## Copyright (C) 2004 David Bateman & Andy Adler
##
## 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, write to the Free Software
## Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
## 02110-1301  USA

## -*- texinfo -*-
## @deftypefn {Function File} {@var{y} =} speye (@var{m})
## @deftypefnx {Function File} {@var{y} =} speye (@var{m}, @var{n})
## @deftypefnx {Function File} {@var{y} =} speye (@var{sz})
## Returns a sparse identity matrix. This is significantly more
## efficient than @code{sparse (eye (@var{m}))} as the full matrix
## is not constructed.
##
## Called with a single argument a square matrix of size @var{m} by
## @var{m} is created. Otherwise a matrix of @var{m} by @var{n} is
## created. If called with a single vector argument, this argument 
## is taken to be the size of the matrix to create.
## @end deftypefn

function s = speye(m,n)
  if (nargin == 1)
    if (isvector (m) && length(m) == 2)
      n = m(2);
      m = m(1);
    elseif (isscalar (m))
      n = m;
    else
      error ("speye: invalid matrix dimension");
    endif
  else
    if (!isscalar (m) || !isscalar (n))
      error ("speye: invalid matrix dimension");
    endif
  endif

  lo = min([m,n]);
  s = sparse(1:lo,1:lo,1,m,n);
endfunction

%!assert(issparse(speye(4)))
%!assert(speye(4),sparse(1:4,1:4,1))
%!assert(speye(2,4),sparse(1:2,1:2,1,2,4))
%!assert(speye(4,2),sparse(1:2,1:2,1,4,2))
%!assert(speye([4,2]),sparse(1:2,1:2,1,4,2))