1. Programming paradigms
Section 3.5
Programming paradigms
Part 2.

2. Standard Programming Techniques
In modern programming, problems are broken into modules that can be programmed easily.
Each module is a solution to an individual problem and each module has to interface with other modules.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

3. Parameters, & local and global variables
A local variable:
This is a variable declared within a procedure or a function of a program and which cannot be accessed by code outside that procedure or function.
A Local variable can only be accessed by code within the procedure or the function where it is declared.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

4. Parameters, & local and global variables
Cont~d
A global variable
This is declared within the main program or within a public module of a program.
It can be accessed from anywhere within the source code of the program.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

5. Parameters, & local and global variables
Cont~d
A function uses PARAMETERS to pass values to the calling module.
Consider a VB6 program to calculate the perimeter of a rectangle.
Public Function PerimeterOfRectangle (X As Integer, Y As Integer) As Integer
X = 2 * X
Y = 2 * Y
PerimeterOfRectangle = X + Y
End Function
Formal parameters
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

6. Parameters, & local and global variables
Cont~d
When this function is called, it is given actual parameters. Values must be stated.
A=4
B=5
Perimeter = PerimeterOfRectangle(A, B)
Alternatively you can call the function by giving the values directly e.g.
Perimeter = PerimeterOfRectangle(4, 5)
Formal parameters
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

7. Passing parameters We have two different ways of passing parameters.
Passing by value
Passing by reference
When a parameter is passed by value,
Only the value of the parameter is passed to the procedure called.
Value of the parameter can be manipulated by the procedure called.
When the procedure called is terminated the new value is discarded and the value of the parameter in the calling procedure returns to the original value
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

8. Passing parameters When parameters are passed by reference,
The parameter is stored in the original location and only a pointer (a reference to the parameter’s memory address) is passed to the procedure called.
Any changes made to the reference passed to the procedure called, will remove the value of the parameter at the original location.
When the procedure called is terminated the new value is available to the calling procedure
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

9. Examples: Passing by value Passing by reference
Procedure1
Declare a
a = 5
Call Procedure2 (a)
Display a
EndProcedure1
Procedure2 (ByVal b)
b = b + 2
EndProcedure2
The output of the parameter will be the display of value 5
Passing by reference
Procedure1
Declare a
a = 5
Call Procedure2 (a)
Display a
EndProcedure1
Procedure2 (ByRef b)
b = b + 2
EndProcedure2
The output of the parameter will be the display of value 7
Both of these examples are coded in Visual basic
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

10. Examples: Global variable The output will be the display of value 7
Main
Declare a
a = 5
Call Procedure2
Display a
EndProcedure1
Procedure2 ( )
a = 7
EndProcedure2
The output will be the display of value 7
Local variable
Main
Declare a
a = 5
Call Procedure2
Display a
EndProcedure1
Procedure2 ( )
a = 7
EndProcedure2
The output will be the display of value 5
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi
Also coded in Visual basic

11. Stacks and Procedures
When a procedure or function is called, the computer needs to know where to return to when the function or procedure is completed. i.e. , the return address must be known.
Furthermore, functions and procedures may call other functions and procedures which means that several return addresses be stored and they must be retrieved in the right order.
This can be achieved by using a stack.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

12. How a stack can be used to handle procedure calling & parameter passing
The return address is placed on stack along with the values of parameters when a procedure is called. If that procedure in turn calls another procedure the returning addresses are placed on the stack in the order so that most recent return address is placed on the top of the stack.
As the procedure(s) execute the parameters are read off the stack
The return values will be passed to the statements by taking the addresses out of those statements out of the stack.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

13. Example: Consider the following Module 1 Module 2 End End End
Main Prog
Module 1
CALL MODULE 1
100
Module 2
CALL MODULE 2
130
End
End
End
In this example, the numbers represent memory addresses.
The addresses will be stored in the stack each time a function is called and will be removed from the stack each time a return instruction is executed.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

14. Example Contd STACK (Last in First Out) 130 100
When module 1 is called by the main program, the address 100 is pushed onto the stack
Then when module 1 calls module 2, the address 130 is also pushed
When module 2 returns a value to module1, the address is popped off the stack. (The pointer is now back at 100)
When the module1 returns the value to the main program, then the location 100 is popped and the pointer is put at NULL
STACK (Last in First Out)
130
100
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

15. Object Oriented Programming
Object oriented languages (like JAVA, Eiffel, Smalltalk, etc) have classes and derived classes, and use concepts of encapsulation, inheritance and polymorphism.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

16. Classes, sub classes and super classes
In the real world, you'll often find many individual objects all of the same kind. E.g. There may be thousands of other bicycles in existence, all of the same make and model. Each bicycle was built from the same set of blueprints and therefore contains the same components.
In object-oriented terms, we say that your bicycle is an instance of the class of objects known as bicycles.
Therefore
A class is the blueprint from which individual objects are created.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

17. Classes, sub classes and super classes
Classes can be derived from other classes. The derived class is called a subclass. The class from which it's derived is called the superclass. The following figure illustrates these two types of classes:
A subclass inherits its state and behavior from all of its ancestors.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

18. Encapsulation (data hiding)
Encapsulation means that the internal representation of an object is generally hidden from view outside of the object's definition.
Only the object's own methods can directly manipulate its fields.
Object
method
Object
DATA
method
method
An object cannot
manipulate the data directly.
Object
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

19. Take an example of a class called Rectangle
To calculate the area, we CAN’T do it like this
area : = myRectangle.width * myRectangle.length;
where myRectangle is an object of type Rectangle
Class Rectangle must provide a suitable method in order to calculate the area of myRectangle.
It can do this when an instance of the class is instantiated.
The class must be provided with the methods to calculate the length and width, which can then be used to calculate the area.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

20. Example Contd.
You can use the following methods to calculate the length and width
integer getWidth( ) {
getWidth := width;
}//end of getWidth method.
integer getLength( ) {
getLength := length;
}//end of getLength method.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

21. Example Contd.
myRectangle can now use these methods to get at the width and length. However, it cannot change their values.
To find the area we can write
myWidth := myRectangle.getWidth( );
myLength := myRectangle.getLength( );
area := myWidth * myLength;
To find the perimeter we can write
myPerimeter := 2 * (myWidth + myLength);
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

22. Inheritance
Object-oriented programming allows classes to inherit commonly used state and behavior from other classes.
It allows the re-use of code and the facility to extend the data and methods without affecting the original code.
Inheritance is the ability of a class to use the variables and methods of a class from which the new class is derived.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

23. Example of inheritance
Consider the class Person that has data about a person's name and address and the methods
outputData( ) that outputs the name and address,
getName( ) that returns the name and
getAddress( ) that returns the address.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

24. Example of inheritance Contd.
If we now want a class called Employee that requires the same data and methods as Person but also needs to store and output an employee's National Insurance Number (NIN).
We don’t need to rewrite the contents of Person class. We simply create a subclass called Employee which inherits the data and methods of class Person.
Person
Employee
Name
Address
NINumber
outputData()getName()
outputData()getNINumber()
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi

25. Polymorphism
The fact that when two or more classes that are inherited from a parent class, can implement an inherited method differently is called polymorphism.
From the previous example, both the Person class and the Employee class have the method called outputData().
If myPerson is an instance of Person class, we can use myperson.outputData()
To use the outputData() method of Employee class, we can say myEmp.outputData(). Where myEmp is an instance of the employee subclass.
VCN CIE COMPUTING 9691/3 ::: Compiled by Benjamin Muganzi