How should comments be used to support code without duplicating what the code already expresses?

• A. They should restate every line of code to ensure comprehension.
• B. They should explain why something is done, document intent, edge cases, and important decisions.
• C. They should be avoided to keep code minimal.
• D. They should replace the need for good variable names.

Answer: (B)

Comments should capture why something is done, document intent, edge cases, and important decisions. They add context that isn’t obvious from the code itself, such as the rationale for choosing a particular approach, the constraints the code relies on, and any assumptions or invariants that must hold. This helps someone reading the code later understand what problem the solution is solving and why a specific path was taken, especially when the code uses clever tricks, special-case handling, or non-obvious algorithms. By focusing on intent and decisions rather than restating what the code already expresses, comments become a signal about design choices, potential risks, and future considerations, which is invaluable during maintenance or refactoring. Keep comments close to the relevant code and concise, and update them if the code changes so they remain accurate. Avoid duplicating every line or using comments to compensate for unclear naming or poor structure; well-named identifiers and clear code reduce the need for extraneous commentary, while comments should complement that clarity by explaining purpose and context.