## Copyright (C) 1996, 1998, 2000, 2002, 2004, 2005, 2007
##               Auburn University.  All rights reserved.
##
## This file is part of Octave.
##
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## under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 3 of the License, or (at
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## 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, see
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## -*- texinfo -*-
## @deftypefn {Function File} {[@var{num}, @var{den}] =} zp2tf (@var{zer}, @var{pol}, @var{k})
## Converts zeros / poles to a transfer function.
##
## @strong{Inputs}
## @table @var
## @item zer
## @itemx pol
## Vectors of (possibly complex) poles and zeros of a transfer
## function.  Complex values must appear in conjugate pairs.
## @item k
## Real scalar (leading coefficient).
## @end table
## @end deftypefn

## Author: A. S. Hodel <a.s.hodel@eng.auburn.edu>
## (With help from students Ingram, McGowan.)

function [num, den] = zp2tf (zer, pol, k)

  if (nargin != 3)
    print_usage ();
  endif  

  ## Find out whether data was entered as a row or a column vector and
  ## convert to a column vector if necessary.

  [rp, cp] = size (pol);
  [rz, cz] = size (zer);

  if (! (isvector (zer) || isempty (zer)))
    error ("zer(%dx%d) must be a vector", rz, cz);
  elseif (! (isvector (pol) || isempty (pol)))
    error ("pol(%dx%d) must be a vector", rp, cp);
  elseif (length (zer) > length (pol))
    error ("zer(%dx%d) longer than pol(%dx%d)", rz, cz, rp, cp);
  endif

  ## initialize converted polynomials

  num = k;
  den = 1;

  ## call __zp2ssg2__ if there are complex conjugate pairs left, otherwise
  ## construct real zeros one by one.  Repeat for poles.

  while (! isempty (zer))
    if (max (abs (imag (zer))))
      [poly, zer] = __zp2ssg2__ (zer);
    else
      poly = [1, -zer(1)];
      zer = zer(2:length(zer));
    endif
    num = conv (num, poly);
  endwhile

  while (! isempty (pol))
    if (max (abs (imag (pol))))
      [poly, pol] = __zp2ssg2__ (pol);
    else
      poly = [1, -pol(1)];
      pol = pol(2:length(pol));
    endif
    den = conv (den, poly);
  endwhile

endfunction