Strange Bugs

I recently went to an introductory course about the new UCL-Dell supercomputer Legion. It was a 2-day course, with plenty of presentations about optimizing/parallelizing code, and some hands-on. Other than a few quirks (the system is still being tested), it worked great- and it’s very exciting to have that (huge) amount of computational power available. The people are also very nice- more info on them here.

Of course (just like the Condor pool in the deparment), these systems aren’t really made for Java. So recently I’ve been porting all my Java collaborative filtering code to C++ (Note to Java haters: it worked fast/don’t be a hater). When I first ran my kNN collaborative filtering experiments, I noticed that the C++ version produced less accurate results (on the MovieLens dataset) than the Java implementation. So the long hunt for what was wrong began, and it turned out to be a very strange.

Here’s the gist of it: any user-user kNN prediction in collaborative filtering is made by a weighted average of neighbour ratings. You take your neighbour’s ratings, subtract the respective user’s mean from each rating, do an average- weighting each rating by similarity (or trust!)- and then add your own mean to that. So neighbours who have not rated the item in question do not contribute to the prediction. In my C++ implementation, neighbours who have nothing to say (rating – mean = 0) did not contribute to the prediction. Result: less accuracy. Lesson learned: the importance in predicting is not only how much your neighbours like or dislike the item, but also whether they have rated it or not! A shrug is just as much of a judgement as a thumbs up or down. (Ok, maybe a silly mistake to make- but I’ve never seen this written explicitly anywhere!)

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