cs-028 — The Heisenbug
| Field | Value |
|---|---|
| id | cs-028 |
| difficulty | Advanced |
| topics | debugging, method |
| lecture | lecture-07 |
| estimated time | 5-minute attempt + 10-minute discussion |
| prerequisites | functions, dictionaries, and earlier algorithmic patterns |
| objective | design the algorithm, verify it on edges, and argue correctness or trade-offs |
| solution | held by the instructor; discussed after the attempt |
Problem
A defect that disappears when a debug print is added. Discuss causes (ordering, mutation during iteration, hidden dependency) and design a strategy to catch it.
Your deliverable is the minimal failing input, the ranked hypotheses, the located mechanism, the minimal fix, and a regression test. If the problem is ambiguous anywhere, state the assumption you are making and carry on — a stated assumption is part of a good answer.
Examples
- Input: defect vanishes when print(i) is added to the loop
- Output: candidates: loop mutates the list it iterates; set iteration order; time-dependent state
Solve for the worked instance first. Then ask what would change if one number, name, or condition were replaced by something stranger.
What to notice
- Print changes timing, not logic - unless the loop iterates something the print rebuilds; enumerate while mutating is the classic.
- A bug is a claim about your mental model; the failing input is the experiment.
- The fix should be smaller than the explanation of the bug.
Discussion starter
After the solution is revealed, the class spends five minutes on this prompt: "Which hypothesis did you rank first, and what evidence moved it down the list?"
Constraints and assumptions
- Difficulty: Advanced (functions, dictionaries, and algorithmic reasoning assumed).
- Topics: debugging, method.
- Classroom-scale inputs unless the problem says otherwise.
- Time-box: about five minutes for a defensible plan; a complete solution may take longer and that is expected - state where you would resume.
- Optional stretch (not required): describe an alternative approach and say when it beats the obvious one.
Hints and extension
- First nudge: identify the pattern the worked instance uses before computing anything.
- If stuck after three minutes, write the input/output line you expect and trace the smallest example by hand.
- After the reveal, attempt the extension below before the next lecture.
- Extension challenge: Design the smallest input that still exposes the defect you fixed.