Vectorized groupby with NumPy

Question

Pandas has a widely-used groupby facility to split up a DataFrame based on a corresponding mapping, from which you can apply a calculation on each subgroup and recombine the

Answer 1

There's probably a faster way than this (both of the operands are making copies right now), but:

np.bincount(np.broadcast_to(groups, X.T.shape).ravel(), X.T.ravel())

array([ 15.,  30.])