cs-025 — The Case of the Missing Sales
| Field | Value |
|---|---|
| id | cs-025 |
| difficulty | Beginner |
| topics | debugging, hypotheses |
| lecture | lecture-07 |
| estimated time | 5-minute attempt + 10-minute discussion |
| prerequisites | none |
| objective | state inputs, outputs, and assumptions, then solve a small instance by hand |
| solution | held by the instructor; discussed after the attempt |
Problem
A receipt program prints a total that is too low by exactly one item's price. Form hypotheses, design the minimal reproducing input, and localise the fault.
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: items [2.00, 3.50, 1.25]; receipt printed 5.50
- Output: one item skipped (1.25 missing); repro input: 3 items, expected 6.75
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
- Difference between expected and printed equals exactly one item: hypothesise a skipped element, not rounding.
- 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: Beginner (no prior experience assumed).
- Topics: debugging, hypotheses.
- Classroom-scale inputs unless the problem says otherwise.
- Time-box: about five minutes for a first attempt; partial answers with clear reasoning are worth more than silent struggling.
- 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.