cs-003 — Two Vending Machines
| Field | Value |
|---|---|
| id | cs-003 |
| difficulty | Intermediate |
| topics | assumptions, specification |
| lecture | lecture-01 |
| 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
Two machines sell the same drink for different prices; one accepts exact coins only, the other gives change. Write precise rules for which machine a student should use given the coins they hold.
Your deliverable is the stated assumptions, the rule list, the decision for the worked instance, and one contested case. 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: drink 2.00; student holds coins 2.00 exactly vs drink 2.30 with coins 2.50 (needs change)
- Output: exact-coin machine when coins match a price; change machine otherwise; never the exact machine if it means overpaying
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
- Rules must cover: exact match, overpay possible, overpay impossible, change machine sold out.
- Writing the rules IS the solution; code is transcription.
- Every assumption is a place where two good answers can differ.
Discussion starter
After the solution is revealed, the class spends five minutes on this prompt: "Propose one extra rule that would change the verdict — is it an improvement?"
Constraints and assumptions
- Difficulty: Intermediate (loops and lists assumed).
- Topics: assumptions, specification.
- 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: Re-solve with a second, different assumption and compare the answers.