← Years 7–8 Creating Digital Solutions

Decomposing a problem into sub-problems

Splitting a big problem into smaller sub-problems, such as input, process and output. Each part can be solved on its own, then the solutions are combined.

Part of: 7-8 Digital Technologies: Defining and decomposing problems — Module 6, IGCSE Computer Science 0478 — 7a Program Development Life Cycle — Module 5, IGCSE Computer Science 0478 — 7b Decomposition and Sub-systems — Module 5, ACARA Digital Technologies Years 9–10 — Modular and Object-Oriented Programming (JavaScript) — Module 8, QCE Digital Solutions Unit 1 — Understanding Digital Problems — Module 6, QCE Digital Solutions Unit 1 — Algorithms and Programming Techniques — Module 60, QCE Digital Solutions Unit 1 — Programmed Solutions — Module 4, QCE Digital Solutions Unit 2 — Data-Driven Problems and Solution Requirements — Module 21, QCE Digital Solutions Unit 3 — Real-World Problems and Solution Requirements — Module 4, QCE Digital Solutions Unit 4 — Complex Digital Data Exchange Problems and Solution Requirements — Module 95, WA Computer Science ATAR Year 11 — Pseudocode and Algorithm Design — Module 9, WA Computer Science ATAR Year 11 — Modular Programming — Module 1

Understanding Decomposing a problem into sub-problems in Years 7–8 Creating Digital Solutions

What it does: Neither part needs the other to be finished first, so they can be solved on their own and at the same time. Combining the small solutions gives a complete solution to the original big problem. Decomposition means breaking a large problem into smaller parts that are easier to solve. Input is the data that goes into the system. Showing a message is the output. Output is the result shown to the user. Counting the steps is the process. Independent sub-problems can be solved separately. The email depends on the form data, so it is not independent. Each part is solved on its own, then the solutions are combined to solve the whole problem.

Common mistakes: The first step is to split the big problem into smaller parts, so the line should start with DECOMPOSE. COMBINE is what you do later, once the parts are solved. Getting the city name is an input sub-problem, not an output. The output sub-problem should show the weather to the user.

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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

Which best describes decomposition?

Making a problem bigger so it is harder to solveSplitting a big problem into smaller sub-problems

Matches

Decompose a quiz app: 1. Make the whole app at once

↓

This does not split the problem into smaller parts. It should be split into sub-problems like input (answers), process (check score) and output (show result).

Applying

Meanings

Term

decomposition

Meaning

Splitting a big problem into smaller sub-problems, such as input, process and output. Each part can be solved on its own, then the solutions are combined.

Practice

Problem: Make a lunch order app

Testing

Gaps

Sub-problem 1: the answers

Check Answer

Spots

1COMBINE Quiz App

2 INPUT: read the answer

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Quick Questions

1. Which best describes decomposition?

A. Splitting a big problem into smaller sub-problems ✓

B. Making a problem bigger so it is harder to solve

Decomposition means breaking a large problem into smaller parts that are easier to solve.

2. A class wants an app to track canteen orders. Which is a sensible first step?

A. Break it into sub-problems like taking orders, working out the total and showing the receipt ✓

B. Try to solve the whole app in one go without any plan

Splitting the app into smaller sub-problems makes each part easier to plan and solve.

3. In a quiz app, which sub-problem is the INPUT?

A. The student types in their answer ✓

B. The app shows 'Correct!' on the screen

Input is the data that goes into the system. Showing a message is the output.

4. In a quiz app, which sub-problem is the PROCESS?

A. The student taps the answer button

B. Checking if the answer matches the right one ✓

Process is what the system does with the data, such as checking or calculating. Tapping is input.

5. In a step counter app, which sub-problem is the OUTPUT?

A. Counting each step from the sensor

B. Displaying the total steps on the screen ✓

Output is the result shown to the user. Counting the steps is the process.

Match the Pairs

decomposition → Splitting a big problem into smaller sub-problems that can each be solved on their own

input, process, output → Three kinds of sub-problems: getting data in, working with it, and showing the result

Sub-problems that are independent can be solved separately by different people → Sub-problem A: design the quiz questions Sub-problem B: design the score screen

After solving each sub-problem, combine the solutions to solve the whole problem → Combine: menu + order form + receipt = canteen ordering app

Decompose a quiz app: 1. Make the whole app at once → This does not split the problem into smaller parts. It should be split into sub-problems like input (answers), process (check score) and output (show result).

Sub-problems: Input = the score is shown on screen → Showing the score is an output sub-problem, not an input. Input is getting the player's answer.

Key Term

decomposition

Splitting a big problem into smaller sub-problems, such as input, process and output. Each part can be solved on its own, then the solutions are combined.

Fill in the Blanks

1. Problem: Make a quiz score app Sub-problem 1: Input the answers Sub-problem 2: Process the score

Hint: Type the first part of input, process, output: the step where data goes into the system.

2. Sub-problem 1: Input the answers Sub-problem 2: Process the score Sub-problem 3: Output the result

Hint: Type the middle step, where the system works on the data it was given.

3. Input: type in the quiz answers Process: add up the correct answers Output: show the final score

Hint: Type the name of the step that shows results to the user.

4. Problem: Plan a class party Decompose into: food, music, invites Solve each part on its own

Hint: Type the word for splitting a big problem into smaller sub-problems.

5. Sub-problem A: solved on its own Sub-problem B: solved on its own solution = combine(A, B)

Hint: Type the function name that joins the separate solutions into one.

Spot the Error

Question 1

A COMBINE Quiz App

B INPUT: read the answer

C PROCESS: check the answer

D OUTPUT: show the score

Line A — The first step is to split the big problem into smaller parts, so the line should start with DECOMPOSE. COMBINE is what you do later, once the parts are solved.

Question 2

A DECOMPOSE Weather App

B INPUT: get the city name

C PROCESS: find the forecast

D OUTPUT: get the city name

E END

Line D — Getting the city name is an input sub-problem, not an output. The output sub-problem should show the weather to the user.

Question 3

A DECOMPOSE Canteen Orders

B PROCESS: read the order

C PROCESS: add up the cost

D OUTPUT: print the receipt

Line B — Reading the order is an input sub-problem. The process sub-problem should do the work on the data, such as adding up the cost.

Question 4

A SUB A: design the board

B SUB B: write the rules

C A must wait for B to finish

D Combine A and B at the end

Line C — Independent sub-problems can each be solved on their own. A should not need to wait for B, so the line should say A can be solved without B.

Question 5

A SUB A: count the votes

B SUB B: make the chart

C COMBINE A and B before solving

D Result: one working program

Line C — You combine the solutions after each sub-problem has been solved. The line should say COMBINE A and B at the end.

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