Repair a Function and Keep Its Test
Verify a correction against ordinary, boundary and previously failing cases.
Supplied materials
- Paper and pencil; an adult may read aloud. No device, account or purchase is needed.
Learn the idea
A regression test is a saved test for behavior that should keep working after changes. Fixing one failing case can accidentally break another branch. A small test suite with explicit expected outputs gives evidence about the supplied cases; passing it does not prove correctness for every imaginable input.
Contract: for integer x from 0 through 10, label(x) returns low if x < 3, mid if 3 ≤ x < 7, and high otherwise. Faulty draft: if x < 3 return low; else if x ≤ 7 return mid; else high. The boundary test 7 → high fails. Repair the second comparison to x < 7, keeping the first branch unchanged.
- Write expected labels for 2, 3, 6, 7 and 8 before tracing.
- Run the faulty draft and mark each pass or fail.
- Repair ≤ 7 to < 7.
- Rerun all five cases and keep 7 → high as the regression test.
See the method in action
Faulty outputs for 2,3,6,7,8: low, mid, mid, mid, high. Only 7 fails its expected high label. Repaired outputs: low, mid, mid, high, high; all five supplied tests pass.
Check your reasoning
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See the answer and explanation
7 → high
The original error was including 7 in mid. Keeping 7 → high detects that same error if it reappears.
Take it further
Change the second boundary to 8: low if x < 3, mid if 3 ≤ x < 8, otherwise high. Expected results at 7,8,9: mid, high, high. Update expected outputs deliberately because the contract changed.
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