cs-027 — The Ghost Duplicate
| Field | Value |
|---|---|
| id | cs-027 |
| difficulty | Intermediate |
| topics | debugging, state |
| lecture | lecture-07 |
| estimated time | 5-minute attempt + 10-minute discussion |
| prerequisites | loops, lists, and tracing from lectures 1-6 |
| objective | choose the technique, trace it faithfully, and defend the boundary behaviour |
| solution | held by the instructor; discussed after the attempt |
Problem
A task list occasionally shows one task twice after edits. Reason about shared references / aliasing as the cause and design the experiment that proves 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: tasks edited twice; list shows one task twice after edit 2
- Output: aliasing: the same list object referenced twice; is-check (a is b) proves it
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
- Check identity, not equality: two entries that ARE the same object vs two equal objects.
- 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: "Write the one-line commit message that explains the mechanism, not the symptom."
Constraints and assumptions
- Difficulty: Intermediate (loops and lists assumed).
- Topics: debugging, state.
- Classroom-scale inputs unless the problem says otherwise.
- Time-box: about five minutes for the plan and trace; finishing the details is a bonus, defending the method is the target.
- 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.