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Scale.ai·Software Engineer·Take-home Assignment·Intermediate

Intermediate
May 2026

Summary

Scale.ai SWE debug round, last of the assign project section. One bug left and it was a loop state issue, the kind that feels obvious in hindsight but takes a second to see when you're under the clock.

Questions Asked (1)

Q1

In a loop where a per-iteration variable (counter, accumulator, or temporary set) is not being reset at the start of each iteration, identify the bug, fix the reset, and verify each iteration runs from a clean state.

Algorithms & Data StructuresRoot Cause Analysis
Author's notes

Took me longer than I'd like to admit.

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AI HintsAI Generated

Suggested Approach

First, reproduce the bug by tracing the loop with a small example to show how the stale variable causes incorrect results. Then, identify the variable that should be reset and move its initialization inside the loop, ensuring each iteration starts with a clean state. Finally, verify the fix by re-tracing and testing edge cases to confirm correctness.

Pro tip: Demonstrate a systematic debugging process: use print statements or a debugger to observe variable states across iterations, and mention that you'd add a regression test to prevent similar bugs. This shows you not only fix the issue but also improve code quality.

1. Reproduce and Identify

Create a minimal example that triggers the bug and trace the variable's value across iterations to pinpoint where it should be reset.

2. Analyze Root Cause

Explain why the variable persists across iterations (e.g., declared outside the loop) and how that leads to incorrect accumulation or state leakage.

3. Implement Fix

Move the variable's initialization inside the loop (or reset it at the start of each iteration) to ensure a clean state.

4. Verify Correctness

Re-run the example and additional test cases, checking that each iteration now starts fresh and the overall output is correct.

5. Prevent Recurrence

Suggest adding a unit test or code review practice to catch similar scope-related bugs in the future.

Key Points to Mention

  • Variable scope and lifetime in loops (e.g., declaring inside vs. outside the loop).
  • Common symptoms: incorrect accumulation, stale data, or unexpected results in later iterations.
  • Debugging techniques: print statements, debugger, or logging to inspect variable state per iteration.
  • The importance of resetting accumulators, counters, and temporary collections at the start of each iteration.
  • Testing strategies: unit tests with multiple iterations and edge cases (e.g., zero iterations, single iteration).
  • Code readability and maintainability: keeping variable declarations close to their usage to avoid such bugs.

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