## Copyright (C) 2006 Michael Creel <michael.creel@uab.es> ## ## This program is free software; you can redistribute it and/or modify ## it under the terms of the GNU General Public License as published by ## the Free Software Foundation; either version 2 of the License, or ## (at your option) any later version. ## ## This program is distributed in the hope that it will be useful, ## but WITHOUT ANY WARRANTY; without even the implied warranty of ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the ## GNU General Public License for more details. ## ## You should have received a copy of the GNU General Public License ## along with this program; If not, see <http://www.gnu.org/licenses/>. ## -*- texinfo -*- ## @deftypefn {Function File} {} unvech (@var{v}) ## Performs the reverse of "vech". Generates a symmetric matrix from the lower ## triangular elements, received as a vector @var{v}. ## @end deftypefn # Note: this uses a double loop. A C version would be a lot faster for large matrices. ## Adapted-By: Ben Hall <benjamin.hall@pw.utc.com> function x = unvech (v) if (nargin != 1) usage ("unvech (v)"); endif if (! isvector(v)) usage ("unvech (v)"); endif # find out dimension of symmetric matrix p = length(v); g = -(1 - sqrt(1 + 8*p))/2; if (mod(g,1) != 0) error("unvech: the input vector does not generate a square matrix"); endif x = zeros(g,g); # fill in the symmetric matrix, the obvious way # k = 1; # for i = 1:g # for j = i:g # x(i,j) = v(k); # if (i != j) x(j,i) = v(k); endif # k = k + 1; # endfor # endfor # fill in the symmetric matrix, a more clever way ii = repmat( 1:g, [g 1] ); idx= find( ii <= ii' ); x(idx) = v; x = x + x' - diag(diag(x)); endfunction

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