← IGCSE Computer Science Paper 2

Deciding which details to keep and which to ignore

Keep only the details the solution needs to work and ignore the rest. A library system needs the book title and borrower, not the book's colour. Too few details fails; too many makes it harder.

Part of: IGCSE Computer Science 0478 — 7b Decomposition and Sub-systems — Module 18

Understanding Deciding which details to keep and which to ignore in IGCSE Computer Science Paper 2

What it does: The loan record stores only the relevant details: which book was borrowed and who borrowed it. Nothing else is needed. The system needs the borrower to track the loan. Cover colour does not help it work, so abstraction ignores it. Abstraction filters out unnecessary detail. Keeping everything makes the problem harder to solve. Missing a needed detail means the solution fails. The borrower is required to track loans. Lesson times are needed to build a timetable. A favourite colour has no effect on how the solution works, so it is ignored. Road names and distances are needed for finding routes. Building colours are not, so ignoring them is good abstraction.

Common mistakes: The library system needs the borrower, so Borrower must be input. The book's colour is an irrelevant detail, and Borrower is never given a value. It should be INPUT Borrower. Colour is an irrelevant detail that was never input. The loan record needs the title and the borrower, so it should be OUTPUT Title, Borrower.

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This module's key terms are also in a printable crossword (Modules 107–118) and word search (Modules 94–118).

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Learning

Duo

A library system records who borrowed a book. Which detail should it keep?

The colour of the book's coverThe borrower's name

Matches

loan <- [bookTitle]

↓

Too little detail: the borrower is missing, so the system cannot tell who has the book and the solution fails

Applying

Meanings

Term

abstraction

Meaning

Keep only the details the solution needs to work and ignore the rest. A library system needs the book title and borrower, not the book's colour. Too few details fails; too many makes it harder.

Practice

// Keep only what the loan needs

Testing

Gaps

BookTitle

Check Answer

Spots

3INPUT Title

4INPUT BookColour

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Quick Questions

1. A library system records who borrowed a book. Which detail should it keep?

A. The borrower's name ✓

B. The colour of the book's cover

The system needs the borrower to track the loan. Cover colour does not help it work, so abstraction ignores it.

2. Which statement about abstraction is correct?

A. Keep every detail so nothing is missed

B. Keep the details the solution needs and ignore the rest ✓

Abstraction filters out unnecessary detail. Keeping everything makes the problem harder to solve.

3. A library system stores the book title but not the borrower. What is the problem?

A. Too little detail, so it cannot show who has the book ✓

B. Too much detail, so the system is slow to understand

Missing a needed detail means the solution fails. The borrower is required to track loans.

4. A library system also stores each book's weight, cover colour and the author's favourite food. What is the effect?

A. Too much detail makes the problem harder to solve ✓

B. More detail always makes the solution better

Irrelevant details add complexity and make the problem harder to understand and solve, without helping the system work.

5. A school timetable program is being designed. Which detail is irrelevant to it?

A. The time of each lesson

B. A teacher's favourite colour ✓

Lesson times are needed to build a timetable. A favourite colour has no effect on how the solution works, so it is ignored.

Match the Pairs

abstraction → Keeping only the details the solution needs to work and ignoring the rest

IF detail NOT needed THEN IGNORE it → Remove a detail that does not help the solution work, such as the colour of a library book

A library system needs the book title and the borrower to record a loan, so these details are kept → loan <- [bookTitle, borrowerName]

The book's colour is irrelevant to lending, so it is left out of the record → book <- [title, author] // no colour

loan <- [bookTitle] → Too little detail: the borrower is missing, so the system cannot tell who has the book and the solution fails

loan <- [title, borrower, colour, weight, coverArt] → Too much detail: colour, weight and cover art are not needed, so the problem is harder than it should be

Key Term

abstraction

Keep only the details the solution needs to work and ignore the rest. A library system needs the book title and borrower, not the book's colour. Too few details fails; too many makes it harder.

Fill in the Blanks

1. INPUT BookTitle

Hint: Type the pseudocode keyword that reads a needed detail from the user.

2. Borrower ← "Mia"

Hint: Type the assignment symbol that stores a value in the variable.

3. INPUT BookTitle OUTPUT BookTitle

Hint: Type the variable that holds the detail the system just read in and needs to show.

4. IF DaysLate > 0 THEN OUTPUT "Fine due" ENDIF

Hint: Type the comparison operator that checks the borrower has days late above zero.

5. PROCEDURE CheckOut(Borrower, BookTitle) OUTPUT Borrower, BookTitle ENDPROCEDURE

Hint: Type the keyword that closes a procedure that only uses the details it needs.

Spot the Error

Question 1

A DECLARE Title : STRING

B DECLARE Borrower : STRING

C INPUT Title

D INPUT BookColour

Line D — The library system needs the borrower, so Borrower must be input. The book's colour is an irrelevant detail, and Borrower is never given a value. It should be INPUT Borrower.

Question 2

A INPUT Title

B INPUT Borrower

C OUTPUT Title, Colour

Line C — Colour is an irrelevant detail that was never input. The loan record needs the title and the borrower, so it should be OUTPUT Title, Borrower.

Question 3

A INPUT Available

B IF Available = TRUE

C OUTPUT "Can borrow"

D ENDIF

Line B — Whether the book is available is a relevant detail, and the program uses it in a decision. Every IF needs the keyword THEN, so it should be IF Available = TRUE THEN.

Question 4

A Fine = 0

B INPUT DaysLate

C Fine ← DaysLate * 0.5

D OUTPUT Fine

Line A — The fine only needs the number of days late, and the book's colour is ignored. Cambridge pseudocode uses the assignment arrow ←, so the line should be Fine ← 0.

Question 5

A INPUT Distance

B INPUT Age

C Price ← Distance * 0.2

D IF Age < 16 DO

E Price ← Price / 2

F ENDIF

Line D — The ticket price only needs the distance and the age, not the bus colour. An IF statement must use THEN, not DO. It should be IF Age < 16 THEN.

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