The assign-then-update loop is ten lines; the signal is whether you handle the three things that bite in production: initialization, an empty cluster, and a real convergence test instead of a fixed iteration count.
Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
The assign-then-update loop is ten lines; the signal is whether you handle the three things that bite in production: initialization, an empty cluster, and a real convergence test instead of a fixed iteration count.
Interviewers are not checking whether you remember the formula. They are checking whether you know k-means minimizes within-cluster squared distance, that random init gives you a bad local optimum often enough that k-means++ is the default, and that an empty cluster will crash a naive mean. A candidate who hardcodes 100 iterations and never mentions convergence has written a demo, not an algorithm.
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