KS3 · Computer Science

Decomposition: splitting problems up

"Plan a day at the Eiffel Tower." Where do you even start? Try "work out how you'll get there" instead. Easier, right? You just made the problem smaller. That's decomposition.

Computer Science · Structure

A day at the Eiffel Tower, split up

Tap any box. Main problem at the top, smaller subproblems underneath.

Tap any part of the diagram to see what it does.

Computer Science · Decomposition

Now you do the splitting

A game's player actions are one big problem. Pick an action, then pick the group it belongs to.

Still to sort

Move (0)

Actions about getting around

Item (0)

Actions about things the player has

Spells (0)

Actions about magic

8 of 8 still to sort.

Computer Science · Decomposition

Decompose a lunch order app

Which subproblem does the lunch order app need first, second and last?

1 · Needed first3 · Needed last

  1. Process the input (make the decision)

  2. Give the price back to the user

  3. Get the user's input

Computer Science · Decomposition

Your turn: a homework app

The Three W's come BEFORE you decompose, because you should understand a problem before you use computational thinking on it. They are three questions: what does the solution need to do? Why does the problem need solving? Who will use the solution?

A homework app needs planning. First answer the Three W's for it. Then write down the subproblems you would split it into. [6 marks]

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

Computer Science · Decomposition

What does decomposition actually do?

Four people are arguing about decomposition after the lesson.

Which one is closest to what you think? Pick one, say how sure you are, then check.
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How one overwhelming job turns into a set of small ones you can actually solve.

What you need to know

  • Decomposition breaks a problem into smaller, more manageable subproblems, a bit like doing a jigsaw piece by piece.
  • A subproblem can be decomposed again, as many times as needed, until everything is easy to solve.
  • A structure diagram shows it: the main problem at the top, branches leading down to the subproblems.
  • Understand the problem first with the Three W's. After decomposing, abstraction and algorithmic thinking take over.

The big picture

Decomposition means breaking a big problem into smaller, more manageable subproblems, and splitting a subproblem again if it still feels too big. A structure diagram shows the result with the main problem at the top. Decomposition is one computational thinking technique, alongside abstraction and algorithmic thinking, and it comes first: it splits the problem but does not solve it.

Key points

1Computational thinking is a structured approach to solving problems, and decomposition is one of its techniques, alongside abstraction and algorithmic thinking.
2A subproblem is a smaller part of the main problem, and it can have subproblems of its own.
3A structure diagram gives a clear overview of the main problem and the subproblems that need solving.
4Decomposition works on trips, game actions and programming projects like apps, with or without digital technology.
5Decomposition only splits the problem. Abstraction then focuses on the important details of each subproblem, and algorithmic thinking creates the step-by-step instructions.

Worked example

Problem

Decompose a lunch order app into subproblems. Then say what you have done and what you haven't done yet.

⚠ Watch out

Thinking that once you've decomposed a problem, you've solved it, or that decomposition is the same as writing the algorithm. It only splits the problem into subproblems. Abstraction and algorithmic thinking come after.

🧠

Memory hook

Think jigsaw: you never solve it in one go, you solve it piece by piece. Big problem on top, smaller problems underneath.

✓

Check yourself

Picture the Eiffel Tower diagram. Say out loud what sits at the top, what sits on the next layer, and what you'd do if a box still felt too big.

Flashcards

(14)
What is decomposition?
Breaking a problem down into smaller, more manageable subproblems.
What is a subproblem?
A smaller part of the main problem. It can be decomposed again into subproblems of its own.
How many times can you decompose a subproblem?
As many times as needed, until the problems are manageable and easy to solve.
What does a structure diagram show?
A decomposed problem: a clear overview of the main problem and the subproblems that need to be solved.
Which box sits at the top of a structure diagram?
The main problem. Branches lead down from it to the subproblems.
Name the three computational thinking techniques in this lesson.
Decomposition, abstraction and algorithmic thinking.
What is abstraction used for after decomposition?
Focusing on the important details of each smaller subproblem and ignoring irrelevant details.
What is algorithmic thinking?
Creating a series of logical, step-by-step instructions (an algorithm) to solve the problem.
What are the Three W's?
What the solution needs to do, why the problem needs to be solved, and who will be using the solution.
When do the Three W's come in?
Before you use computational thinking: a problem should be understood first.
How does the trip to the Eiffel Tower decompose on the first layer?
Into transport, tickets and food.
How do a game's player actions decompose?
Into move (forward, turn left, turn right), item (pick up, use, drop) and spells (cast, learn).
How can a homework app be decomposed?
Into adding homework, viewing the list of homework tasks and ticking homework off once done.
Does decomposition need digital technology?
No. The principles of computational thinking can be applied in various aspects of life, even without any digital technology.

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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