GCSE · Computer Science · Edexcel · Spec 1CP2

Abstraction

Try putting a whole football match into a computer: every blade of grass, every fan, every shout. Impossible, and pointless. The real skill is keeping only what your problem needs.

One situation · two problems

Same football match, two different programs

Here is one situation: a football match between two teams. Two programs are being built about it. The results app shows the final score and updates the league table. The match game lets you play the match yourself. For each detail below, tick the program that needs it: one, both, or neither.

A Needed by the results app
B Needed by the match game

The names of the two teams
How many goals each team has scored
Each team's points in the league table so far
Where each player and the ball are on the pitch
The size and markings of the pitch
The price of a ticket to watch
What the crowd was singing

From situation to program

Build the model, then the data

Example: a cinema wants a program that lets customers book seats for a film online. Go from the real cinema to the data the program needs. Fill in the missing decisions.

Situation: a cinema screen with rows of seats, showing a film. Problem: let customers book a seat online.
Ask what the problem needs: a customer must choose a free seat, for the right showing.
missing step

Which line is step 3?

Keep: the film's title and start time, so each booking is for the right showing.Keep: the film's plot, so customers know what they are booking.Leave out: the start time, as every showing of a film is the same.

Why bother?

Reason it through

Why does abstraction make a problem easier to solve?

Link 1 of 4

First link · your turn

You've left out every detail the problem doesn't need. What are you left with?

Locked — reveal the link above first

Don't mix these two up

DecompositionvsAbstraction

They're both ways of making a big problem manageable, which is exactly why people confuse them.

Focus

What it does to the problem

Decomposition

Breaks it into smaller sub-problems.

Abstraction

Removes the detail the problem doesn't need.

The insight

One splits, the other strips. If you remember one thing from this section, make it this.

The question you ask

Decomposition

What smaller problems is this made of?

Abstraction

What does this problem actually need?

What you end up with

Decomposition

Several sub-problems, each easier to tackle.

Abstraction

A simpler problem, or model, that keeps only what matters.

How they work together

Decomposition

Break the big problem into sub-problems first.

Abstraction

Then remove the unnecessary detail from the problem and from each sub-problem.

Check your thinking

What is abstraction, really?

Which of these is closest to what you think abstraction is right now?

Removing as much detail as you possibly can, so the problem is as simple as possible.Deciding, for the problem you're solving, which details to keep and which to leave out.Getting rid of the details that just don't matter, whatever the problem is.Breaking a big problem down into smaller parts.

How sure are you?

I'm sureI think soI'm guessing

Removing as much detail as you possibly can, so the problem is as simple as possible.

This is a common misconception

Close, but that goes too far. Strip out a detail the problem needs and the model stops working. The cinema program without its record of booked seats is simpler, and useless. The aim is to keep everything the problem needs and nothing it doesn't.

Deciding, for the problem you're solving, which details to keep and which to leave out.

This matches the evidence

Yes. It's a decision made for a particular problem: keep the relevant details, leave out the rest. Change the problem and the decision can change too.

Getting rid of the details that just don't matter, whatever the problem is.

This is a common misconception

There's no fixed list of details that 'don't matter'. Think back to the pitch size: pointless for the results app, essential for the game. Whether a detail matters depends on the problem.

Breaking a big problem down into smaller parts.

This is a common misconception

That's decomposition, a different technique. Decomposition splits a problem into sub-problems; abstraction removes the detail each one doesn't need. They're used together, but they're not the same thing.

Abstraction = keep what this problem needs, leave out what it doesn't. Not 'as simple as possible', and not 'splitting it up'.

Your turn

Abstract a new problem

The sports centre has 4 badminton courts, numbered 1 to 4. Courts can be booked for one hour at a time, between 9 am and 9 pm. Each court has a floor colour and a net. Each member has a membership number, a name, a favourite sport and a photo on their card.

A sports centre wants a program that lets its members book a badminton court for an hour. Using the details below, explain how abstraction would be applied when designing this program, and give one benefit of doing so. [4 marks]

Your answer

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

The mark scheme

Keeps at least one detail the booking needs, e.g. the court number, the booking time or the member's membership number. (1 mark)
Leaves out at least one detail the booking doesn't need, e.g. the floor colour, the net, the favourite sport or the photo. (1 mark)
Explains that each detail is kept or left out depending on whether booking a court needs it. (1 mark)
Gives a benefit, e.g. it reduces complexity, so the program is easier to design or quicker to write. (1 mark)

Model answer

Abstraction means deciding which details the booking problem needs. The program would keep the court number, the hour being booked and the member's membership number, so it knows which court is booked, when, and by whom. It would leave out the floor colour, the net, the member's favourite sport and their photo, because none of them is needed to book a court. This reduces complexity, so the program is easier to design and quicker to write.

Keep, leave out, say why (because this problem does or doesn't need it), then name the benefit.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The trick isn't keeping less. It's keeping exactly what this problem needs.

What you need to know

Abstraction is removing or hiding details that are not needed for the problem being solved, so attention stays on the details that matter.
Carrying out abstraction means deciding, for a particular problem, which details are relevant (keep them) and which are irrelevant (leave them out).
Relevance depends on purpose. The same real-world situation can be abstracted differently for different problems, keeping different details each time.
Abstraction builds a model: a simplified representation of part of the real world that keeps only the features needed, so a problem about it can be analysed and solved, including by a computer program.
When a situation is turned into a program, abstraction decides which data the program needs to store and process. Everything else is left out of the program.
Abstraction reduces complexity. A simpler model or problem is easier to understand, easier to design a solution for and quicker to program, and the solution is less likely to contain unnecessary work.
Abstraction is used alongside decomposition. Decomposition breaks a large problem into sub-problems; abstraction removes the unnecessary detail from the problem and each sub-problem, so each can be understood and solved.

The big picture

Abstraction means removing or hiding the details a problem doesn't need, so you can focus on the ones that matter. It's a decision: for this problem, what must be kept and what can be left out? Relevance depends on the problem, so one situation can be abstracted in different ways. What's left is a model, and in a program that model decides which data is stored and processed. Abstraction reduces complexity, and it works alongside decomposition, which breaks a problem into sub-problems.

Key points

1Abstraction: remove or hide the details the problem doesn't need.

2It's a decision: keep what's relevant, leave out what isn't.

3Different problem, different details kept, even for the same situation.

4The result is a model: a simplified representation that keeps only what's needed.

5For a program, the model decides what data is stored and processed.

6It reduces complexity: easier to understand, design and program.

7Decomposition splits; abstraction strips. Use both together.

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Worked example

Problem

A school library wants a program that lets students search for books and borrow them. Show how decomposition and abstraction could be used together on this problem.

⚠ Watch out

Leaving a detail out because it 'doesn't matter', without asking: doesn't matter to what? Judge every detail against the problem you've been given, not against how important it feels. The same detail can be useless in one question and essential in the next.

🧠

Memory hook

Keep what THIS problem needs. Leave out the rest. Change the problem, change what you keep.

✓

Check yourself

Think of a bus. Name one detail a timetable program needs that a seating-plan program doesn't, and one detail the other way round. Why do the two programs keep different details?

Flashcards

(13)

What is abstraction?

Removing or hiding the details that aren't needed for the problem being solved, so you can focus on the ones that matter.

What decision do you make when you carry out abstraction?

For this particular problem: which details are relevant (keep them) and which are irrelevant (leave them out).

Can the same situation be abstracted in more than one way?

Yes. Different problems need different details, so each abstraction keeps different ones.

Why is there no fixed list of 'irrelevant' details?

A detail that one problem doesn't need can be essential to another, like pitch size: useless to a results app, vital to a match game.

In abstraction, what is a model?

A simplified representation of part of the real world that keeps only the features needed for the problem.

What is a model used for?

To analyse and solve a problem about the real situation, including by a computer program.

When a situation becomes a program, what does abstraction decide?

Which data the program needs to store and process. Everything else is left out of the program.

What is the main benefit of abstraction?

It reduces complexity.

Give two ways a simpler model helps when writing a program.

The solution is easier to design, and it's quicker to program.

How does abstraction affect the finished solution?

It's less likely to contain unnecessary work, because it only handles what the problem needs.

Decomposition vs abstraction: what does each do?

Decomposition breaks a problem into smaller sub-problems. Abstraction removes the detail a problem doesn't need.

How are decomposition and abstraction used together?

Decompose the problem into sub-problems, then remove the unnecessary detail from the problem and each sub-problem, so each can be understood and solved.

Is the aim of abstraction to make a problem as simple as possible?

No. Remove a detail the problem needs and the model can't solve it. Keep everything needed, and nothing else.

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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Same football match, two different programs

Build the model, then the data

What is abstraction, really?

Abstract a new problem

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