Testing & Test Data

Spec & Goals 3 min

AQA Spec 3.2.11 · Robust and secure programming

By the end of this lesson you can:

1. Explain why testing makes a program robust.
2. Choose normal, boundary and erroneous test data for an input.
3. State the difference between a syntax error and a logic error.

Warm-Up 5 min

Last lesson you wrote validation to reject silly input. Today we test a program to prove it actually works.

Quick starter

A program should accept exam marks from 0 to 100. Which single value is most likely to expose a bug at the edge?

• 50
• 100
• 72

Key Concept — testing and kinds of test data 14 min

Testing runs a program with chosen data to find errors. A well-tested program is robust — it copes with unexpected input without crashing.

Three kinds of test data

For any input you should test all three kinds. They catch different mistakes.

Syntax errors vs logic errors

Two very different kinds of error:

A trace table records each variable as the program runs. It is the best tool for hunting a logic error.

Worked Example — test data and a trace table 12 min

Problem: a program accepts a test mark that must be in the range 0–100. Choose test data for it.

Now a buggy routine that should total marks 1, 2 and 3 (answer should be 6):

01  total ← 0
02  FOR i ← 1 TO 3
03      total ← total + i
04  ENDFOR
05  total ← total + 1
06  OUTPUT total

Trace it to find the logic error:

The loop already gives 6. Line 05 wrongly adds 1 more, so the output is 7. Removing line 05 fixes the logic error.

The corrected loop in Python

total = 0
for i in range(1, 4):
    total = total + i
print(total)

This corrected version outputs 6 — the loop alone gives the right total.

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

Recap & Key Terms 3 min

Testing finds errors using normal, boundary and erroneous data. A syntax error stops the program running; a logic error gives a wrong result. A trace table helps locate logic errors.

Testing

Running a program with planned data to find errors and check it works.

Normal test data

Typical, valid data the program should accept.

Boundary test data

Values at the edge of the allowed range, which should still be accepted.

Erroneous test data

Invalid data the program should reject.

Syntax error

An error that breaks the rules of the language, so the program will not run.

Logic error

An error where the program runs but produces the wrong result.

Homework 1 min

Task (≤ 15 min): A field accepts a day of the month from 1 to 31. Give one normal, two boundary and two erroneous test values, and say what should happen to each.