## Copyright (C) 1996 Auburn University.  All rights reserved.
##
## 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, 59 Temple Place, Suite 330, Boston, MA 02111 USA.

## -*- texinfo -*-
## @deftypefn {Function File} {} zpout (@var{zer}, @var{pol}, @var{k}, @var{x})
## print formatted zero-pole form to the screen.
## @var{x} defaults to the string @code{"s"}
## @end deftypefn
## @seealso{polyval, polyvalm, poly, roots, conv, deconv, residue,
## filter, polyderiv, polyinteg, and polyout}

## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## Created: June 1995

function zpout (zer, pol, k, x)

  save_empty = empty_list_elements_ok;
  empty_list_elements_ok = 1;

  if (nargin < 3 ) | (nargin > 4) | (nargout != 0 )
    usage("zpout(zer,pol,k[,x])");
  endif

  if( !(isvector(zer) | isempty(zer)) | !(isvector(pol) | isempty(pol)) )
    error("zer, pol must be vectors or empty");
  endif

  if(!isscalar(k))
    error("zpout: argument k must be a scalar.")
  endif

  if (nargin == 3)
    x = "s";
  elseif( ! isstr(x) )
    error("zpout: third argument must be a string");
  endif

  numstring = num2str(k);

  if(length(zer))
    ## find roots at z,s = 0
    nzr = sum(zer == 0);
    if(nzr)
      if(nzr > 1)
        numstring = [numstring,sprintf(" %s^%d",x,nzr)];
      else
        numstring = [numstring,sprintf(" %s",x)];
      endif
    endif
    zer = sortcom(-zer);
    for ii=1:length(zer)
      if(zer(ii) != 0)
        numstring = [numstring,sprintf(" (%s %s)",x,com2str(zer(ii),1) ) ];
      endif
    endfor
  endif

  if(length(pol))
    ## find roots at z,s = 0
    nzr = sum(pol == 0);
    if(nzr)
      if(nzr > 1)
        denomstring = [sprintf("%s^%d",x,nzr)];
      else
        denomstring = [sprintf("%s",x)];
      endif
    else
      denomstring = " ";
    endif
    pol = sortcom(-pol);
    for ii=1:length(pol)
      if(pol(ii) != 0)
        denomstring = [denomstring,sprintf(" (%s %s)",x,com2str(pol(ii),1))];
      endif
    endfor
  endif

  len = max(length(numstring),length(denomstring));
  if(len > 0)
    y = strrep(blanks(len)," ","-");
    disp(numstring)
    if(length(denomstring))
      disp(y)
      disp(denomstring)
    endif
  else
    error ("zpout: empty transfer function")
  end

  empty_list_elements_ok = save_empty;

endfunction