Instacart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jul 2026

Summary

Debugging round at Instacart for a software engineering role. One question, one bug, pretty contained scope but the detail that tripped people up was subtle enough that I can see why it makes a good filter.

Questions Asked (1)

You're given a function that should return an athlete's personal best score across completed attempts of an event. A test expects 17 but the function returns 13. Find the bug and fix it.

Root Cause AnalysisAlgorithms & Data Structures

Author's notes

The fix itself is one line basically, but I spent way too long staring at the score comparison logic before I even thought to check whether the function was filtering on attempt status.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the function's expected behavior and the test case. Then, systematically trace the code with the given inputs to identify where the logic deviates from the expected result, focusing on common pitfalls like filtering, initialization, or comparison errors. Finally, propose a fix and verify it against the test case and edge cases.

Pro tip: Demonstrate a methodical debugging process: verbalize your assumptions, test them with small examples, and consider edge cases like empty inputs or negative scores. This shows maturity and thoroughness beyond just finding the bug.

1. Understand the requirements

Restate the function's purpose: return the maximum score among completed attempts. Clarify what 'completed' means (e.g., status flag) and confirm the expected output for the given test case.

2. Trace the code with the test input

Walk through the function line by line using the specific input that yields 13 instead of 17. Track variable values and identify where the logic diverges from the expected behavior.

3. Identify the bug

Pinpoint the root cause, such as incorrect filtering (including incomplete attempts), wrong initialization (e.g., starting max at 0), or faulty comparison (e.g., using < instead of >).

4. Propose and implement a fix

Describe the necessary code change, such as adjusting the filter condition, initializing max to the first valid score, or correcting the comparison operator.

5. Verify the fix and consider edge cases

Re-run the test case to confirm it now returns 17. Also, think about edge cases like no completed attempts, all scores negative, or ties, and ensure the fix handles them correctly.

Key Points to Mention

Filtering logic: ensure only completed attempts are considered (e.g., status == 'completed').
Initialization of the maximum variable: avoid initializing to 0 if scores can be negative; use the first valid score or -Infinity.
Comparison operator: use > to update max when a higher score is found.
Edge cases: empty list of completed attempts, negative scores, and multiple attempts with the same score.
Testing: validate the fix with the given test case and additional cases to prevent regressions.
Communication: clearly explain the debugging process and reasoning behind the fix.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.