What is the "hashing trick"? - MetaOptimize Q+ALet's say we want to design a function v = phi(x), which from a d-dimensional vector x = (x(1), x(2), ..., x(d)) outputs a new m-dimensional vector v, with m either greater or smaller than d. In other words, phi can be used either for reducing dimensionality of x (d > m) or for sparsifying x (m > d). One way to do so is to use a hash function h to map x(1) to v(h(1)), x(2) to v(h(2)), ..., x(d) to
