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function [vU2,dU2] = eigsort(U,desI)
% E = EIGSORT(X,desI) is a vector containing the eigenvalues of a square
% matrix X, where E is sorted descendingly if desI=1 or ascendingly otherwise
%
% [V,D] = EIG(X) produces a diagonal matrix D of eigenvalues and a
% full matrix V whose columns are the corresponding eigenvectors so
% that X*V = V*D.
%
% Written 9/6/98 by Tao Zha
%
% Copyright (C) 1997-2012 Tao Zha
%
% This 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 3 of the License, or
% (at your option) any later version.
%
% It 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.
%
% If you did not received a copy of the GNU General Public License
% with this software, see <http://www.gnu.org/licenses/>.
%
[vU,dU]=eig(U);
dUvec = diag(dU); % If dU is SPD, use "abs" in front of diag. This is because,
% even though dU is supposed to be positive, the numerical solution
% for a near-zero eigenvalue can be negative.
% *******************
% Second thought: "abs" is not necessary, I think. If negative, it
% implies zero already -- thus the smallest number
[dUvec2,dUI2] = sort(dUvec); % ascending
if desI==1 % if descending is required
dUvec2 = flipud(dUvec2);
dUI2 = flipud(dUI2);
end
if nargout==1
vU2 = dUvec2;
dU2 = NaN;
else
vU2 = vU(:,dUI2);
dU2 = diag(dUvec2); % put it back to matrix form
end
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