Producing Robust Programs Exam Practice

This lesson provides comprehensive exam practice for the Producing Robust Programs topic, covering the key content from OCR J277 Section 2.4 that appears on Paper 2: Computational Thinking, Algorithms and Programming. This lesson brings together defensive design, input validation, authentication, maintainability, error types, testing, test data, trace tables, and optimisation.

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What to Expect on Paper 2

The Producing Robust Programs section typically accounts for 10–20 marks on Paper 2. Questions test your understanding of how to write reliable, well-tested, maintainable code.

Question Type	Typical Marks	What You Need to Do
Define a term	1–2 marks	Define validation, verification, syntax error, etc.
Identify validation checks	2–3 marks	Name and explain appropriate checks for a scenario
Select test data	3–4 marks	Choose normal, boundary, and erroneous data
Identify error type	1–2 marks	State whether an error is syntax, logic, or runtime
Improve maintainability	2–4 marks	Suggest specific improvements to given code
Debug with trace table	4–6 marks	Find and fix a logic error
Write validation code	3–5 marks	Write pseudocode for input validation

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Practice Question 1: Definitions (3 marks)

Question: Define the following terms: (a) Syntax error (1 mark) (b) Logic error (1 mark) (c) Runtime error (1 mark)

Model Answer: (a) A syntax error is a mistake in the code that breaks the rules of the programming language, preventing the program from running. (b) A logic error is a mistake in the algorithm's logic that causes the program to produce incorrect output, but the program still runs without crashing. (c) A runtime error is an error that occurs while the program is running, causing it to crash (e.g. division by zero or accessing an invalid array index).

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Practice Question 2: Input Validation (4 marks)

Question: A program asks the user to enter a UK phone number (must start with "0", must be exactly 11 digits, must contain only numbers). Identify two appropriate validation checks and explain how each would be applied.

Model Answer:

1. Length check — check that the phone number contains exactly 11 characters. If the length is not 11, display an error message and ask the user to re-enter.
2. Format check — check that the first character is "0" and that all 11 characters are digits. If the format is wrong, display a specific error message explaining the required format.

(Also acceptable: type check to ensure only numeric characters, presence check to ensure it is not empty.)

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Practice Question 3: Test Data (4 marks)

Question: A program accepts a percentage mark from 0 to 100. Provide four items of test data, one of each type (normal, boundary valid, boundary invalid, erroneous), stating the expected outcome for each.

Model Answer:

Test Data	Type	Expected Outcome
65	Normal	Accepted — processed normally
0	Boundary (valid)	Accepted — lower boundary
101	Boundary (invalid)	Rejected — "Mark must be 0–100"
"abc"	Erroneous	Rejected — "Please enter a number"

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Practice Question 4: Validation vs Verification (2 marks)

Question: Explain the difference between validation and verification. (2 marks)

Model Answer: Validation checks that data is reasonable, sensible and meets certain rules (e.g. an age must be between 0 and 150) — it checks if data could be correct, but not that it IS correct. Verification checks that data has been entered or transmitted accurately (e.g. asking the user to enter their email address twice and comparing the two entries to confirm no typing errors were made).

OCR Exam Tip: This is one of the most frequently asked questions in the OCR exam. Learn a clear one-sentence definition for each. Validation = checks data is reasonable. Verification = checks data is accurate.

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Practice Question 5: Error Identification (3 marks)