a Sub when you don't need anything back

Example: The user inputs her name. The program displays a welcome message.

ExitWelcome

Enter your name

Delphine

Welcome to VB, Delphine!

Use a Sub to display the message

110-G11

Private Sub btnWelcome_Click( …) 'Call the Welcome Sub to display 'a welcome message Welcome (txtName.Text)End SubPrivate Sub Welcome(strName As String) 'Display a welcome message using the 'label lblMessage lblMessage. Text = "Welcome to VB, " _ & strName & "!" lblMessage.Visible = TrueEnd Sub

A Sub Example (2)

To use theSub:write its nameand give it its argument.

110-G12

A Function Example(1)Goal: Write a program to display if a year is a leap year or not.

blnLeapYear

How? : Use a function: Given the year, the function returns True if the year is aleap year and False otherwise.

The function returns a BooleanblnLeapYear(1999) is False blnLeapYear(1996) is True

ExitCheck

Is 1996 a leap year?

Yes!

110-G13

Private Function blnLeapYear(intYear As Integer) _As Boolean

'Check if intYear is a leap year If (intYear Mod 4 = 0 And intYear Mod 100 <> 0) _ Or (intYear Mod 400 = 0) Then Return True Else Return False 'blnLeapYear = False is OK as well End IfEnd Function

Function can be only accessed within the module

Function argument

Function type

Function Name

A Function Example (2)

Returnedvalue

Create a Function named blnLeapYear

110-G14

A Function Example (3)Using a function from inside a SubProcedure:Private Sub btnLeapYear_Click( ..)

'Get the year

Dim intYear As Integer

intYear = CInt(txtYear.Text)

'Use the function blnLeapYear

'to find out if the year is a leap year

If blnLeapYear(intYear) Then

lblYesNo.Text = "Yes!"

Else

lblYesNo.Text = "No!"

End If

'Display the answer

lblYesNo.Visible = True

End Sub

To use thefunction:write its nameand give it its argumentbetween ( )

110-G15

Order of execution

Private Sub cmdLeapYear_Click(…)

...

If blnLeapYear(intYear) Then

...

End Sub

1

Private Function blnLeapYear...

...

End Function

3

52

What is blnLeapYear ?

4

Send backTrue or False

Control Flow