AQA A-Level Computer Science AS Paper 1, June 2025: Question 8

Exception Handling in the Skeleton Program

📌 What this question tests

This question evaluates your ability to navigate, inspect, and analyse the Skeleton Program (Assessment Objective AO2). Specifically, it assesses:

Identifying programming constructs such as structured exception handling (e.g. try...except / try...catch blocks).
Understanding the operational purpose of defensive programming: preventing runtime crashes during data type conversion (casting) of user inputs.

Question 08.1

1 Mark • AO2 (Analyse)

Identify the subroutine using exception handling

✅ Correct Answer

CheckDistance

Case and spacing are ignored by examiners, but exact spelling is strictly required.

🧠 Exam Technique

Use Ctrl+F: Quickly search the Skeleton Program file for language-specific keywords:
• Python: try or except
• C# / Java: try or catch
• VB.NET: Try or Catch
Give only the identifier name. Do not include parentheses or parameters unless explicitly asked.

❌ Common Errors & Mark Scheme Guidance

Reject (R): Providing extra code alongside the subroutine identifier (e.g. writing Function CheckDistance() or CheckDistance = True ).
Reject (R): Misspelling the name (e.g. CheckDistances or ChekDistance ).
Ignore (I): Differences in case or surrounding whitespace (e.g. checkdistance is fully credited).

Mark Scheme Breakdown: 1 mark for precisely stating CheckDistance .

Question 08.2

2 Marks • AO2 (Analyse)

Explain the purpose of exception handling in CheckDistance

✅ Model Answer (Full 2 Marks)

To secure both marks, your response must cover the cause of the potential error and the consequence avoided:

Mark 1 (Input/Type issue): The string or character entered by the user may not be valid or convertible to an integer.
Mark 2 (System outcome): Without exception handling, this conversion would produce a runtime error and cause the program to crash / terminate unexpectedly.

💡 Key Knowledge: Why Catch Errors?

In user-facing programs, input always enters as a string/text format:

Converting "15" → integer 15 succeeds.
Converting "fifteen" or "" → throws an exception (e.g. ValueError or FormatException ).
Graceful Recovery: Exception handling intercepts this runtime error, allowing the program to display an error prompt and ask again rather than halting execution.

🧠 Exam Technique: "Explain Why" Pattern

For a 2-mark explanation question on error handling, always follow the two-part structure:

[Specific Trigger] What user action creates the fault? (e.g. "entering non-numeric data that cannot cast to an int")
+
[Program Consequence] What bad outcome does the handler avoid? (e.g. "prevents an unhandled exception causing a program crash").

❌ Common Misconceptions & Lost Marks

Vague answers: Simply writing "to stop errors" or "to validate data" scores 0 marks without explaining which error or how it crashes.
Missing the conversion aspect: Forgetting to mention the data type mismatch (string cannot be converted to integer).
Confusing validation with exception handling: Saying "it checks if the distance is between 1 and 10" misses the point. Exception handling specifically traps runtime conversion failures, not range boundaries.

Mark Scheme Breakdown:
• 1 Mark: The string/character supplied may not be (convertible to) an integer.
• 1 Mark: Without exception handling the program would crash / halt execution.