AS · Practice questions · Abstraction and decomposition

Simplify, then split.

Six original Cambridge-style questions on abstraction and decomposition, with mark-scheme style answers.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Remove, then divide.

Abstraction: remove details not needed; keep only the essential ones; the result is an abstract model.
Benefits: less complex, easier to understand, quicker to design and program, less data, reusable model.
Decomposition: break a problem into sub-problems, again and again, until each can be solved.
Modules: each sub-problem becomes a procedure (CALLed) or a function (returns a value, used in an expression).

Explain

[2 marks]

Describe the purpose of abstraction when designing a solution to a problem.

Removing / filtering out details that are not relevant to the problem. ✓
To produce a simplified (abstract) model that keeps only the essential details. ✓

Recall

[3 marks]

State three benefits of using abstraction.

The problem is less complex / easier to understand. ✓
The solution is quicker to design and program (less development time). ✓
Less data needs to be collected and stored / the program is smaller; the model may be reused (any one). ✓

Apply

[4 marks]

A cinema wants a program to sell seats online. Produce an abstract model of a seat for this program by identifying three essential details. Give one detail that should be left out and justify your choice.

Seat identifier, e.g. row letter and seat number. ✓
Whether the seat is booked or available (for the chosen showing). ✓
Price band / seat type (e.g. standard, premium, wheelchair space). ✓
Left out: e.g. colour of the seat fabric, because it does not affect booking or pricing. ✓

Explain

[2 marks]

Explain how decomposition leads to the use of procedures and functions in a program.

The problem is broken down into smaller sub-problems (repeatedly). ✓
Each sub-problem is small enough to be coded as one module: a procedure or a function. ✓

Analysis

[3 marks]

A team is writing a payroll program. Give three advantages of decomposing the program into modules.

Modules can be written by different programmers at the same time. ✓
Each module can be tested on its own, so errors are easier to find and fix. ✓
Modules can be reused (called many times or in other programs), so code is not repeated; maintenance is easier. ✓

Write

[4 marks]

Write pseudocode for a function Larger that takes two integer parameters and returns the larger of the two. Then write a statement that uses the function to output the larger of Score1 and Score2.

Header FUNCTION Larger(A : INTEGER, B : INTEGER) RETURNS INTEGER. ✓
IF A > B THEN RETURN A ELSE RETURN B ENDIF (or equivalent; either value if equal). ✓
ENDFUNCTION. ✓
OUTPUT Larger(Score1, Score2) (no CALL, used in an expression). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.