python - Efficient way to fill up a 4d array from entries of a product of two matrices -


title might not precise hoped, here problem. i'm filling 4d numpy array entries of product of 2 matrices. right code following :

m = p.dot(u) c_arr = np.zeros((b_size,b_size,n,n))   alpha in xrange(b_size):       beta in xrange(b_size):           in xrange(n):               j in xrange(n):                   c_arr[alpha,beta,i,j] = np.conjugate(m[i,alpha])*m[j,beta]

it turns out function called quite ofen , appears time-consumming. i'm beginning python , suspect there more efficient way write function avoiding loops, haven't been able figure out myself...

you can use numpy.einsum:

c = np.einsum('ia,jb->abij', m.conj(), m)

or, since there no actual sum being computed (i.e. form of outer product), can use numpy broadcasting regular array multiplication after reshaping input matrix m appropriately:

nrows, ncols = m.shape c = m.t.reshape(1, ncols, 1, nrows) * m.t.conj().reshape(ncols, 1, nrows, 1)

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