Edexcel 4CP0 · Topic 2 · 2.1

Testing &
Debugging

Types of testing · Test data · Error types · Debugging

CSZone Edexcel iGCSE Computer Science 4CP0

Why Test Programs?

Finding and fixing problems

"Testing is the process of running a program with a range of test data to check it produces the correct output and behaves as expected under all conditions."

GOALS OF TESTING

Verify the program works as intended

Find errors (bugs) before the user does

Confirm the program handles all types of input correctly

IF NOT TESTED PROPERLY

Programs crash with unexpected inputs

Security vulnerabilities left open

Users lose trust / data corrupted

Test Data Types

Three types of test data

✅

Normal (Valid)

Typical, expected input. The program should produce the correct, expected output.

Age: 16, Score: 75

⚠️

Boundary (Edge)

At the very edge of valid range. Tests the limits — both inside and just outside the boundary.

Age: 0, 17, 18, 120

❌

Erroneous (Invalid)

Data the program should reject. Tests that invalid input is handled gracefully with an error message.

Age: -5, "hello", 999

Test Table Example

Testing an age input (valid range: 16–65)

Test data	Type	Expected result
25	Normal	Accepted
40	Normal	Accepted
16	Boundary (lower)	Accepted
65	Boundary (upper)	Accepted
15	Boundary (just below)	Rejected
66	Boundary (just above)	Rejected
-1	Erroneous	Error message
"abc"	Erroneous	Error message

Types of Errors

Three types of programming error

SYNTAX ERROR

Breaking the rules of the programming language. Caught before the program runs. Example: WHILE x = 5 with no ENDWHILE, or a missing bracket.

LOGIC ERROR

The program runs but produces incorrect output. The code is legal but the algorithm is wrong. Example: using + instead of * to calculate area.

RUNTIME ERROR

The program crashes while running. Often caused by dividing by zero, accessing an invalid index, or running out of memory.

Testing Approaches

White Box vs Black Box Testing

WHITE BOX TESTING

🔍

Tester can see the code. Tests are designed to execute all branches and paths through the code. Also called structural testing. Done by developers.

BLACK BOX TESTING

📦

Tester cannot see the code. Tests based only on the expected inputs and outputs (specification). Done by users or independent testers.

Debugging

Finding and fixing bugs

DEBUGGING TECHNIQUES

Trace tables — manually follow algorithm step by step

Print statements — output variable values at key points

Breakpoints — pause execution at specific lines in a debugger

Code walkthroughs — review code with another person

ITERATIVE TESTING

Test throughout development, not just at the end. Each iteration: write code → test → fix → test again. This catches errors early when they are cheapest to fix.

Exam Practice

Question 1

A program accepts ages between 0 and 120. State one suitable boundary test value and explain why it is a boundary value.

2 marks

✓ MARK SCHEME

1 mark

Any of: 0, 120, -1, 121 (or 1, 119)

1 mark

It is at (or just outside) the edge of the valid range / it is the minimum (or maximum) acceptable value / tests the boundary condition of the validation

Exam Practice

Question 2

State the difference between a syntax error and a logic error. Give an example of each.

4 marks

✓ MARK SCHEME

Syntax def

Breaking the rules of the programming language / prevented from running

Syntax ex.

Missing END IF / misspelled keyword / missing bracket

Logic def

Program runs but gives incorrect results / algorithm is wrong

Logic ex.

Using + instead of * / wrong condition in IF / loop counting wrong number of times

Common Mistakes

Don't lose marks

1

Confusing boundary and erroneous data. Boundary data is at the very edge of the valid range (e.g. age = 0 or 120). Erroneous data is clearly outside/invalid (e.g. age = -5 or "abc").

2

Saying syntax errors happen while the program is running. Syntax errors are caught before execution by the compiler/interpreter. Runtime errors crash the program during execution.

3

Forgetting that boundary testing includes values just outside the boundary. For a range 0–120, you should test -1 and 121 as boundary values, not just 0 and 120.

Topic Summary — 2.1

Key Takeaways

TEST DATA TYPES

Normal — typical valid input
Boundary — at the edges of valid range
Erroneous — data that should be rejected

TESTING TYPES

White box — can see code
Black box — cannot see code (spec only)

ERROR TYPES

Syntax — breaks language rules, stops before running
Logic — runs but wrong output
Runtime — crashes during execution

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Next Lesson

2.2

Sequence, Selection & Iteration

Program control structures in Edexcel pseudocode

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