cs-050 — Course Rosters

Problem

Given two course rosters, output students in both, either, and only one. Set operations as direct vocabulary.

Your deliverable is the mapping/set answers for the worked instance, the key choice justified, and the tie-break rule stated. 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: CS = {ada, bo, cy}; DS = {bo, dee, ada}
• Output: both: {ada, bo}; either: {ada, bo, cy, dee}; only CS: {cy}; only DS: {dee}

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

• Translate the English ('both', 'either', 'only') into set operators before coding.
• A dictionary turns 'search for the item' into 'look it up' — that is the whole win.
• Sets answer yes/no questions; dictionaries answer how-many questions.

Discussion starter

After the solution is revealed, the class spends five minutes on this prompt: "What breaks if two different real-world things map to the same key — and how would you detect it?"

Constraints and assumptions

• Difficulty: Foundational (early-semester techniques assumed: precise statements, traces, simple loops).
• Topics: sets, operations.
• Classroom-scale inputs unless the problem says otherwise.
• Time-box: about five minutes; naming the pattern and stating the assumption is already a pass, completing the calculation is the stretch.
• 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: Invert the lookup (value to keys) and state when collisions appear.