def from_tri_2_sym(tri, dim):
    """convert a upper triangular matrix in 1D format
       to 2D symmetric matrix


    Parameters
    ----------

    tri: 1D array
        Contains elements of upper triangular matrix

    dim : int
        The dimension of target matrix.


    Returns
    -------

    symm : 2D array
        Symmetric matrix in shape=[dim, dim]
    """
    symm = np.zeros((dim, dim))
    symm[np.triu_indices(dim)] = tri
    return symm
convert a upper triangular matrix in 1D format
       to 2D symmetric matrix


    Parameters
    ----------

    tri: 1D array
        Contains elements of upper triangular matrix

    dim : int
        The dimension of target matrix.


    Returns
    -------

    symm : 2D array
        Symmetric matrix in shape=[dim, dim]