numpy: multiply arrays rowwise

Question

I have those arrays:

a = np.array([      [1,2],      [3,4],      [5,6],      [7,8]])  b = np.array([1,2,3,4]) 

and I want them to multiply like so:

[[1*1, 2*1], [3*2, 4*2], [5*3, 6*3], [7*4, 8*4]] 

... basically out[i] = a[i] * b[i], where a[i].shape is (2,) and b[i] then is a scalar.

What's the trick? np.multiply seems not to work:

>>> np.multiply(a, b) ValueError: operands could not be broadcast together with shapes (4,2) (4) 

Answer 1

add an axis to b:

>>> np.multiply(a, b[:, np.newaxis]) array([[ 1,  2],        [ 6,  8],        [15, 18],        [28, 32]])