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The shorthand “WTF” is common among developers. It does not actually stand for “worse than failure” but I run a clean ship here people. [Hint: let's just say that WTF is a somewhat crude interrogative. Sample usage: "Dude, WTF?" or, if you prefer, comic form.]

I just spent two days looking for a bug in my code whose fix was simply replacing

A = A + np.dot(features, features - env.gamma * newfeatures)

with

A = A + np.outer(features, features - env.gamma * newfeatures)

In my defense, I didn’t spend two full days looking for this mistake, but the process was so demoralizing that I couldn’t find much willpower to do productive work when I wasn’t actively debugging.

Takeaway lesson, triple check your linear algebra. If you get something wrong, you’re likely to have a hard time figuring out why, or worse, you may mistake the junk for the correct answer. [Aside: note that I was really burned by the fact that '+' is overloaded in this case, and so is valid for both matrix, matrix addition and scalar, matrix addition. Despite the scorch marks, I do like this feature.]

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