I am worrying that this might be a really stupid question. However I can't find a solution. I want to do the following operation in python without using a loop, because I am dealing with large size arrays. Is there any suggestion?
import numpy as np
a = np.array([1,2,3,..., N]) # arbitrary 1d array
b = np.array([[1,2,3],[4,5,6],[7,8,9]]) # arbitrary 2d array
c = np.zeros((N,3,3))
c[0,:,:] = a[0]*b
c[1,:,:] = a[1]*b
c[2,:,:] = a[2]*b
c[3,:,:] = ...
...
...
c[N-1,:,:] = a[N-1]*b
To avoid Python-level loops, you could use np.newaxis to expand a (or None, which is the same thing):
>>> a = np.arange(1,5)
>>> b = np.arange(1,10).reshape((3,3))
>>> a[:,None,None]*b
array([[[ 1, 2, 3],
[ 4, 5, 6],
[ 7, 8, 9]],
[[ 2, 4, 6],
[ 8, 10, 12],
[14, 16, 18]],
[[ 3, 6, 9],
[12, 15, 18],
[21, 24, 27]],
[[ 4, 8, 12],
[16, 20, 24],
[28, 32, 36]]])
Or np.einsum, which is overkill here, but is often handy and makes it very explicit what you want to happen with the coordinates:
>>> c2 = np.einsum('i,jk->ijk', a, b)
>>> np.allclose(c2, a[:,None,None]*b)
True
Collected from the Internet
Please contact [email protected] to delete if infringement.
Comments