← Cohere Interview Insights

Cohere·Machine Learning Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
Jun 2026

Summary

Second round coding interview for an MLE role at Cohere. They asked me to implement top-K sampling using NumPy and I completely fell apart because I'd only practiced PyTorch. The interviewer was patient and dropped hints, but I left feeling like I'd wasted everyone's time.

Questions Asked (1)

Q1

Implement top-K sampling from a probability distribution using NumPy.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I froze.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

Start by clarifying the input format (logits or probabilities) and whether to return indices or sampled values. Then implement top-K filtering by selecting the K highest probabilities, zeroing out the rest, renormalizing, and sampling using np.random.choice. Discuss trade-offs like handling ties, numerical stability, and efficiency for large vocabularies.

Pro tip: Mention that in practice, top-K is often applied to logits before softmax to avoid underflow, and that using np.argpartition is more efficient than full sorting for large K. Also, note that sampling should be done with replacement=False if returning multiple samples, but typically it's one sample.

1. Clarify input and output

Ask whether the input is a probability distribution or logits, and whether the output should be the sampled index or value. Confirm if K is a fixed integer and if sampling is with or without replacement.

2. Select top-K elements

Use np.argpartition to find the indices of the K largest probabilities (or logits) in O(n) time, then optionally sort those K for determinism. Alternatively, use np.argsort if simplicity is preferred.

3. Filter and renormalize

Create a new array with only the top-K probabilities, set others to zero, and renormalize by dividing by the sum to ensure it sums to 1. If working with logits, apply softmax after filtering.

4. Sample from the filtered distribution

Use np.random.choice with the filtered probabilities to draw a sample. If returning multiple samples, specify replace=False and ensure K >= number of samples.

5. Discuss edge cases and trade-offs

Address cases like K > number of non-zero probabilities, ties in probabilities, and numerical stability. Mention time complexity and memory considerations for large distributions.

Key Points to Mention

  • Difference between top-K sampling and other sampling methods like top-p (nucleus) sampling.
  • Use of np.argpartition for efficient selection of top-K elements without full sort.
  • Renormalization step to ensure the filtered distribution sums to 1.
  • Handling of logits vs probabilities, including numerical stability (e.g., subtracting max logit before softmax).
  • Edge cases: K larger than vocabulary size, ties, and zero probabilities.
  • Trade-offs: deterministic vs stochastic sampling, and the effect of K on diversity vs quality.

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