2.1 Testing and debugging — Edexcel 4CP0

Why Testing Matters

Testing is the process of running a program with carefully chosen data to check that it behaves correctly. A well-tested program should handle normal data, boundary data, and erroneous data correctly. The goal of testing is to find and fix errors before the program is deployed.

Types of Errors

Error Type	When it occurs	Example
Syntax error	During translation — breaks language rules	Writing ENDWHILE (no space) instead of END WHILE (correct Edexcel form); missing END IF
Logic error	During execution — program runs but gives wrong result	Using > instead of ≥ in a condition
Runtime error	During execution — program crashes	Division by zero; array index out of bounds

Test Data

Good test data covers three categories:

Type	Description	Example (age input, valid range 1–120)
Normal (valid)	Typical data the program should accept and process correctly	25, 40, 67
Boundary (extreme)	Values at the edge of what is acceptable — tests the boundary condition	1, 120 (just inside); 0, 121 (just outside)
Erroneous (invalid)	Data the program should reject — wrong type or out of range	"hello", -5, 200, 3.14

Trace Tables for Debugging

A trace table (also called a dry run) is used to track variable values through an algorithm step by step. It helps identify logic errors by showing exactly what the program does at each stage. Create a column for each variable and update values row by row as you execute the algorithm.

Debugging Strategies

Desk checking (dry run): Manually trace the algorithm with test values using a trace table
Print statements / SEND to DISPLAY: Insert temporary output statements to check variable values at different points
Breakpoints: Pause execution at a specific line and inspect variable values in an IDE
Stepping through code: Execute one line at a time to observe the effect of each statement

📝 Exam Tip: For "identify a suitable test value" questions, always give at least one normal, one boundary, and one erroneous value. State WHY each value was chosen — e.g., "120 is a boundary value because it is the maximum valid age".

⚠️ Common Mistakes

Only giving normal test data — boundary and erroneous data are equally important
Confusing a logic error with a runtime error — a logic error produces a wrong answer; a runtime error crashes the program
Saying syntax errors occur "when the program runs" — they occur during translation (before running)

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Worksheet — 2.1 Testing and Debugging

Answer each question then submit for marking against the Edexcel mark scheme.

Q1State the three types of error that can occur in a program and explain when each occurs.[6 marks]

✅ Mark Scheme

2 marks each:

• Syntax error (1) — occurs during translation / when the code breaks the rules of the language (1)
• Logic error (1) — occurs during execution, program runs but produces incorrect output (1)
• Runtime error (1) — occurs during execution, program crashes / performs an impossible operation (1)

Q2A program asks the user to enter an exam score between 0 and 100. Give one example of each type of test data and explain why each was chosen.[6 marks]

✅ Mark Scheme

1 mark for value + 1 mark for reason, ×3:

• Normal: e.g. 50 (1) — typical valid value to check the program processes it correctly (1)
• Boundary: e.g. 0 or 100 (1) — at the edge of the valid range, tests the boundary condition (1)
• Erroneous: e.g. -1, 101, or "hello" (1) — outside valid range / wrong type, should be rejected (1)

Q3Explain the purpose of a trace table and describe how it is used to debug a program.[4 marks]

✅ Mark Scheme

Mark scheme:

• A trace table is used to track the values of variables as an algorithm executes (1)
• Each column represents a variable; each row represents a step / change in value (1)
• The algorithm is dry-run manually with test data, recording each variable change (1)
• Comparing expected values with actual trace values reveals where a logic error occurs (1)

Q4Identify the type of error in each situation: (a) a program divides by a variable that turns out to be zero; (b) a loop counts from 1 to 9 instead of 1 to 10; (c) a keyword is misspelled as "ENDWILE".[3 marks]

✅ Mark Scheme

1 mark each:

• (a) Runtime error — division by zero causes the program to crash during execution (1)
• (b) Logic error — program runs but the loop count is wrong, producing incorrect output (1)
• (c) Syntax error — misspelled keyword breaks language rules, detected at translation (1)

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Edexcel 4CP0 · Topic 2

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Term	Definition
Syntax error	An error caused by breaking the rules of the programming language; detected at translation
Logic error	An error where the program runs but produces incorrect output due to a flaw in the algorithm
Runtime error	An error that causes the program to crash during execution (e.g. division by zero)
Normal test data	Typical, valid data the program should accept and process correctly
Boundary test data	Values at the extreme edge of the valid range; tests the boundary condition
Erroneous test data	Invalid data the program should detect and reject (wrong type or out of range)
Trace table	A table used to manually track variable values as an algorithm is dry-run step by step

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Why Testing Matters

Types of Errors

Test Data

Trace Tables for Debugging

Debugging Strategies

Worksheet — 2.1 Testing and Debugging

🎯 Mini Test