packmat

 PACKMAT assumes a symmetric matrix on input and returns its 'L' packed 
 representation i.e., a dense vector of length n*(n+1)/2 set up
 by columns of the lower triangle of A

0001 function [Ap] = packmat(A)
0002 % PACKMAT assumes a symmetric matrix on input and returns its 'L' packed
0003 % representation i.e., a dense vector of length n*(n+1)/2 set up
0004 % by columns of the lower triangle of A
0005 
0006 % This file is a part of PENLAB package distributed under GPLv3 license
0007 % Copyright (c) 2013 by  J. Fiala, M. Kocvara, M. Stingl
0008 % Last Modified: 27 Nov 2013
0009 
0010   [n m] = size(A);
0011   if (n~=m)
0012     error('Input matrix needs to be square.')
0013   end
0014 
0015   Ap = zeros(n*(n+1)/2,1);
0016   offset=1;
0017   for j=1:n
0018     len=n-j;
0019     Ap(offset:offset+len)=A(j:n,j);
0020     offset=offset+len+1;
0021   end
0022