Copy flat list of upper triangle entries to full matrix?

Question

I have the upper triangle entries (including diagonal) of a symmetric matrix in a flat list (concatenated rows), and I want to use them to fill in the full matrix, including the

Answer 1

Assuming that you have a vector containing the upper triangular values of a symmetric matrix (n x n) then you can re-build the full matrix as follows:

import numpy as np

# dimension of the full matrix
n = 80

# artificial upper triangle entries n(n-1) / 2 if matrix is symmetric
entries = np.array(range((80*79) / 2))

full_matrix = np.zeros((n,n))
inds = np.triu_indices_from(full_matrix, k = 1)
full_matrix[inds] = entries
full_matrix[(inds[1], inds[0])] = entries

print(full_matrix)

Verify:

np.allclose(full_matrix, full_matrix.T)