Variables & Constants — AQA GCSE CS (8525)

Spec & Goals 3 min

AQA Spec 3.2.2 · Variables and constants

By the end of this lesson you can:

Define the terms variable and constant.
Use assignment with ← to store a value.
Choose sensible identifier names for variables and constants.

Warm-Up 5 min

Last lesson we met the five data types. A program needs a place to keep those values — that place has a name.

Quick starter

In the line score ← 0, what does the arrow ← do?

Reveal the answer

It assigns — it stores the value 0 in a named store called score. We are about to give that store its proper name.

Key Concept — variables, constants and names 14 min

A variable is a named store whose value can change while the program runs.

A constant is a named value that cannot change once set. We declare it with CONSTANT.

Assignment

Assignment stores a value in a variable using the arrow ←. The name goes on the left, the value on the right.

AQA pseudo-code	What it means
`age ← 15`	Store `15` in the variable `age`.
`age ← age + 1`	Change `age` to one more than it was.
`CONSTANT sst ← 0.06`	Declare a constant `sst` fixed at `0.06`.

Why use a constant?

It makes the program clearer — sst reads better than a bare 0.06.
If the value ever changes, you edit it in one place.
It prevents accidental changes to a value that must stay fixed.

Sensible identifier names

An identifier is the name of a variable or constant. Good names are clear and follow the rules.

Describe the data: totalPrice, not x.
No spaces; start with a letter; usually no symbols.
Be consistent in style across the program.

Worked Example — a total with SST 12 min

Problem: read a price, add Malaysian SST at 6%, and output the total.

The tax rate never changes, so it is a constant. The price changes each run, so it is a variable.

AQA pseudo-code

CONSTANT sst ← 0.06
price ← STRING_TO_REAL(USERINPUT)
tax ← price * sst
total ← price + tax
OUTPUT total

The same algorithm in Python

SST = 0.06
price = float(input("Enter the price: "))
tax = price * SST
total = price + tax
print(total)

For a price of 100, the tax is 6.0 and the total is 106.0.

Try It Yourself 12 min

🟢 Easy

Goal: Write AQA pseudo-code that stores your name in a variable called userName and outputs it.

Hint: use assignment with ←, then OUTPUT.

🟡 Medium

Goal: Declare a constant pi ← 3.142, read a radius, then output the area of a circle (pi * radius * radius).

Hint: use CONSTANT for pi and a variable for the radius.

🔴 Stretch

Goal: A friend names a variable x for a shopping total. Suggest a better identifier and give one reason it is better.

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

Define[1 mark]

Define the term variable.

Mark scheme

A named store (in memory) whose value can change while the program runs (1).

Give[1 mark]

Give one reason a programmer uses a constant rather than a variable.

Mark scheme

Any one: the value cannot be changed by mistake / it makes code clearer / it is edited in one place (1).

State[1 mark]

State a suitable variable name to store the number of lives left in a game.

Mark scheme

Any clear, descriptive name, e.g. livesLeft / numLives (1).

Identify[1 mark]

Which one is a valid and sensible variable name?

A 2nd score B total price C highScore D x@1

Mark scheme

C — highScore (1). The others use a space, a leading digit or a symbol.

Recap & Key Terms 3 min

A variable is a named store whose value can change; a constant is a fixed named value. We store values with assignment (←) and name them with clear identifiers.

Variable: A named store in memory whose value can change while the program runs.
Constant: A named value, fixed when written, that cannot change while the program runs.
Assignment: Storing a value in a variable using the arrow ←.
Identifier: The name a programmer gives to a variable, constant or subroutine.

Homework 1 min

Task (≤ 15 min): Write, in AQA pseudo-code, a program that declares a constant for the price of one ticket (RM12), reads how many tickets are wanted, and outputs the total cost.

Model answer

CONSTANT ticketPrice ← 12
quantity ← STRING_TO_INT(USERINPUT)
total ← ticketPrice * quantity
OUTPUT total

Award marks for: a constant for the price (1), input cast to an integer (1), correct total output (1).