Representing Algorithms — AQA GCSE CS (8525)

Spec & Goals 3 min

AQA Spec 3.1.1 · Representing algorithms

By the end of this lesson you can:

Explain what an algorithm is, and use decomposition and abstraction to plan one.
Read and write a simple algorithm in AQA pseudo-code.
Represent the same algorithm as a flowchart, using the correct symbols.

Warm-Up 5 min

This is your first algorithms lesson. Good news: you already follow algorithms every day — a recipe is one.

Quick starter

Aisyah wrote the steps for making teh tarik, but they are jumbled. Put them in a sensible order:

Pour the tea between two cups to make it frothy.
Add the condensed milk.
Brew the tea leaves in hot water.
Serve.

Reveal a sensible order

Brew the tea → add the condensed milk → pour between two cups → serve. The order matters — change it and the result is wrong. That is exactly what an algorithm cares about.

Key Concept — what an algorithm is 14 min

An algorithm is a precise sequence of steps that solves a problem or completes a task. The steps must be clear, in order, and finite — they come to an end.

Two thinking tools: decomposition and abstraction

Before writing an algorithm, we make the problem easier in two ways:

To build a quiz game you decompose it into "ask a question", "check the answer", and "keep the score". A map app uses abstraction — it hides the houses and trees and shows only the roads you need.

Two ways to represent an algorithm

Pseudo-code — structured, code-like English. AQA has its own pseudo-code that the exam uses.
Flowcharts — a diagram of boxes and arrows.

AQA pseudo-code — the basics

A few rules you will use all course:

Pseudo-code	What it means
`x ← 5`	Assign (store) the value 5 in `x`. The `←` is the assignment arrow.
`OUTPUT x`	Display the value of `x`.
`x ← USERINPUT`	Read a value typed by the user into `x`.
`IF … THEN … ELSE … ENDIF`	Make a decision (this is selection).

Flowchart symbols

Each shape has a fixed meaning — using the right one earns the mark.

The four flowchart symbols you must know. Arrows show the order the steps run in.

Worked Example — the larger of two numbers 12 min

Problem: input two numbers and output the larger one.

Decompose: (1) get two numbers, (2) compare them, (3) output the bigger. Abstract: it does not matter what the numbers represent — only which is larger.

AQA pseudo-code

num1 ← USERINPUT
num2 ← USERINPUT
IF num1 > num2 THEN
    OUTPUT num1
ELSE
    OUTPUT num2
ENDIF

The same algorithm drawn as a flowchart:

Inputs, then one decision that branches to two outputs, then stop. The pseudo-code and the flowchart describe the same algorithm.

The same algorithm in Python

num1 = int(input("Enter a number: "))
num2 = int(input("Enter a number: "))
if num1 > num2:
    print(num1)
else:
    print(num2)

Try It Yourself 12 min

🟢 Easy

Goal: Write AQA pseudo-code that inputs two numbers and outputs their total.

Hint: read two values with USERINPUT, add them with +, then OUTPUT the result.

🟡 Medium

Goal: Write pseudo-code that inputs an age and outputs 'Adult' if it is 18 or over, otherwise 'Minor'.

Hint: you need an IF … THEN … ELSE … ENDIF with the test age ≥ 18.

🔴 Stretch

Goal: Draw a flowchart for a program that inputs an exam mark and outputs 'Pass' if the mark is 50 or more, or 'Fail' otherwise. Use the correct symbol for each step.

📝 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 decomposition.

Mark scheme

Breaking a (larger) problem down into smaller sub-problems / tasks (1).

Describe[2 marks]

Describe the difference between an algorithm and a program.

Mark scheme

An algorithm is a sequence of steps / a plan to solve a problem (1).
A program is that algorithm implemented / written in a programming language (1).

State[1 mark]

State the name of the flowchart symbol used to represent a decision.

Mark scheme

Decision box / diamond / rhombus (1).

State[1 mark]

Look again at the worked-example pseudo-code. State the output when the user enters num1 = 6 and num2 = 9.

Mark scheme

9 (1) — because 6 > 9 is false, so the ELSE branch runs.

Recap & Key Terms 3 min

An algorithm is an ordered set of steps to solve a problem. We plan it with decomposition and abstraction, then represent it as pseudo-code or a flowchart. A program is the algorithm written in a real language.

Algorithm: A sequence of steps, in order, that solves a problem or carries out a task.
Decomposition: Breaking a problem into smaller, easier sub-problems.
Abstraction: Removing unnecessary detail to focus only on what matters.
Pseudo-code: A structured, code-like way of writing an algorithm (AQA has its own version for the exam).
Flowchart: A diagram of an algorithm using terminal, process, input/output and decision symbols joined by arrows.
Selection: Choosing between paths with a decision — written as IF … THEN … ELSE … ENDIF.

Homework 1 min

Task (≤ 15 min): Write, in AQA pseudo-code, an algorithm that inputs two numbers and outputs their average.

Model answer

num1 ← USERINPUT
num2 ← USERINPUT
average ← (num1 + num2) / 2
OUTPUT average

Award marks for: two inputs (1), a correct mean calculation with brackets (1), output the result (1).

Spec & Goals 3 min

AQA Spec 3.1.1 · Representing algorithms

By the end of this lesson you can:

Explain what an algorithm is, and use decomposition and abstraction to plan one.
Read and write a simple algorithm in AQA pseudo-code.
Represent the same algorithm as a flowchart, using the correct symbols.

Warm-Up 5 min

This is your first algorithms lesson. Good news: you already follow algorithms every day — a recipe is one.

Quick starter

Aisyah wrote the steps for making teh tarik, but they are jumbled. Put them in a sensible order:

Pour the tea between two cups to make it frothy.
Add the condensed milk.
Brew the tea leaves in hot water.
Serve.

Reveal a sensible order

Brew the tea → add the condensed milk → pour between two cups → serve. The order matters — change it and the result is wrong. That is exactly what an algorithm cares about.

Key Concept — what an algorithm is 14 min

An algorithm is a precise sequence of steps that solves a problem or completes a task. The steps must be clear, in order, and finite — they come to an end.

Two thinking tools: decomposition and abstraction

Before writing an algorithm, we make the problem easier in two ways:

Two ways to represent an algorithm

Pseudo-code — structured, code-like English. AQA has its own pseudo-code that the exam uses.
Flowcharts — a diagram of boxes and arrows.

AQA pseudo-code — the basics

A few rules you will use all course:

Pseudo-code	What it means
`x ← 5`	Assign (store) the value 5 in `x`. The `←` is the assignment arrow.
`OUTPUT x`	Display the value of `x`.
`x ← USERINPUT`	Read a value typed by the user into `x`.
`IF … THEN … ELSE … ENDIF`	Make a decision (this is selection).

Flowchart symbols

Each shape has a fixed meaning — using the right one earns the mark.

The four flowchart symbols you must know. Arrows show the order the steps run in.

Worked Example — the larger of two numbers 12 min

Problem: input two numbers and output the larger one.

Decompose: (1) get two numbers, (2) compare them, (3) output the bigger. Abstract: it does not matter what the numbers represent — only which is larger.

AQA pseudo-code

num1 ← USERINPUT
num2 ← USERINPUT
IF num1 > num2 THEN
    OUTPUT num1
ELSE
    OUTPUT num2
ENDIF

The same algorithm drawn as a flowchart:

Inputs, then one decision that branches to two outputs, then stop. The pseudo-code and the flowchart describe the same algorithm.

The same algorithm in Python

num1 = int(input("Enter a number: "))
num2 = int(input("Enter a number: "))
if num1 > num2:
    print(num1)
else:
    print(num2)

Try It Yourself 12 min

🟢 Easy

Goal: Write AQA pseudo-code that inputs two numbers and outputs their total.

Hint: read two values with USERINPUT, add them with +, then OUTPUT the result.

🟡 Medium

Goal: Write pseudo-code that inputs an age and outputs 'Adult' if it is 18 or over, otherwise 'Minor'.

Hint: you need an IF … THEN … ELSE … ENDIF with the test age ≥ 18.

🔴 Stretch

Goal: Draw a flowchart for a program that inputs an exam mark and outputs 'Pass' if the mark is 50 or more, or 'Fail' otherwise. Use the correct symbol for each step.

📝 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 decomposition.

Mark scheme

Breaking a (larger) problem down into smaller sub-problems / tasks (1).

Describe[2 marks]

Describe the difference between an algorithm and a program.

Mark scheme

An algorithm is a sequence of steps / a plan to solve a problem (1).
A program is that algorithm implemented / written in a programming language (1).

State[1 mark]

State the name of the flowchart symbol used to represent a decision.

Mark scheme

Decision box / diamond / rhombus (1).

State[1 mark]

Look again at the worked-example pseudo-code. State the output when the user enters num1 = 6 and num2 = 9.

Mark scheme

9 (1) — because 6 > 9 is false, so the ELSE branch runs.

Recap & Key Terms 3 min

Algorithm: A sequence of steps, in order, that solves a problem or carries out a task.
Decomposition: Breaking a problem into smaller, easier sub-problems.
Abstraction: Removing unnecessary detail to focus only on what matters.
Pseudo-code: A structured, code-like way of writing an algorithm (AQA has its own version for the exam).
Flowchart: A diagram of an algorithm using terminal, process, input/output and decision symbols joined by arrows.
Selection: Choosing between paths with a decision — written as IF … THEN … ELSE … ENDIF.

Homework 1 min

Task (≤ 15 min): Write, in AQA pseudo-code, an algorithm that inputs two numbers and outputs their average.

Model answer

num1 ← USERINPUT
num2 ← USERINPUT
average ← (num1 + num2) / 2
OUTPUT average

Award marks for: two inputs (1), a correct mean calculation with brackets (1), output the result (1).