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+## Copyright (C) 1996, 1998 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} {} tf (@var{num}, @var{den}, @var{tsam}, @var{inname}, @var{outname})
+## build system data structure from transfer function format data
+##
+## @strong{Inputs}
+## @table @var
+## @item  num
+## @itemx den
+## coefficients of numerator/denominator polynomials
+## @item tsam
+## sampling interval. default: 0 (continuous time)
+## @item inname
+## @itemx outname
+## input/output signal names; may be a string or cell array with a single string
+## entry.
+## @end table
+##
+## @strong{Outputs}
+## @var{sys} = system data structure
+##
+## @strong{Example}
+## @example
+## octave:1> sys=tf([2 1],[1 2 1],0.1);
+## octave:2> sysout(sys)
+## Input(s)
+##         1: u_1
+## Output(s):
+##         1: y_1 (discrete)
+## Sampling interval: 0.1
+## transfer function form:
+## 2*z^1 + 1
+## -----------------
+## 1*z^2 + 2*z^1 + 1
+## @end example
+## @end deftypefn
+
+## Author: R. Bruce Tenison <btenison@eng.auburn.edu>
+## Created: July 29, 1994
+## Name changed to TF2SYS July 1995
+## updated for new system data structure format July 1996
+
+function outsys = tf (num, den, tsam, inname, outname)
+
+  ## Test for the correct number of input arguments
+  if ((nargin < 2) || (nargin > 5))
+    usage ("outsys = tf (num, den [, tsam, inname, outname])");
+    return
+  endif
+
+  ## check input format
+  if( ! ( (isvector(num) || isscalar(num)) && ...
+        (isvector(den) || isscalar(den))) )
+    error(["num (",num2str(rows(num)),"x",num2str(columns(num)), ...
+      ") and den (",num2str(rows(den)),"x",num2str(columns(den)), ...
+      ") must be vectors"])
+  endif
+
+  ## strip leading zero coefficients
+  num = __tfl__ (num);
+  den = __tfl__ (den);
+
+  if (length(num) >  length(den))
+    error("# of poles (%d) < # of zeros (%d)",length(den)-1, length(num)-1);
+  endif
+
+  ## check sampling interval (if any)
+  if(nargin <= 2)           tsam = 0;           # default
+  elseif (isempty(tsam))    tsam = 0;           endif
+  if ( (! (isscalar(tsam) && (imag(tsam) == 0) )) || (tsam < 0) )
+    error("tsam must be a positive real scalar")
+  endif
+
+  outsys.num = num;
+  outsys.den = den;
+
+  ## Set the system vector:  active = 0(tf), updated = [1 0 0];
+  outsys.sys = [0, 1, 0, 0];
+
+  ## Set defaults
+  outsys.tsam = tsam;
+  outsys.n = length(den)-1;
+  outsys.nz = 0;
+  outsys.yd = 0;        # assume discrete-time
+  ## check discrete time
+  if(tsam > 0)
+    [outsys.n,outsys.nz] = swap(outsys.n, outsys.nz);
+    outsys.yd = 1;
+  endif
+
+  outsys.inname  = __sysdefioname__ (1, "u");
+  outsys.outname = __sysdefioname__ (1, "y");
+  outsys.stname  = __sysdefstname__ (outsys.n, outsys.nz);
+
+  ## Set name of input
+  if (nargin > 3)
+    ## make sure it's a cell array of a single string
+    if(!isempty(inname))
+      if(!iscell(inname))  inname = {inname};  endif
+      if( !is_signal_list(inname) )
+        error("inname must be a string or cell array of strings");
+      endif
+      if(length(inname) > 1)
+        warning("tf: %d input names provided; first used",length(inname));
+        inname = inname(1);
+      endif
+      outsys = syssetsignals(outsys,"in",inname);
+    endif
+  endif
+
+  ## Set name of output
+  if (nargin > 4)
+    if(!isempty(outname))
+      if(!iscell(outname))  outname = {outname};  endif
+      if(!is_signal_list(outname))
+        error("outname must be a string or a cell array of strings");
+      endif
+      if(length(outname) > 1)
+        warning("tf: %d output names provided; first used",length(outname));
+        outname = outname(1);
+      endif
+      outsys = syssetsignals(outsys,"out",outname);
+    endif
+  endif
+
+endfunction