3.2.2a Variables, Constants & Operators — AQA 8525

Variables

A variable is a named location in memory that stores a value that can change during program execution.

// Declaring and using variables
score ← 0          // starts at 0
score ← score + 10  // changed to 10
name ← "Alice"    // a string variable

The AQA assignment operator is ← (left-arrow). The right-hand side is evaluated first, then stored in the variable on the left.

Constants

A constant is a named value that cannot change once set. Constants make code more readable and easier to maintain — if a value needs updating, you only change it in one place.

CONSTANT PI ← 3.14159
CONSTANT MAX_SCORE ← 100
CONSTANT GREETING ← "Hello"

Feature	Variable	Constant
Value can change?	Yes	No
Naming convention	camelCase or lowercase	UPPER_CASE
Example	score ← 0	CONSTANT MAX ← 100

Arithmetic Operators

Operator	Meaning	Example	Result
+	Addition	5 + 3	8
-	Subtraction	10 - 4	6
*	Multiplication	3 * 4	12
/	Division (real)	7 / 2	3.5
DIV	Integer division (floor)	7 DIV 2	3
MOD	Remainder (modulus)	7 MOD 2	1
^	Exponentiation (power)	2 ^ 3	8

DIV and MOD in Practice

// Convert 150 minutes to hours and minutes
hours ← 150 DIV 60    // 2
mins  ← 150 MOD 60    // 30
OUTPUT hours " hours " mins " minutes"

MOD is also useful for checking if a number is even (n MOD 2 = 0) or for cycling through array indices.

Comparison Operators

Operator	Meaning	Example	Result
==	Equal to	5 == 5	True
!=	Not equal to	5 != 3	True
<	Less than	3 < 7	True
>	Greater than	7 > 3	True
<=	Less than or equal to	5 <= 5	True
>=	Greater than or equal to	4 >= 6	False

INPUT and OUTPUT

name ← INPUT("Enter your name: ")
age  ← int(INPUT("Enter your age: "))
OUTPUT "Hello " + name
OUTPUT "You are " + str(age) + " years old"

Exam tip: AQA always uses ← for assignment (never =). DIV and MOD are AQA keywords — always capitalise them. Many marks are lost by writing % for MOD.

⚠️ Common Mistakes

Using = instead of ← for assignment in pseudo-code
Writing % for modulus — AQA always uses MOD
Forgetting that / gives a real result while DIV gives an integer
Using the same name for a constant and a variable

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Video coming soon

Key points

Difference between variables and constants
The ← assignment operator in AQA pseudo-code
DIV vs / and MOD with worked examples
How INPUT and OUTPUT work in AQA

Click slide or press arrow keys to navigate

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Worksheet — 3.2.2a

8 questions · 21 marks · Mark schemes on submit

Q1What is the difference between a variable and a constant? Give one example of each.[3]

✅ Mark scheme

Mark scheme

Variable: value can change during execution [1]; Constant: value is set once and cannot change [1]; One example of each e.g. score ← 0 / CONSTANT MAX ← 100 [1].

Q2Calculate: (a) 17 DIV 5 (b) 17 MOD 5 (c) 2 ^ 4 (d) 13 MOD 4[4]

✅ Mark scheme

Mark scheme

(a) 3 [1]; (b) 2 [1]; (c) 16 [1]; (d) 1 [1].

Q3Write AQA pseudo-code to convert 500 seconds into minutes and seconds and output both values.[3]

✅ Mark scheme

Mark scheme

minutes ← 500 DIV 60 [1]; seconds ← 500 MOD 60 [1]; OUTPUT minutes and seconds [1].

Q4What does this code output? x ← 10, y ← 3, OUTPUT x DIV y, OUTPUT x MOD y[2]

✅ Mark scheme

Mark scheme

3 [1]; 1 [1].

Q5State the AQA assignment operator and explain why it differs from the equals sign (=).[2]

✅ Mark scheme

Mark scheme

← (left arrow) [1]; = means equality (comparison) whereas ← means "assign this value to the variable" [1].

Q6How can you check if a number n is even using MOD? Write the condition.[2]

✅ Mark scheme

Mark scheme

n MOD 2 == 0 [1]; if this is True, n is even [1].

Q7Write pseudo-code to ask the user for two numbers and output their sum, difference, and product.[3]

✅ Mark scheme

Mark scheme

a ← int(INPUT()); b ← int(INPUT()) [1]; OUTPUT a+b [1]; OUTPUT a-b, a*b [1].

Q8State two reasons why using constants is better than writing the value directly in code (magic numbers).[2]

✅ Mark scheme

Mark scheme

Any 2: makes code more readable/self-documenting [1]; easier to change — only one place to update [1]; prevents accidental change to the value [1].

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Variables

Constants

Arithmetic Operators

DIV and MOD in Practice

Comparison Operators

INPUT and OUTPUT

Key points

Worksheet — 3.2.2a

Mini Test — 3.2.2a