Choose tests that catch a rule error
Use ordinary and boundary inputs to check a small function.
Supplied materials
- Pseudocode for a fictional ticket classifier: if age < 12, return “child”; otherwise return “standard.” The supplied rule says ages below 12 are child tickets; age 12 and above are standard.
- Use only invented integer ages; do not submit your age or anyone else’s.
Learn the idea
A test combines an input with an expected output. Choose an ordinary example and examples at a boundary where the rule changes. A passing ordinary case does not prove that the boundary is implemented correctly.
The comparison < excludes the boundary; ≤ includes it. If a programmer accidentally replaces < 12 with ≤ 12, age 11 and age 13 still pass, but age 12 reveals the error.
- Write expected outputs for invented ages 11, 12 and 13.
- Trace the supplied < 12 classifier for each.
- Change the comparison to ≤ 12 and repeat.
- Identify the smallest test set here that distinguishes the two comparisons.
See the method in action
Correct rule: 11 → child, 12 → standard, 13 → standard. The mistaken ≤ rule labels 12 as child, so the test 12 → standard exposes the boundary error.
Check your reasoning
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See the answer and explanation
Age 12 should return standard.
Only the boundary input 12 has a different result under those two comparisons. Both rules classify the other supplied ages the same way.
Take it further
Add a requirement that negative ages are rejected rather than classified. Write a test for −1 and describe the new validation step needed.
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Longer courses for learners ages 14+
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