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

You can use slicing to accomplish this:

>>> M = np.matrix([
...         [-1,-2,-3],
...         [-4,-5,-6]
...     ])
>>> M[1,:] *= 2          # multiply all elements in second row by 2
>>> M
matrix([[ -1,  -2,  -3],
        [ -8, -10, -12]])
>>> M[:,1] *= 2          # multiply all elements in second column by 2
>>> M
matrix([[ -1,  -4,  -3],
        [ -8, -20, -12]])

To assign a given column or row to a list:

>>> M[:,1] = [[0], [0]]        # note the nested lists to reassign column
>>> M
matrix([[ -1,   0,  -3],
        [ -8,   0, -12]])
>>> M[1,:] = [2, 2, 2]         # flat list to reassign row
>>> M
matrix([[-1,  0, -3],
        [ 2,  2,  2]])