← Years 7–8 Creating Digital Solutions

Decomposition diagrams

A tree with the whole problem at the top and boxes below for its smaller sub-problems. Each level adds detail, and you stop splitting when a part is simple.

Part of: 7-8 Digital Technologies: Defining and decomposing problems — Module 7, IGCSE Computer Science 0478 — 7b Decomposition and Sub-systems — Module 20, QCE Digital Solutions Unit 1 — Understanding Digital Problems — Module 8

Understanding Decomposition diagrams in Years 7–8 Creating Digital Solutions

What it does: Action phrases begin with a verb such as Book, so the box says what job must be done. Splitting a box such as Make a poster into smaller boxes adds another level of detail to the tree. A decomposition diagram is a tree. The whole problem sits at the top, and the smaller sub-problems branch out below it. 'Build the whole app' is big and has many parts, so split it. 'Play one song when tapped' is already simple. You stop decomposing once a part is simple. Splitting further just adds clutter and makes the diagram harder to read.

Common mistakes: Boxes in a decomposition diagram should be labelled with action phrases. 'Questions' is just a noun. It should say what to do, like 'Write the questions'. The top of the tree must be the whole problem. 'Buy water bottles' is only one small part. It should be 'Plan a sports day'.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 3–47) and word search (Modules 3–125).

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Learning
Duo
Where does the whole problem go in a decomposition diagram?
In the box at the topIn a box at the bottom
Matches
Box label: Music
↓
The label is just a noun, not an action phrase. It should say what to do, such as Choose the music.
Applying
Meanings
Term
decomposition diagram
Meaning
A tree with the whole problem at the top and boxes below for its smaller sub-problems. Each level adds detail, and you stop splitting when a part is simple.
Practice
Build a canteen ordering app
Testing
Gaps
Level : Make a quiz app
Check Answer
Spots
1L0: Make a quiz game
2 L1: Questions

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Quick Questions

1. Where does the whole problem go in a decomposition diagram?
A. In the box at the top ✓
B. In a box at the bottom
A decomposition diagram is a tree. The whole problem sits at the top, and the smaller sub-problems branch out below it.
2. Which box label is better for a school canteen app diagram?
A. Show the menu ✓
B. Menu
Boxes are labelled with action phrases. 'Show the menu' tells you what the part must do, but 'Menu' is just a thing.
3. What do the boxes on the level under the top box show?
A. The final answer to the whole problem
B. The smaller sub-problems that make up the whole problem ✓
Each box below holds a sub-problem. Solving all the sub-problems together solves the whole problem.
4. What does each new level down in the diagram give you?
A. More detail about the problem ✓
B. A completely different problem
Each level splits a box into smaller parts, so you see more detail. The parts are still all about the same whole problem.
5. A box says 'Check the password is correct'. This is simple enough to do in one step. What should you do?
A. Keep splitting it into tiny steps forever
B. Stop splitting this box ✓
You stop splitting when a part is simple. Extra boxes would only make the diagram messy.

Match the Pairs

Plan a school disco → The whole problem, written in the single box at the top of the diagram
Choose music | Set up speakers → Two sub-problem boxes on the level below the top box
Label a box with an action phrase that starts with a verb → Book the hall
Level 2 adds more detail by splitting one box into smaller parts → Make a poster -> Draw art | Add date and time
Box label: Music → The label is just a noun, not an action phrase. It should say what to do, such as Choose the music.
Keep splitting: Press play -> Move finger -> Bend knuckle → The splitting did not stop when the part was simple. Press play is already easy enough to do, so it needs no more boxes.

Key Term

decomposition diagram
A tree with the whole problem at the top and boxes below for its smaller sub-problems. Each level adds detail, and you stop splitting when a part is simple.

Fill in the Blanks

1. Level 1: Make a quiz app Level 2: Write questions | Show score
Hint: Type the level number for the whole problem at the very top of the tree.
2. Box label = verb + noun, e.g. Book hall
Hint: Type the kind of word an action phrase starts with.
3. Stop splitting when a part is simple
Hint: Type the word that describes a part that does not need splitting any more.
4. Level 2: Plan food Level 3: Buy drinks | Buy snacks
Hint: Type the number of the level that sits directly below level 2.
5. Sub-problems go below the whole problem
Hint: Type the word that says where sub-problem boxes sit compared with the whole problem.

Spot the Error

Question 1
A L0: Make a quiz game
B L1: Questions
C L1: Check the answers
D L1: Show the score
Line B — Boxes in a decomposition diagram should be labelled with action phrases. 'Questions' is just a noun. It should say what to do, like 'Write the questions'.
Question 2
A L0: Buy water bottles
B L1: Choose the events
C L1: Book the oval
D L1: Tell the students
Line A — The top of the tree must be the whole problem. 'Buy water bottles' is only one small part. It should be 'Plan a sports day'.
Question 3
A L0: Build a school website
B L0: Design the layout
C L1: Write the page text
D L1: Test the links
Line B — Only the whole problem is at level 0. A sub-problem directly below it should be level 1, so this line should start with L1.
Question 4
A L0: Make a class poster
B L1: Collect the facts
C L1: Draw the picture
D L1: Pick up the red pen
Line D — Boxes on the same level should have a similar level of detail. 'Pick up the red pen' is a tiny step. It should be a level 1 part like 'Write the title'.
Question 5
A L0: Create a pet care app
B L1: Record feeding times
C L1: Bake a chocolate cake
D L1: Store pet photos
Line C — Every sub-problem must be a part of solving the whole problem. Baking a cake has nothing to do with a pet care app. It should be something like 'Show the vet details'.

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