numpy: efficiently summing with index arrays

Question

Suppose I have 2 matrices M and N (both have > 1 columns). I also have an index matrix I with 2 columns -- 1 for M and one for N. The indices for N are unique, but the indic

Answer 1

Using also m_ind, n_ind = w.T, just do M += np.bincount(m_ind, weights=N[n_ind], minlength=len(M))