Representational Abstraction, Generalisation and Information Hiding

Overview

Recall questions on abstraction in computing, representational abstraction, abstraction by generalisation and its 'is a kind of' hierarchy, and information hiding with its benefits.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is abstraction, as the term is used in computing?

   Model answer

   Abstraction is the process of building a simplified model of something real by keeping only the detail that matters for the problem in hand and setting aside the rest. Computer science builds clean abstract models of messy, noisy real-world objects and phenomena, so the computer scientist must decide what to include and what to discard — the minimum detail needed to solve the given problem to the accuracy required.

2. 2

   Define representational abstraction.

   Model answer

   Representational abstraction strips away whatever detail is irrelevant to the purpose in hand, and what survives that pruning is the representation. A tube map is the standard illustration: true distances, the curves of the track and everything at street level are all discarded, because a passenger needs only the order of the stations and the points where lines meet.

3. 3

   Give a worked example of representational abstraction and say exactly what was removed and what was kept.

   Model answer

   An underground railway map is a representational abstraction of the rail network. What is kept is the order of the stations on each line, which lines they are on and where lines meet, because that is all a passenger needs to plan a journey. What is removed is the true geographical position of each station, the real distances between them, the curves of the track and everything above ground. The result is a diagram that is geographically wrong but far easier to use for its purpose. A second example: a satnav stores a road network as junctions joined by roads with a travel cost, discarding road width, surface and scenery.

4. 4

   Define abstraction by generalisation or categorisation.

   Model answer

   Here things are sorted into groups according to the characteristics they have in common, and those groups nest inside wider groups, producing a hierarchy of the 'is a kind of' sort. A saloon is a kind of car, a car is a kind of road vehicle: each level upwards retains only the properties shared by everything beneath it, and each level downwards adds detail.

5. 5

   Give a worked example of abstraction by generalisation and show the 'is a kind of' hierarchy.

   Model answer

   Take saloons, hatchbacks, motorbikes and lorries. Saloons and hatchbacks share four wheels, an engine, seats and a steering wheel, so they are grouped as cars; cars, motorbikes and lorries share an engine and the ability to carry things on roads, so they are grouped as road vehicles. The hierarchy is: a saloon is a kind of car, a car is a kind of road vehicle, a road vehicle is a kind of vehicle. Each level up keeps only the characteristics common to everything below it. In a program the same hierarchy appears as classes: a Square is a kind of Rectangle, a Rectangle is a kind of Shape, so anything true of every Shape can be written once in Shape.

6. 6

   Distinguish representational abstraction from abstraction by generalisation.

   Model answer

   Representational abstraction works on a single thing and throws away detail from it — one map of one network, with the geography stripped out. Abstraction by generalisation works on many things and groups them by the characteristics they share, producing an 'is a kind of' hierarchy — many vehicles collected into the category 'car'. One reduces detail within an item; the other creates categories across items.

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