vech.m

JAVA 数学程序库 提供常规的数值计算程序包

## Copyright (C) 1995, 1996  Kurt Hornik
##
## This file is part of Octave.
##
## Octave 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, or (at your option)
## any later version.
##
## Octave 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 Octave; see the file COPYING.  If not, write to the Free
## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301, USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} vech (@var{x})
## Return the vector obtained by eliminating all supradiagonal elements of
## the square matrix @var{x} and stacking the result one column above the
## other.
## @end deftypefn

## See Magnus and Neudecker (1988), Matrix differential calculus with
## applications in statistics and econometrics.

## Author KH <[e-mail]>
## Created: 8 May 1995
## Adapted-By: jwe

function v = vech (x)

  if (nargin != 1)
    print_usage ();
  endif

  if (! issquare (x))
    error ("vech: x must be square");
  endif

  ## This should be quicker than having an inner `for' loop as well.
  ## Ideally, vech should be written in C++.
  n = rows (x);
  v = zeros ((n+1)*n/2, 1);
  count = 0;
  for jj = 1 : n
    ii = jj : n;
    v (count + ii) = x (ii, jj);
    count = count + n - jj;
  endfor

endfunction

/*
@GROUP
LinearAlgebra
@SYNTAX
vech()
@DOC
.
@EXAMPLES
<programlisting>
</programlisting>
@NOTES
@SEE
*/