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pythonnumpynumpy-ndarrayarray-broadcastingelementwise-operations

Element-wise multiply of multiple numpy 2d arrays

To simplify my question, let's say I have these arrays:

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

I would like to use element-wise multiplication on them so the result will be:

array([[  2,   4,  18],
       [ 48,  15, 108]])

I know I can do a*b*c, but that won't work if I have many 2d arrays or if I don't know the number of arrays. I am also aware of numpy.multiply but that works for only 2 arrays.

Solution

  • Use stack and prod.

    stack will create an array which can be reduced by prod along an axis.

    import numpy as np

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

unknown_length_list_of_arrays = [a, b, c]

d1 = a * b * c
stacked = np.stack(unknown_length_list_of_arrays)
d2 = np.prod(stacked, axis=0)

np.testing.assert_equal(d1, d2)