GCSE · Computer Science · AQA · Spec 8525

Decomposition

Nobody writes a whole game in one go. The trick: stop asking 'where do I start?', ask 'what jobs does this need doing?', and give each job its own piece.

Computer Science · Decomposition

Break down a quiz game

The whole problem: make a quiz game. It has been broken into four smaller problems — sub-problems — and each one has a job. Pick a small task, then put it under the sub-problem whose job it helps to do.

Still to sort

Ask a question (0)

Its job: put a question in front of the player and collect their answer.

Mark the answer (0)

Its job: decide whether the player's answer is right.

Update the score (0)

Its job: keep an accurate count of right answers.

Where the line is: Marking makes the decision. Updating the score records it. Two different jobs, so two different sub-problems.

Show the result (0)

Its job: tell the player how they did at the end.

Where the line is: Asking a question and showing the result both use the screen. Asking gets an answer from the player; showing the result reports how the game went.

8 of 8 still to sort.

This is one sensible way to break the quiz game down, not the only one. What makes it work is that every column does one job you can name.

Predict, then check

Here is an earlier attempt at breaking down the same quiz game.

The earlier attempt split the quiz game into just three sub-problems: 'Ask a question', 'Deal with the answer' and 'Show the result'. Which one is still too big to count as one identifiable task?

Check your thinking

What does decomposition actually mean?

Three students were asked to say what decomposition means.

Which answer is closest to what you think right now?

It's writing down the steps of the solution in the order they happen.It's splitting a problem into a few pieces. Once it's split, you're done.It's breaking a problem into sub-problems that each do one identifiable task, and splitting any that are still too big.

How sure are you?

I'm sureI think soI'm guessing

It's writing down the steps of the solution in the order they happen.

This is a common misconception

This is the most common mix-up, so it's worth pinning down. A list of steps answers 'what do I do first, then next?'. Decomposition asks a different question: 'what jobs does this problem need doing?'. Each answer is a part of the problem — a sub-problem with its own job. In the quiz game, 'Show the result' is a part because it has a job you can name, not because of when it happens. Parts, not steps.

It's splitting a problem into a few pieces. Once it's split, you're done.

This is a common misconception

Half right. Splitting into pieces is the start, but it isn't always the end. A piece that is still too big gets split again — just as 'Deal with the answer' had to become 'Mark the answer' and 'Update the score'.

It's breaking a problem into sub-problems that each do one identifiable task, and splitting any that are still too big.

This matches the evidence

Yes — that's the whole idea. Sub-problems, each with a job you can name, and parts inside parts whenever a part is still too big to solve in one go.

Decomposition breaks a problem into parts, not a list of steps — and any part that is still too big becomes a problem to break down in its own right.

Your turn

Explain it on a new problem

A cinema wants a program for its ticket machine. Explain what is meant by decomposition, using the ticket machine program as your example. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Says decomposition means breaking a problem into a number of smaller sub-problems. (1 mark)
Says each sub-problem does an identifiable task (one job you can name). (1 mark)
Says a sub-problem can itself be broken down into smaller sub-problems. (1 mark)
Names at least two sensible sub-problems of the ticket machine, such as 'choose a film and time', 'take the payment' or 'print the ticket'. (1 mark)

Model answer

Decomposition means breaking a problem into a number of smaller sub-problems, where each sub-problem does an identifiable task. For the ticket machine, the sub-problems could be 'choose a film and time', 'take the payment' and 'print the ticket'. If a sub-problem is still too big to solve in one go, it can be broken down further — for example, 'take the payment' could be split into 'work out the price' and 'collect the money'.

Three ideas carry the explanation — sub-problems, an identifiable task for each, and breaking down further. The example is where you prove you can use them.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Big problems get solved in pieces — and a piece that's still too big gets broken up again.

What you need to know

Decomposition means breaking a problem into a number of sub-problems.
Each sub-problem accomplishes an identifiable task: you can say what its one job is.
A sub-problem might itself be broken down further into smaller sub-problems.

The big picture

Decomposition means breaking a problem into a number of sub-problems, so that each sub-problem does an identifiable task: a job you can name. If a sub-problem is still too big, it can be broken down again into smaller sub-problems, so a decomposition can have parts inside parts. It is about the parts a problem needs, not the order in which things happen.

Key points

1Start by asking 'what jobs does this problem need doing?', not 'what do I do first?'.

2A small task belongs with the sub-problem whose job it helps to do, not with tasks that just look alike.

3Keep breaking down until every piece is one job you can name.

4There is no set number of sub-problems, and an example breakdown is one sensible way, not the only one.

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Worked example

Problem

Break down this problem: a program for a number-guessing game. The computer picks a secret number, and the player keeps guessing, getting a clue after each wrong guess, until they get it right.

⚠ Watch out

Writing a list of steps in the order they happen and calling it a decomposition. Decomposition breaks a problem into parts — sub-problems that each do a job — and any part can be broken down again.

🧠

Memory hook

Parts, not steps — and parts can have parts.

✓

Check yourself

Pick an app you use, such as a music player. Without looking back, name three sub-problems it could be broken into and the job each one does. Then break one of them down again.

Flashcards

(5)

What is decomposition?

Breaking a problem into a number of sub-problems, each of which accomplishes an identifiable task.

What does it mean for a sub-problem to have an identifiable task?

You can say what its one job is, in a few words.

A sub-problem is still too big to solve in one go. What do you do?

Break it down again into smaller sub-problems of its own. A decomposition can have parts inside parts.

Is a list of steps in the order they happen a decomposition?

No. Decomposition breaks a problem into parts that each do a job. It is not about the order things happen in.

Two tasks both put something on the screen. Must they go in the same sub-problem?

No. A task goes with the sub-problem whose job it helps to do, not with tasks that use the same action.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Break down a quiz game

What does decomposition actually mean?

Explain it on a new problem

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