Further programming

Paper 4 · Unit 20. Programming paradigms, OOP, Prolog, random files and exception handling.

1. Programming paradigms

Paradigm	Description
Low-level	Assembly or machine code, close to the hardware; uses addressing modes (immediate, direct, indirect, indexed, relative)
Imperative (procedural)	A sequence of statements that change the program state, organised into procedures and functions
Object-oriented	Programs are built from objects that combine data (attributes) and behaviour (methods)
Declarative	States facts and rules; the system works out the answer to a query (e.g. Prolog)

2. Object-oriented programming

Term	Meaning
Class	A blueprint defining attributes and methods
Object	An instance of a class
Constructor	A method that runs when an object is created, setting initial values
Encapsulation	Keeping attributes private and accessing them through methods (getters and setters)
Inheritance	A subclass takes on the attributes and methods of a superclass and can add its own
Polymorphism	A method with the same name behaves differently in different classes (overriding)
Containment / aggregation	An object contains other objects as attributes

CLASS Pet
   PRIVATE Name : STRING
   PUBLIC PROCEDURE NEW(GivenName : STRING)
      Name ← GivenName
   ENDPROCEDURE
   PUBLIC FUNCTION GetName() RETURNS STRING
      RETURN Name
   ENDFUNCTION
   PUBLIC FUNCTION Speak() RETURNS STRING
      RETURN "..."
   ENDFUNCTION
ENDCLASS

CLASS Dog INHERITS Pet
   PUBLIC FUNCTION Speak() RETURNS STRING   // overrides
      RETURN "Woof"
   ENDFUNCTION
ENDCLASS

MyDog ← NEW Dog("Rex")

3. Declarative programming

In Prolog, facts state what is true, rules define relationships, and queries ask the system to find solutions by matching (backtracking through the facts).

parent(ali, sara).
parent(sara, omar).
grandparent(X, Z) :- parent(X, Y), parent(Y, Z).

?- grandparent(ali, Who).
Who = omar

4. File processing and exceptions

Random (direct-access) files store records at calculated positions:

OPENFILE "students.dat" FOR RANDOM
SEEK "students.dat", Address
GETRECORD "students.dat", Student
PUTRECORD "students.dat", Student
CLOSEFILE "students.dat"

An exception is an unexpected event during execution (e.g. file not found, division by zero, invalid input). Exception handling catches it so the program can recover or end cleanly instead of crashing:

TRY
   OPENFILE "data.txt" FOR READ
EXCEPT
   OUTPUT "File not found"
ENDTRY

5. Examiner tips

1. Encapsulation: mention private attributes and public methods.
2. Inheritance vs polymorphism: reusing vs overriding.
3. Prolog: facts end with a full stop; variables start with a capital letter.
4. Random files use SEEK, GETRECORD and PUTRECORD.
5. Exception handling prevents crashes and gives useful messages.

6. Practice questions

7. Topic test: predict your grade