Multiply each number of an array with its aligned row of a matrix in numpy?

Question

Suppose I have an arr = (n item array) and a matrix mat = nxm matrix. I want to multiply each item of arr with its aligned row of the matrix mat. How can I do it at numpy?

Answer 1

You would use broadcasting for this:

M = np.arange(1, 13).reshape(3, 4)
print M
# [[ 1  2  3  4]
#  [ 5  6  7  8]
#  [ 9 10 11 12]]

a = np.arange(1, 4)
print a
# [1, 2, 3]

# we broadcast a (3, 1) vector against a (3, 4) matrix
print a[...,None].shape, M.shape
# (3, 1) (3, 4)

print a[..., None] * M
# [[ 1  2  3  4]
#  [10 12 14 16]
#  [27 30 33 36]]