how to 3-way outer product in numpy?

Question

About the numpy.outer [link] .

Given two vectors, a = [a0, a1, ..., aM] and b = [b0, b1, ..., bN], the outer product will be M*N matrix.

Answer 1

You can use numpy.ix_; it adds axes to its operands such that they form an open grid. Below the shapes of A,B,C are (2, 1, 1), (1, 3, 1) and (1, 1, 4), so just multiplying them together results in the outer product.

a = np.arange(1, 3)
b = np.arange(1, 4)
c = np.arange(1, 5)
A,B,C = np.ix_(a,b,c)
A*B*C
# array([[[ 1,  2,  3,  4],
#         [ 2,  4,  6,  8],
#         [ 3,  6,  9, 12]],
#
#        [[ 2,  4,  6,  8],
#         [ 4,  8, 12, 16],
#         [ 6, 12, 18, 24]]])