📘 CIE IGCSE Computer Science

💻 Programming Concepts – Detailed Notes

1️⃣ Variables and Constants

🔹 Variable

A variable is a named storage location in memory whose value can change during program execution.

✅ Declaration

You must:

• Choose a meaningful identifier
• Specify the data type
• Assign a value (optional at declaration in some languages)

Example (pseudocode):

DECLARE age : INTEGER
age ← 16

🧠 Exam Tip

• Always use meaningful names:
  • ❌ x
  • ✅ studentAge
• Variables must be declared before use.

🔹 Constant

A constant is a named value that does NOT change during program execution.

Example:

DECLARE PI : REAL
PI ← 3.142

Or:

CONSTANT MAX_MARK ← 100

🧠 Exam Tip

• Use constants for fixed values (tax rate, maximum size, etc.)
• Makes program easier to maintain.

2️⃣ Basic Data Types

You must know these:

Data Type	Description	Example
INTEGER	Whole numbers	5, -2
REAL	Decimal numbers	3.14, -1.5
CHAR	Single character	'A'
STRING	Multiple characters	"Hello"
BOOLEAN	True or False	TRUE

🔹 Examples

DECLARE score : INTEGER
DECLARE temperature : REAL
DECLARE initial : CHAR
DECLARE name : STRING
DECLARE isPassed : BOOLEAN

🧠 Common Exam Mistakes

• Using STRING instead of CHAR
• Storing decimals in INTEGER
• Confusing BOOLEAN with STRING

3️⃣ Input and Output

🔹 Input

Used to get data from the user.

INPUT name
INPUT age

🔹 Output

Used to display data.

OUTPUT "Welcome"
OUTPUT name

🧠 Exam Tip

• Always prompt clearly:

OUTPUT "Enter your age:"
INPUT age

4️⃣ Control Structures

These are the three fundamental programming constructs.

🔹 (a) Sequence

Instructions executed in order.

INPUT number
square ← number * number
OUTPUT square

Execution flows top → bottom.

🔹 (b) Selection

Used to make decisions.

1️⃣ IF Statement

IF mark >= 50 THEN
OUTPUT "Pass"
ELSE
OUTPUT "Fail"
ENDIF

2️⃣ CASE Statement

Used when there are many possible values.

CASE grade OF
"A": OUTPUT "Excellent"
"B": OUTPUT "Good"
"C": OUTPUT "Average"
OTHERWISE OUTPUT "Invalid"
ENDCASE

🧠 Use CASE when:

• Comparing ONE variable to MANY values.

🔹 (c) Iteration (Loops)

Used to repeat instructions.

1️⃣ Count-Controlled Loop

Repeats a fixed number of times.

FOR i ← 1 TO 5
OUTPUT i
NEXT i

2️⃣ Pre-condition Loop

Condition checked BEFORE loop runs.

WHILE number > 0 DO
INPUT number
ENDWHILE

If condition is false initially → loop never runs.

3️⃣ Post-condition Loop

Condition checked AFTER loop runs.

REPEAT
INPUT number
UNTIL number = 0

Runs at least once.

🧠 Exam Tip

Know the difference between:

• WHILE (pre-condition)
• REPEAT UNTIL (post-condition)

🔹 (d) Totalling and Counting

Very common in exams.

Counting Example

count ← 0
FOR i ← 1 TO 5
INPUT number
IF number > 10 THEN
count ← count + 1
ENDIF
NEXT i

Totalling Example

total ← 0
FOR i ← 1 TO 5
INPUT number
total ← total + number
NEXT i

🧠 Important Rule

• Initialize counters and totals to 0
• Update inside loop

🔹 (e) String Handling

Know these functions:

Function	Meaning
LENGTH(string)	Returns length
SUBSTRING(string, start, length)	Extracts part
UPPER(string)	Converts to uppercase
LOWER(string)	Converts to lowercase

Example

name ← "Austin"

OUTPUT LENGTH(name) // 6
OUTPUT SUBSTRING(name,1,3) // Aus
OUTPUT UPPER(name) // AUSTIN

⚠️ The first character position may be 0 or 1 depending on language.

🔹 (f) Operators

Arithmetic Operators

Operator	Meaning
+	Addition
-	Subtraction
*	Multiply
/	Divide
^	Power
MOD	Remainder
DIV	Integer division

Example:

10 DIV 3 = 3
10 MOD 3 = 1

Relational Operators

Operator	Meaning
=	Equal
<>	Not equal
<	Less than
>	Greater than
<=	Less or equal
>=	Greater or equal

Logical Operators

Operator	Meaning
AND	Both true
OR	At least one true
NOT	Opposite

Example:

IF age >= 16 AND citizen = TRUE THEN

5️⃣ Nested Statements

A statement inside another statement.

Example (nested IF):

IF age >= 16 THEN
IF hasID = TRUE THEN
OUTPUT "Allowed"
ENDIF
ENDIF

Nested loop example:

FOR i ← 1 TO 3
FOR j ← 1 TO 2
OUTPUT i, j
NEXT j
NEXT i

⚠️ Maximum 3 levels in exam.

6️⃣ Procedures and Functions

🔹 Procedure

A subprogram that performs a task.
Does NOT return a value.

🔹 Function

Returns a value.

🔹 Parameters

Values passed into procedures/functions.

Up to 3 parameters in exam.

🔹 Local vs Global Variables

Type	Meaning
Global	Accessible everywhere
Local	Only inside procedure/function

Example:

7️⃣ Library Routines

Built-in functions provided by language.

You must know:

• MOD
• DIV
• ROUND
• RANDOM

Example:

number ← RANDOM(1,10)
rounded ← ROUND(3.6)

8️⃣ Writing Maintainable Programs

Very important for high marks.

✅ Meaningful Identifiers

✅ Commenting

Use comments to explain logic:

Comment:

• Complex logic
• Procedures
• Important formulas

Do NOT:

• Comment obvious lines unnecessarily

✅ Use Procedures and Functions

Break large programs into smaller parts.

Benefits:

• Easier debugging
• Easier testing
• Easier modification
• Improves readability

⭐ Common Exam Question Types

1. Trace the code.
2. Complete missing code.
3. Write an IF or loop.
4. Identify data type.
5. Explain difference between loop types.
6. Identify errors.
7. Write a procedure/function.

🚨 Very Common Mistakes

❌ Forgetting to initialise total
❌ Using = instead of ←
❌ Infinite loops
❌ Wrong loop type
❌ Not using meaningful identifiers
❌ Mixing up MOD and DIV

🎯 Final Exam Advice

✔ Read question carefully
✔ Underline key words (count, total, repeat until, etc.)
✔ Use indentation properly
✔ Check loop conditions
✔ Check variable names are consistent