What does it mean that a sort is stable, and why does it matter?

Deep dive

A sort is *stable* if elements with equal sort keys keep their original relative order. Stability doesn’t change whether the final sequence is correctly ordered by the key; it changes what happens inside ties.

Why it matters

• Multi-key sorting: sort by secondary key first, then sort by primary key with a stable sort.
• UX: when many items tie, stability avoids “jumping” rows between refreshes.

Example

Input: (dept=A, name=Z), (dept=A, name=B)
Sort by dept only:
- Stable => Z stays before B (same as input)
- Unstable => the two may swap

Common pitfalls

• Assuming your language’s default sort is stable without checking docs.
• Relying on stability to simulate multi-key sorting, then switching to an unstable algorithm (e.g., some QuickSort variants).