Subtract a column vector from matrix at specified vector of columns using only broadcast

Question

I want to subtract a column vector from a numpy matrix using another vector which is index of columns where the first column vector needs to be subtracted from the main mat

Answer 1

One can use bincount and outer

>>> M - np.outer(V, np.bincount(I, None, M.shape[1]))
array([[ 0,  1,  0, -3],
       [ 1,  0,  0, -3],
       [ 0,  0,  1, -3],
       [ 1,  0,  0, -3],
       [ 0,  0,  0, -2]])

or subtract.at

>>> out = M.copy()
>>> np.subtract.at(out, (np.s_[:], I), V[:, None])
>>> out
array([[ 0,  1,  0, -3],
       [ 1,  0,  0, -3],
       [ 0,  0,  1, -3],
       [ 1,  0,  0, -3],
       [ 0,  0,  0, -2]])