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

This version is a slight variation of yours:

import numpy as np

def utri2mat(utri):
    n = int(-1 + np.sqrt(1 + 8*len(utri))) // 2
    iu1 = np.triu_indices(n)
    ret = np.empty((n, n))
    ret[iu1] = utri
    ret.T[iu1] = utri
    return ret

I replaced

    ret = ret + ret.transpose() - np.diag(ret.diagonal())

with an assignment of utri directly to the transpose of ret:

    ret.T[iu1] = utri

I also removed the argument ntotal and instead computed what n has to be based on the length of utri.