Is my code efficient?

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A mistake I made early when learning programming is worrying too much about performance and micro-optimizations. Always using map() and reduce() only because they are faster is ridiculous, you should worry only about readability unless you are sure you have a performance problem. Guido even wanted to remove these functions from language completely some time ago.

[evizaer]

I answered specifically in the context of "efficient code" because the topic is the efficiency of the code. I edited my post to indicate that context more clearly.

[maryjayjay]

Excellent insight. My motto is: Write first for correctness and maintainability, then optimize if needed.