Devising an Abstract Model

Key Point

Design consideration	Questions to ask
Problem definition	What exactly needs solving? Which features are essential?
Model usage	How will it be used in practice? Is it practical and accessible?
Target audience	Who will use it and what is their expertise level?
Relevance vs simplicity	Which details are essential, and which can be removed?

Worked Example

Library System Abstract Model

Real-world complexities we ignore:
• Physical layout of the library
• Colour of the carpet
• Temperature of the building
• Staff break schedules

Abstraction we keep:

pseudocode

record Book:
    string ISBN
    string title
    string author
    boolean isAvailable
end record

record User:
    string userID
    string name
    list borrowedBooks
end record

record Loan:
    string bookISBN
    string userID
    date borrowedDate
    date dueDate
end record

Key operation	Purpose
borrowBook(userID, ISBN)	Create a loan and mark a book unavailable
returnBook(userID, ISBN)	Close loan and mark book available
searchBooks(query)	Find books by title/author/ISBN
checkOverdueLoans()	Identify late returns

This abstraction captures what matters for the system while ignoring irrelevant physical details.

Exam Tip

Exam Tips for Abstraction:
• Be able to explain why abstraction is necessary (simplification, efficiency, layered architecture, accessibility)
• Identify similarities and differences between real-world and abstract representations
• Explain the four types: representational, generalisation, data, and procedural abstraction
• Devise appropriate abstract models for given scenarios
• Understand how abstraction relates to OOP objects, data structures, and layered systems

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Question

Graph Theory

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Answer

Modelling a problem as a set of nodes joined by edges and leaving out all other detail, for example towns as nodes and roads as weighted edges, so that standard algorithms can find solutions such as the shortest path.

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Browse all 4 flashcards on Devise an Abstract Model for a Variety of Situations

Graph Theory
Modelling a problem as a set of nodes joined by edges and leaving out all other detail, for example towns as nodes and roads as weighted edges, so that standard algorithms can find solutions such as the shortest path.
Abstract Model
A simplified representation of a real system, held as data and rules that a program can work with, such as a road network stored as junctions and distances. It includes only the features that affect the solution.
Devising an Abstract Model
Deciding what the solution is for, identifying the real-world features and data that affect it, discarding everything else, and choosing how to represent what is kept, for example as symbols, variables or a data structure.
Example: Chess Program
The board is modelled as an 8 by 8 two-dimensional array and each piece as an object with a type, colour and position. The material and size of the real set, the players' names and the surroundings are left out as irrelevant.

Exam questions on Devise an Abstract Model for a Variety of Situations

1 OCR past paper question and 1 practice question (8 marks) with mark schemes

Question 1H446/024 marks
Explain how abstraction is used when designing a class diagram for an online shopping system.
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Focus on relevant attributes (name, price) not physical properties / Group similar items into classes / Hide implementation details / Use inheritance to share common properties / Show relationships not internal workings (4 marks)
Mark yourself: how many marks did you get?
/ 4
Question 2Practice question · H446/02 style4 marks
A school wants a program that produces its lesson timetable. Devise an abstract model for this problem by identifying two features that must be included and two real-world details that can be ignored. Justify your choices.
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Include (1 mark each, max 2), e.g.: • Rooms and their capacities • Teachers and when they are available • Classes / subjects and the number of periods each needs Ignore (1 mark each, max 2), e.g.: • The colour or decoration of rooms • The names of individual students (only class sizes matter) • The weather / what teachers wear Justification: included features affect whether a timetable is valid; ignored details do not change the solution (4 marks)
Mark yourself: how many marks did you get?
/ 4

Exam questions on Devise an Abstract Model for a Variety of Situations

Past paper questions