1. Introduction to Visual Programming
XIV Lecture 4: Methods
Introduction to Visual Programming
Lecture 4 – Methods
XIV – Ian elcoate
Ian Elcoate - May 2007

2. Methods by Structure Two basic types of structured methods Sub Method
Can be used for any standalone block of code
Header uses keyword: Sub
Footer is always: End Sub
Function Method
Used when we know that the standalone block of code must calculate and return a result
Header uses keyword: Function
Footer is always: End Function
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

3. Methods by Generation Event Method Intrinsic Method (Built-in methods)
Created by double-clicking on a control at design-time
E.g. btnExit_Click
Structure: Usually a Sub Method (99.9%)
Into whose body we write relevant code
Intrinsic Method (Built-in methods)
Already created and defined as part of VB.NET
Structure: Can be either Sub or Function Methods
We do not code them, but use them by calling
E.g. CStr()
User Defined Methods (UD)
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

4. User Defined Methods User Defined Methods (UD) Sub Methods
XIV Lecture 4: Methods
User Defined Methods
User Defined Methods (UD)
Are created, coded and called by programmer
Structure: Can be either Sub or Function Methods
Sub Methods
Also known as Subs or Procedures
Self contained block of code
Function Methods
Simply known as Functions
Basically a Sub with a hole in it
i.e. should always returns a result
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -
Ian Elcoate - May 2007

5. User Defined Sub Method
XIV Lecture 4: Methods
User Defined Sub Method
Format:
Private Sub MethodName()
‘statements go here – in body
End Sub
Note:
The MethodName should start in Uppercase
Besides this, follow the variable naming rules / style
If header is correctly typed, VBN will auto complete the footer for you, after pressing the Enter key
Each word in Method name should start with a Uppercase letter
Method name should not be same as a keyword or start with an number
Where to declare method
Avoid using same identifier already used, e.g. control or variable name.
Type header and footer automatically follows
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -
Ian Elcoate - May 2007

6. User defined Function Method
XIV Lecture 4: Methods
User defined Function Method
Format:
Private Function MethodName() As Return-Type
‘Optional Statements . . .
Return Return-Value
End Function
Note:
Inside the function some return-Value is always returned, using the keyword Return
Thus the function header must specify the data-type of this Return-Value, i.e. the Return-Type e.g. As Integer
If you type the header correctly, VBN will auto complete the footer for you, after pressing the Enter key
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -
Ian Elcoate - May 2007

7. Sub and Function examples
Sub Method
Private Sub ShowCircleArea()
lblArea.Text = CStr(Math.PI * 10 * 10)
End Sub
Function Method
Private Function GetCircleArea() As Double
Return (Math.PI * 10 * 10)
Note the result of calculation is the return-value
Thus return-type is specified as Double
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

8. Calling Subs and Functions
Both Subs and Functions are called by name
A Sub Method call is a full statement in its own right
ShowCircleArea()
A Function Method call is an expression, i.e. it should be one part of a statement
Usually the RHS of an assignment statement
area = GetCircleArea()
lblArea.Text = CStr(GetCircleArea())
When using functions, the LHS should be the same data-type as the RHS
If not then remember to Convert the Function result
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

9. Parameters
Parameters are the means by which a method or function receives extra information
Parameters can be considered to be valve in the top of the code-block
The data will only be allowed to enter, if it satisfies certain conditions
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

10. Parameter List Parameters use memory-space, thus need to be defined
This is done by specifying a parameter list
Inside the round brackets of a function or method header
Format:
Private Sub MethodName(paramList)
Private Function FunctionName(paramList) As Return-Type
The paramList is simply a list of parameters
There can be none, one or more parameters
If there are no parameters then the round brackets are left empty
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

11. Defining Parameters
Within the Parameter list, each parameter need to be formally defined
On the most part this definition is very similar to variable declarations
Except we do not use the Dim (or Private) Keywords
Examples:
Private Sub Display(pBalance As Double)
Private Function Cube(pNum As Integer) As Integer
Parameter names are prefixed with a lowercase p
Besides this standard variable name rules apply
By default VBN will precede each parameter with the ByVal keyword
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

12. Arguments For any parameter specified by a method / function
XIV Lecture 4: Methods
Arguments
For any parameter specified by a method / function
The call must supply a corresponding argument
Arguments are specified within the round brackets of the method / function calls
The argument must be of same type as parameter
Arguments must be provided in correct order
Any value can be used as an argument
Variables, e.g. result = Cube(num)
Literal values, e.g. Display(7)
Expressions, e.g. Cube(CInt(txtNum.Text))
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -
Ian Elcoate - May 2007

13. ByVal and ByRef
Did you notice that we did not use the Dim keyword when declaring the parameter pRadius
Instead parameters require either
ByVal - Data Passed By Value
ByRef - Data Passed By Reference
If we use neither keyword then VBN assumes ByVal
I.e. the parameter is assigned the value of the argument
For a variable argument this means:
The value stored in the variable argument will be passed to the parameter
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

14. Passing By Reference (1)
A ByRef parameter is not passed the (value of the) data stored by a variable argument
Instead it is passed the memory address of the argument
This is why ByRef arguments must be variables
This means that the parameter and argument will both share the same memory address
Note: Memory Addresses can only store one item of data
Thus whatever is the value of the parameter at the end of the method
Will also be the new value of the variable argument
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

15. Passing By Reference (2)
ByRef is a difficult concept to appreciate and as such is used in very specific circumstances
When a method needs to return two or more results
Customised validation
Manipulating arrays
For the time being lets consider the first option
Functions return a result
But only one value can be returned
Making use of ByRef parameters allows for more than one value to be returned
Note: to avoid confusion I will use ByRef parameters with Sub Methods only
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

16. Example use of ByRef
A method to calculate and return the area and circumference of a circle
The radius is provided as a ByVal parameter
To return two results the area and circumference are provided as ByRef parameters
Private Sub CalcCircle(pRadius As Double,
ByRef pArea as Double, ByRef pCir As Double)
pArea = Math.PI * pRadius * pRadius
pCir = 2 * Math.Pi * pRadius
End Sub
Remember in the call, variable arguments must be used to match up with the ByRef parameters
CalcCircle(radius, area, circumference)
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -

17. Summary
Modular programming creates faster and more efficient applications
This requires blocks of code to be written as distinct methods
We have already used methods (i.e. Intrinsic and Event methods)
Now we will also use Sub Methods and Function Methods
We pass variable data to methods by means of an argument within the method call
To receive this data, matching parameters must be declared within header of method
Lecture 4 "Methods" - Ian Elcoate - based on work by Asher Rashid -