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Openai·Machine Learning Engineer·Technical Phone Screen·Senior

Senior
Apr 2026

Summary

Interviewed for an ML Engineer role at OpenAI and got hit with a NumPy vectorization puzzle. Pretty niche stuff but makes sense given the role. Left feeling okay about it but not totally confident.

Questions Asked (1)

Q1

Solve an array manipulation problem using only vectorized NumPy operations (broadcasting, slicing, boolean masking) with no explicit Python loops, then discuss the time and space complexity of your solution.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

This tripped me up more than I expected.

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AI HintsAI Generated

Suggested Approach

First, restate the problem and clarify the expected input/output shapes and constraints. Then, walk through a vectorized solution using broadcasting, slicing, and boolean masking, explaining each operation. Finally, analyze the time and space complexity, noting that vectorized operations are O(n) in time and may use O(n) extra space, but with lower constant factors than Python loops.

Pro tip: Emphasize that vectorization shifts work to optimized C loops, but beware of temporary arrays that can blow up memory; mention in-place operations or chunking when appropriate.

1. Clarify the problem

Restate the problem in your own words, confirm input/output shapes, data types, and any constraints (e.g., in-place, memory limits).

2. Outline the vectorized approach

Describe how to use broadcasting, slicing, and boolean masking to achieve the manipulation without explicit loops. Mention specific NumPy functions if helpful.

3. Walk through the implementation

Explain the code step-by-step, highlighting how each operation avoids Python-level iteration and leverages NumPy's optimized routines.

4. Analyze complexity

Discuss time complexity (typically O(n) for element-wise operations) and space complexity, noting any temporary arrays created and their impact.

5. Discuss trade-offs and edge cases

Mention potential memory overhead, alternatives (e.g., in-place ops, chunking), and how the solution handles edge cases like empty arrays or broadcasting mismatches.

Key Points to Mention

  • Broadcasting rules and how they enable element-wise operations without loops
  • Boolean masking for conditional selection or assignment
  • Slicing and views vs. copies: memory implications
  • Time complexity: O(n) for vectorized operations, but with lower constant factors
  • Space complexity: O(n) for output and possible O(n) for temporaries; discuss in-place options
  • Trade-offs: vectorization vs. readability, memory usage, and performance for large arrays

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