AQA A-Level Computer Science AS Paper 1, June 2025: Question 13

Mark scheme

Show the mark scheme

13 3 marks for AO2 (analyse) 3

Direction/NumberOfSquares/distance might be valid (independently);

But the combination of these …;

… could result in Row and/or Column to be outside the boundaries of the map;

(CheckPath) would attempt to address elements beyond the bounds of the (Map)

data structure // would then malfunction/crash;

Max 3

How to answer it

Skeleton Code Analysis: PirateWalks Validation Logic

📌 What this question tests

This question assesses your ability to analyse skeleton code logic (AO2) in Paper 1. Specifically, it tests your understanding of:

Multi-stage input validation: Why validating inputs in isolation is not always sufficient.
Boundary checking & 2D array constraints: How coordinate calculations can cause runtime errors (such as IndexOutOfBoundsException ).
Subroutine interaction: The relationship between checking format/compass direction ( CheckDirection ) versus verifying actual path execution ( CheckPath / grid traversal).

Question 13 • [3 Marks]

Boundary Checking in PirateWalks

Explaining why ValidDirection must be reassessed

💡 Key Knowledge

Isolated vs. Contextual Validity: A direction (e.g., 'N', 'S', 'E', 'W') might be a valid input on its own, and the number of squares entered might be a valid positive integer.
Array Bounds: A grid or board is represented by a finite 2D array with specific row and column limits (e.g., 0 to MaxRow, 0 to MaxColumn).
Runtime Exceptions: If code attempts to access indices outside valid array indices (index < 0 or index ≥ length), the program crashes or malfunctions.

✅ Model Answer (Full 3 Marks)

To gain all 3 marks, your explanation must connect the inputs to the array boundary and the resulting system failure:

Mark 1: The direction and number of squares/distance entered might each be valid on their own (independently)...
Mark 2: ...but the combination of this direction and distance from the current position would cause the new Row and/or Column to fall outside the boundaries of the map.
Mark 3: Consequently, attempting to inspect the path ( CheckPath ) or update the grid would try to address elements beyond the bounds of the Map data structure, causing the program to crash or malfunction.

🧠 Exam Technique & Mark Breakdown

Think in cause-and-effect chains:
1. Input status: Valid individually.
2. Calculated effect: Coordinates move out-of-bounds.
3. Technical consequence: Array index error / program crash.
Name specific variables and structures: Always reference identifiers directly from the skeleton code, such as Row , Column , Map , and CheckPath .
AO2 Requirement: Broad statements like "it's not allowed" will get 0 marks. You must analyse why the code structure needs the boolean reset.

❌ Common Misconceptions & Errors

Vagueness: Saying "the move is invalid" without explaining why (i.e. moving off the edge of the board).
Confusing the checks: Thinking CheckDirection already checks if the pirate stays on the board. CheckDirection only checks if the character entered is a valid compass point (e.g., 'N', 'E', 'S', 'W').
Omitting technical impact: Forgetting to mention the data structure ( Map ) or the failure mode (out-of-bounds index / crash).

Mark Scheme Reference (Max 3):

Direction / NumberOfSquares / distance might be valid (independently);
But the combination of these ... could result in Row and/or Column being outside the boundaries of the map;
( CheckPath ) would attempt to address elements beyond the bounds of the ( Map ) data structure // would then malfunction / crash.

Topics

4.1 Fundamentals of programming · 4.2 Fundamentals of data structures · 4.4 Theory of computation · 4.1.1 Programming · 4.2.1 Data structures and abstract data types · 4.4.1 Abstraction and automation