AQA A-Level Computer Science AS Paper 1, June 2025: Question 2

Mark scheme

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02 2 marks for AO1 (understanding) 2

(Code within) WHILE/Figure 3 loop structure is not executed // final value of X remains

the same;

(Code within) REPEAT/Figure 4 loop structure will be executed (once) // the value of X

changes/decreases;

The WHILE loop tests the condition at the start of the loop whereas the REPEAT loop

tests the condition at the end of the loop;

Max 2

How to answer it

Comparing Pre-Condition and Post-Condition Loops

📌 What this question tests

This question assesses your understanding of iteration constructs (AO1 - Knowledge and Understanding). Specifically, it evaluates whether you understand the fundamental behavioural difference between a pre-condition loop ( WHILE...ENDWHILE ) and a post-condition loop ( REPEAT...UNTIL ) when provided with boundary and invalid data (negative numbers).

Question 02 • [2 marks]

Analysis of Figure 3 (WHILE) vs Figure 4 (REPEAT...UNTIL)

Explaining differing behaviour with negative integer inputs

Figure 3: Pre-condition Loop

INPUT X WHILE X > 0 X ← X - 1 ENDWHILE

Figure 4: Post-condition Loop

INPUT X REPEAT X ← X - 1 UNTIL X ≤ 0

💡 Key Knowledge

Pre-condition Loop ( WHILE ): The boolean condition is evaluated before entering the loop body. If the condition evaluates to False initially, the loop body executes 0 times.
Post-condition Loop ( REPEAT...UNTIL ): The body executes first, and the condition is evaluated at the end. Therefore, the loop body always executes at least once.
Loop Termination Condition: A REPEAT...UNTIL loop terminates when the condition becomes True .

📐 Step-by-Step Trace (Input: X = -3)

Figure 3:
• Initial check: -3 > 0 evaluates to False.
• Loop body is skipped completely.
• Result: X remains -3 (0 iterations).
Figure 4:
• Body executes: X ← -3 - 1 (so X = -4 ).
• Condition test: -4 ≤ 0 evaluates to True.
• Loop terminates immediately after 1 iteration.
• Result: X becomes -4 (decreased by 1).

✅ Acceptable Points (Any 2 for Full Marks)

The code inside the WHILE loop (Figure 3) is not executed / the final value of X remains unchanged. [1 mark]
The code inside the REPEAT loop (Figure 4) will be executed (once) / the value of X decreases / changes. [1 mark]
The WHILE loop tests the condition at the start of the loop, whereas the REPEAT loop tests the condition at the end of the loop. [1 mark]

🧠 Exam Technique

State the difference for both loops: Always explicitly reference both Figure 3 and Figure 4 (or both loop keywords) rather than giving an open-ended statement.
Focus on execution count or condition timing: You can either explain when the condition is checked (start vs end) or what happens to the data/loop body (0 times vs 1 time / unchanged vs decremented).

❌ Common Errors & Misconceptions

Stating that Figure 4 enters an infinite loop: Some students incorrectly assume that because X is negative, subtracting 1 will cause an infinite loop. However, the exit condition is X ≤ 0 , which is immediately satisfied after the first decrement.
Vague comparisons: Writing "one checks first and the other checks later" without identifying which figure does which loses easy marks. Always pair the name/figure with its behaviour.
Forgetting to address negative inputs: The question specifically asked about behaviour when inputs are negative. Explanations must clearly relate to what happens under this specific condition.

Mark Scheme Summary: Max 2 marks for AO1. 1 mark per valid distinction regarding loop execution (0 executions vs 1 execution), state of variable X (unchanged vs decremented), or location of test condition (start vs end).

Topics

4.1 Fundamentals of programming · 4.1.1 Programming