AQA GCSE Computer Science Paper 1 (1B), June 2025: Question 9
2 marks · Medium difficulty · Multiple Choice
Identify which of the five pseudo-code algorithms correctly calculate the total of five values stored in an array.
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![Figure 9 shows five pseudo-code algorithms labelled A to E, each attempting to sum the five values in the array numbers = [44, 1, 2, 14, 68]. Algorithm A uses a WHILE loop from counter = 0 to counter < 5. Algorithm B uses a FOR loop from 0 to LEN(numbers) - 1. Algorithm C uses a FOR loop from 1 to 7. Algorithm D uses a WHILE loop with condition total > 0. Algorithm E uses a REPEAT UNTIL loop with x = 1 and UNTIL x = 5 without incrementing x. Below, a table asks the student to tick whether each algorithm A, B, C, D, and E totals the five values or does not total the five values.](/.netlify/images?url=https%3A%2F%2Ffirebasestorage.googleapis.com%2Fv0%2Fb%2Fexamquestions-bf477.firebasestorage.app%2Fo%2Fquestions%252FAQA-GCSE-CS.Q25.L1.09.SRC.aqa-8525-gcse-1%25257C2025%25257CJUN%25257Cuntiered%25257C85251B-1790883242170.png%3Falt%3Dmedia&w=1000)
Question text
09 Figure 9 shows five algorithms, represented using pseudo-code.
Figure 9
numbers [44, 1, 2, 14, 68]
counter 0
total 0
WHILE counter < 5
A index counter
total total + numbers[index]
counter counter + 1
ENDWHILE
OUTPUT total
numbers [44, 1, 2, 14, 68]
total 0
FOR a 0 TO LEN(numbers) - 1
B
total total + numbers[a]
ENDFOR
OUTPUT total
numbers [44, 1, 2, 14, 68]
total 0
FOR a 1 TO 7
C
total total + numbers[a]
ENDFOR
OUTPUT total
numbers [44, 1, 2, 14, 68]
total 0
WHILE total > 0
D
total total + numbers[total]
ENDWHILE
OUTPUT total
numbers [44, 1, 2, 14, 68]
total 0
x 1
E REPEAT
total total + numbers[x]
UNTIL x = 5
OUTPUT total25
The algorithms in Figure 9 have been written to calculate the total of the five values
in the array numbers. Not all of the algorithms work.
Complete the table by ticking ( ) one box in each column (A–E).
[2 marks]
Algorithm
*24* A B C D E
Algorithm totals the five values
Algorithm does not total the five values
Mark scheme
Show the mark scheme
Total
Question Part Marking guidance
marks
09 2 marks for (AO2 apply) 2
MP1: for any three columns correct;
MP2: for all columns correct;
A B C D E
Algorithm totals the five values ✔ ✔
Algorithm does not total the five values ✔ ✔ ✔
R. column if more than 1 tick in that column
How to answer it
Evaluating Pseudo-Code Algorithms: Array Totalling
This question assesses your ability to trace pseudo-code, understand iteration (loops), and recognise how array indexing works in computer science. You must spot syntax logic bugs such as infinite loops, off-by-one index errors, and condition test failures.
Question 09 Breakdown
Total Marks: 2 (AO2 - Application)
✅ Correct Completed Table
| Outcome | Algorithm | ||||
|---|---|---|---|---|---|
| A | B | C | D | E | |
| Algorithm totals the five values | ✓ | ✓ | |||
| Algorithm does not total the five values | ✓ | ✓ | ✓ |
- 1 mark: Any 3 columns correctly ticked.
- 2 marks: All 5 columns correctly ticked.
- Note: If you put more than one tick in any column, that column is rejected and cannot count towards your mark!
📐 Algorithm-by-Algorithm Walkthrough
The array is numbers ← [44, 1, 2, 14, 68] . Because arrays are zero-indexed, the 5 valid indices are 0, 1, 2, 3, 4.
Algorithm A: Works ✓
Loop: WHILE counter < 5 starting at 0 .
Trace: Runs for counter = 0, 1, 2, 3, 4 . Increments by 1 each time. Stops when counter = 5 .
Result: Adds elements at indices 0, 1, 2, 3, and 4. All 5 numbers are totalled correctly.
Algorithm B: Works ✓
Loop: FOR a ← 0 TO LEN(numbers) - 1 .
Trace: LEN(numbers) is 5, so the loop runs from 0 TO 4 inclusive.
Result: Accesses each valid index from 0 to 4 in turn. Successfully totals all five values.
Algorithm C: Fails ✗
Loop: FOR a ← 1 TO 7 .
Trace: Starts at index 1 (skipping index 0: 44 ). Then tries to access index 5, 6, 7 which don't exist!
Result: Misses the first element and causes an Index Out of Bounds / Range Error.
Algorithm D: Fails ✗
Loop: WHILE total > 0 .
Trace: total is set to 0 immediately before the loop. The condition 0 > 0 is False right from the start.
Result: The loop body never executes even once! Also incorrectly uses numbers[total] .
Algorithm E: Fails ✗
Loop: REPEAT ... UNTIL x = 5 .
Trace: x starts at 1, but x is never incremented inside the loop!
Result: Causes an infinite loop adding numbers[1] forever. It also starts at 1 instead of 0.
💡 Key Knowledge to Remember
- Zero-Indexing: Array items are indexed starting at 0 . For an array of size N, valid indices run from 0 to N - 1 .
- Array Length: If LEN(numbers) = 5 , the last item is at position LEN(numbers) - 1 = 4 .
- Condition-Controlled Loops: A WHILE loop tests the condition before running. If the condition is false initially, the loop never runs.
- Post-Condition Loops: A REPEAT...UNTIL loop always runs at least once, testing the condition at the end.
🧠 Exam Technique: How to Trace Fast
- Check Start & End Values: Does the index variable start at 0 ? Does it reach 4 without exceeding it?
- Look for Missing Updates: Check whether loop counters ( counter or x ) actually get updated inside WHILE or REPEAT loops. If not, it's an infinite loop!
- One Tick per Column: Never tick both rows in the same column—the mark scheme strictly instructs examiners to reject multi-ticked columns.
❌ Common Errors Made by Students
- Assuming 1-based indexing: Thinking Algorithm C is correct because it starts counting at 1.
- Overlooking uninitialised / unmet conditions: Believing Algorithm D adds values without checking that total > 0 is immediately false.
- Missing the infinite loop in E: Skimming Algorithm E and seeing UNTIL x = 5 , assuming x increases automatically like a FOR loop.
Topics
3.1 Fundamentals of algorithms · 3.2 Programming · 3.1.1 Representing algorithms · 3.2.2 Programming concepts · 3.2.6 Data structures
Question and mark scheme from the AQA GCSE Computer Science examination, Paper 1 (1B), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.