cs-041 — The Queue Reorder

Problem

Move the last k people of a queue to the front, preserving order. Slicing plus concatenation; discuss negative indices.

Your deliverable is the rearranged list for the worked instance, the slice arithmetic, and a note on what aliasing would change. 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: queue [A, B, C, D, E], k = 2 (last 2 to front)
• Output: [D, E, A, B, C]; k = 0 -> same; k = 5 -> same; k = 7 -> same (wrap)

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

• q[-k:] + q[:-k] with k = k % n; test k=0 before celebrating (negative slice surprises).
• Slices are half-open: they make boundaries explicit instead of hidden in loop conditions.
• Aliasing is the difference between copying a list and copying a name.

Discussion starter

After the solution is revealed, the class spends five minutes on this prompt: "Find the smallest change to the task that makes the slice solution wrong."

Constraints and assumptions

• Difficulty: Beginner (no prior experience assumed).
• Topics: lists, slicing.
• 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: Convert your in-place solution to a copying one and compare the trade-offs.