[ivanhoe]

For me it's OK to use i,j,k as loop counters, a and b for sorting algorithms, e for events or exceptions, and x,y,z for coordinates or in simple math operations. Everything beyond that is really out of "everyone is doing it" area, depends on your background, books/people you've been learning from, programming language, etc.

[manojlds]

Also, things like X, Y in ML. Explicitly calling this training_data etc will not really aid readers.