Abstraction and decomposition

Step 1 Warm-up recall

From Module 5: comparing every pair among 4 items needs how many comparisons?

Your answer

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Step 2 Learning goal

By the end of this lesson you can:

• define abstraction and decomposition (Foundation);
• split a task into parts (Core);
• describe a part by its inputs and output (Core).

Step 3 A familiar example

You can drive a car without knowing how the engine works. You use the pedals and the wheel, and the rest is hidden. That hiding is abstraction.

Step 4 Clear explanation

Abstraction means describing something by what it does and hiding how it does it. Decomposition means splitting a large task into smaller parts, each of which can be understood and checked on its own.

The figure splits an invented receipt task into four parts: read the prices, add them up, add tax and print the receipt. Each part has clear inputs and a clear output, and the output of one is the input of the next. A good decomposition has parts that do one job each, and that can be described in a sentence without mentioning how they work inside. When a fault appears, you can then test the parts one at a time instead of searching the whole task. The entry on the algorithm shows the same idea applied to a step-by-step method.

Step 5 Worked example (4 marks)

Split the task “produce a monthly attendance report” into parts.

1. Part 1: collect the daily attendance records. (1 mark)
2. Part 2: count the days each person attended. (1 mark)
3. Part 3: lay out the counts as a report. (1 mark)
4. Each part has one job and its output feeds the next. (1 mark)

Step 6 Learner practice (instant-marked)

Step 7 Recap

• Abstraction hides how and shows what.
• Decomposition splits a task into parts.
• Each part has clear inputs and an output.

Step 8 Exit ticket (ungraded)

Split the task “send a reminder to every late borrower” into three parts.

Your answer

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Step 9 Go further (Extension)

Read the entry on the algorithm and find an everyday example of a method built from smaller methods.