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findsym.m

## Copyright (C) 2003 Willem J. Atsma
##
## 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, or (at your option) any later version.
##
## This software 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 software; see the file COPYING.  If not,
## see <http://www.gnu.org/licenses/>.

## -*- texinfo -*-
## @deftypefn {Function File} {} @var{vars} = findsym (@var{f}, @var{n})
## Find symbols in expression @var{f} and return them comma-separated in
## string @var{vars}. The symbols are sorted in alphabetic order. If @var{n}
## is specified, the @var{n} symbols closest to "x" are returned.
##
## Example:
## @example
## symbols
## x=sym("x"); y=sym("y"); f=x^2+3*x*y-y^2;
## vars = findsym (f);
## vars2 = findsym (f,1);
## @end example
##
## This is intended for m****b compatibility, calls findsymbols().
## @end deftypefn
## @seealso{findsymbols}

## Author: Willem J. Atsma <watsma(at)users.sf.net>
## Created: 18 April 2003

function VARS = findsym(F,Nout)

symlist = findsymbols(F);
Nlist = length(symlist);
if Nlist==0
      warning("No symbols were found.")
      VARS = "";
      return
endif

if exist("Nout")!=1
      VARS = disp(symlist{1});
      for i=2:Nlist
            VARS = ;
      endfor
      return
else
      # If Nout is specified, sort anew from x.
      symstrings = disp(symlist{1});
      for i=2:Nlist
            symstrings = ;
      endfor

      symasc = toascii(symstrings);
      
      if Nlist<Nout
            warning("Asked for %d, variables, only %d found.",Nout,Nlist);
            Nout=Nlist;
      endif
      symasc(:,1) = abs(toascii("x")-symasc(:,1));

      # Sort by creating an equivalent number for each entry
      Nc = length(symasc(1,:));
      powbase=zeros(Nc,1); powbase(Nc)=1;
      for i=(Nc-1):-1:1
            powbase(i) = powbase(i+1)*128;
      endfor
      [xs,I]=sort(symasc*powbase);
      
      VARS = deblank(symstrings(I(1),:));
      
      for i=2:Nout
            VARS = ;
      endfor

endif
% test the example
%!test
%! symbols
%! x=sym("x"); y=sym("y"); f=x^2+3*x*y-y^2;
%! vars = findsym (f);
%! assert(vars, 'x,y');
%! vars2 = findsym (f,1);
%! assert(vars2, 'x');


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