favorite favourite

What's your favourite standard library function?

Mine is Matlab's squeeze. Every time I use it I can imagine my hyperdimensional manifestation pushing the matrix until all those useless dimensions become shiny drops of acid that are slowly absorbed into the surrounding space.

Mine is Python's reduce. Every time I abuse it to perform iteration in a single expression, I can imagine the guy who'll have to maintain my code weeping at the horrible monstrosity I've created.