Representing algorithms, decomposition & abstraction – GCSE Computer Science exam questions with mark scheme

1Grade 1–34 marks

Three flowchart symbols, A, B and C, are shown in the diagram.

(a) State the purpose of each symbol. [3]

(b) Define the term algorithm. [1]

(a) A (parallelogram): input / output; accept 'input' or 'output' on its own(1)
(a) B (diamond): decision / selection / a question with a yes/no (True/False) outcome(1)
(a) C (rectangle): process (e.g. a calculation or assignment); accept an action / instruction to carry out(1)
(b) A sequence of steps / instructions that can be followed to complete a task / solve a problem. Do not accept 'a program' on its own(1)
Answer: (a) A input/output; B decision; C process. (b) A sequence of steps that completes a task.
Examiner tip: An algorithm is not the same as a program: a program is an algorithm written in a programming language, so 'a program' alone gets no mark.

2Grade 1–33 marks

(a) Define the term decomposition. [1]

(b) Define the term abstraction. [1]

(c) A student is writing a program for a quiz game. Give one example of how the student could use abstraction when designing the game. [1]

(a) Breaking a problem down into a number of smaller / sub-problems (each of which is easier to solve / accomplishes an identifiable task)(1)
(b) Removing / hiding unnecessary details from a problem (to focus on the details that matter)(1)
(c) Any sensible example of leaving out unnecessary detail, related to the quiz, e.g. only storing the question, correct answer and score / not modelling the player's appearance / ignoring the colour or design of the screen background. Allow hiding detail, e.g. calling a checkAnswer subroutine without needing to know how it works. Reject a generic definition with no quiz context(1)
Answer: (a) Breaking a problem into smaller sub-problems. (b) Removing unnecessary detail. (c) e.g. store only question, correct answer and score.
Examiner tip: Don't swap the two definitions: decomposition is about breaking up, abstraction is about leaving out.

3Grade 6–75 marks

Look at this algorithm, written in AQA pseudocode.

nums ← [4, 7, 2, 9, 5]
max ← nums[0]
pos ← 0
FOR i ← 1 TO 4
   IF nums[i] > max THEN
      max ← nums[i]
      pos ← i
   ENDIF
ENDFOR
OUTPUT pos

(a) Complete a trace table for the algorithm with the columns i, max, pos and OUTPUT. Start with a first row showing the initial values max = 4 and pos = 0. [4]

(b) State the purpose of the algorithm. [1]

(a) i column: 1, 2, 3, 4(1)
(a) max column: 4, then 7, then 9 (changes only when i = 1 and i = 3)(1)
(a) pos column: 0, then 1, then 3(1)
(a) Output: 3 (only once, at the end)(1)
(b) Finds / outputs the position (index) of the largest value in the array. Do not accept 'finds the largest number'(1)
Answer: i: 1,2,3,4; max: 4→7→9; pos: 0→1→3; output 3. Purpose: outputs the index of the largest value.
Examiner tip: The algorithm outputs pos, not max, so its purpose is to find the position of the largest value.

Still losing marks on this topic?

Book a free introductory session and we'll find exactly which step is costing you.

Book a free intro