Programming Fundamentals

Programming Fundamentals — CIE A Level Computer Science 9618

1. Variables, Constants, and Data Types

Variable: Named memory location that stores a value which can change during execution.

Constant: Named value that cannot change during execution (e.g., PI = 3.14159).

Standard data types in 9618 pseudocode:

| Data Type | Description | Example |

|---|---|---|

| INTEGER | Whole numbers | 42, −7, 0 |

| REAL | Decimal numbers | 3.14, −0.5 |

| CHAR | Single character | 'A', '5', '!' |

| STRING | Sequence of characters | "Karachi", "A Level" |

| BOOLEAN | TRUE or FALSE | TRUE, FALSE |

Declaring variables:

~~~

DECLARE Name : STRING

DECLARE Age : INTEGER

DECLARE Score : REAL

DECLARE Passed : BOOLEAN

~~~

Type casting: Converting between types.

• INT(3.7) → 3 (truncates)
• STRING(42) → "42"
• REAL("3.14") → 3.14

2. Selection Statements

IF-THEN-ELSE:

~~~

IF Score >= 70 THEN

Grade ← 'A'

ELSEIF Score >= 60 THEN

Grade ← 'B'

ELSE

Grade ← 'C'

ENDIF

~~~

CASE-OF (switch):

~~~

CASE OF Day

1 : OUTPUT "Monday"

2 : OUTPUT "Tuesday"

...

7 : OUTPUT "Sunday"

OTHERWISE : OUTPUT "Invalid"

ENDCASE

~~~

3. Iteration (Loops)

FOR loop: Used when number of iterations is known.

~~~

FOR i ← 1 TO 10

OUTPUT i * i

NEXT i

~~~

WHILE loop: Tests condition before each iteration.

~~~

WHILE Total < 100 DO

Total ← Total + Next

ENDWHILE

~~~

REPEAT-UNTIL: Executes at least once; tests condition after.

~~~

REPEAT

INPUT Number

UNTIL Number > 0

~~~

Key difference: WHILE can execute zero times; REPEAT-UNTIL executes at least once.

4. Arrays

1D Array:

~~~

DECLARE Marks : ARRAY[1:30] OF INTEGER

Marks[1] ← 85

OUTPUT Marks[1]

~~~

2D Array (e.g., seating plan):

~~~

DECLARE Seats : ARRAY[1:10, 1:20] OF CHAR

Seats[3, 7] ← 'X'

~~~

Traversing an array:

~~~

FOR i ← 1 TO 30

Sum ← Sum + Marks[i]

NEXT i

Average ← Sum / 30

~~~

5. Procedures and Functions

Procedure: A named block of code that performs an action. No return value.

~~~

PROCEDURE PrintHeading(Title : STRING)

OUTPUT "=== " & Title & " ==="

ENDPROCEDURE

CALL PrintHeading("Results")

~~~

Function: Returns a single value.

~~~

FUNCTION Factorial(N : INTEGER) RETURNS INTEGER

IF N <= 1 THEN

RETURN 1

ELSE

RETURN N * Factorial(N - 1)

ENDIF

ENDFUNCTION

OUTPUT Factorial(5) // outputs 120

~~~

Parameters:

• Pass by value: Procedure receives a copy — changes don't affect original.
• Pass by reference: Procedure receives the original variable — changes persist.

6. Recursion

A function that calls itself. Every recursive function must have:

1. A base case (terminating condition — stops recursion)
2. A recursive case (calls itself with modified parameters moving toward base case)

Example — Factorial:

• f(0) = 1 (base case)
• f(n) = n × f(n−1) (recursive case)

f(4) = 4 × f(3) = 4 × 3 × f(2) = 4 × 3 × 2 × f(1) = 4 × 3 × 2 × 1 × f(0) = 24

Recursion vs iteration:

• Recursion: elegant, matches mathematical definition — but uses more memory (call stack grows)
• Iteration: more efficient in memory and speed — preferred for performance-critical code
• Stack overflow: too many recursive calls can exhaust the call stack

7. Object-Oriented Programming (OOP)

Class: A blueprint/template defining attributes (data) and methods (behaviour).

Object: An instance of a class with specific attribute values.

Four pillars of OOP:

1. Encapsulation: Bundling data and methods within a class; controlling access via PUBLIC/PRIVATE.

• PRIVATE attributes: only accessible within the class (via getter/setter methods)
• PUBLIC methods: accessible from outside

1. Inheritance: A subclass inherits attributes and methods from a superclass (parent class).

• Enables code reuse
• Subclass can add new attributes/methods or override inherited ones

1. Polymorphism: Same method name behaves differently in different classes.

• Method overriding: subclass provides different implementation of inherited method

1. Abstraction: Hiding complex implementation details; exposing only necessary interface.

Pseudocode example:

~~~

CLASS Animal

PRIVATE Name : STRING

PUBLIC PROCEDURE SetName(N : STRING)

Name ← N

ENDPROCEDURE

PUBLIC FUNCTION GetName() RETURNS STRING

RETURN Name

ENDFUNCTION

PUBLIC PROCEDURE Speak()

OUTPUT "..."

ENDPROCEDURE

ENDCLASS

CLASS Dog INHERITS Animal

PUBLIC PROCEDURE Speak() // method overriding

OUTPUT "Woof!"

ENDPROCEDURE

ENDCLASS

~~~

UML class diagram: Shows classes, attributes (+public, −private), methods, and relationships (inheritance arrow, association lines).