Count number of 1's in binary representation

Question

Efficient way to count number of 1s in the binary representation of a number in O(1) if you have enough memory to play with. This is an interview question I found on an onli

Answer 1

I had to golf this in ruby and ended up with

l=->x{x.to_s(2).count ?1}

Usage :

l[2**32-1] # returns 32

Obviously not efficient but does the trick :)