How to multiply a given row `i` or column `j` with a scalar?

Question

import numpy as np

M = np.matrix([
        [-1,-2,-3],
        [-4,-5,-6]
    ])

print(M)

1. How to multiply a given row i or column j with a scalar?
2. How to acces a given column or row as a list?
3. How to set a given column or row, given a list (of the appropiate length)?

Answer 1

To multiply a particular column:

M[:,colnumber] *= scalar

Or a row:

M[rownumber,:] *= scalar

And of course, accessing them as an iterable is the same thing:

col_1 = M[:,1]

Although, that gives you a new matrix, not a list. Although, honestly, I can't quite seem to figure out all of these operations with matrix objects -- And these don't really seem like matrix type operations. Is there a reason that you're using matrix instead of array objects? If you want matrix multiplication, you can always use np.dot(array_mat1, array_mat2)