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+# Copyright (C) 1996 A. Scottedward Hodel 
+#
+# 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, 675 Mass Ave, Cambridge, MA 02139, USA. 
+ 
+function sys = sysupdate(sys,opt)
+# function retsys = sysupdate(sys,opt)
+# Update the internal representation of a system.
+# inputs:
+#  sys: system data structure
+#  opt: string:  "tf" -> update transfer function
+#                "zp" -> update zero-pole form
+#                "ss" -> update state space form
+#                "all" -> all of the above
+# outputs: retsys: contains union of data in sys and requested data.
+#     if requested data in sys is already up to date then retsys=sys.
+#
+# conversion to tf or zp exits with an error if the system is
+# mixed continuous/digital
+#
+# see also: tf2sys, ss2sys, zp2sys, sysout, sys2ss, sys2tf, sys2zp
+
+# Written by John Ingram  7-9-96
+# $Revision: 1.1.1.1 $
+
+  # check for correct number of inputs 
+  if (nargin != 2)
+    usage("newsys = sysupdate(sys,opt)");
+  elseif(! is_struct(sys) )
+   error("1st argument must be system data structure")
+  elseif(! (strcmp(opt,"tf") + strcmp(opt,"zp") + ...
+	strcmp(opt,"ss") + strcmp(opt,"all")) )
+    error("2nd argument must be \"tf\", \"zp\", \"ss\", or \"all\"");
+  endif
+
+  # check to make sure not trying to make a SISO system out of a MIMO sys
+  if ( (strcmp(opt,"tf") + strcmp(opt,"zp") + strcmp(opt,"all")) ...
+	& (sys.sys(1) == 2) &  (! is_siso(sys) ) )
+    error("MIMO -> SISO update requested");
+  endif
+
+  # update transfer function if desired
+  if ( (strcmp(opt, "tf") + strcmp(opt,"all"))&&  (!sys.sys(2)))
+    # check to make sure the system is not discrete and continuous
+    is_digital(sys);
+
+    # if original system zero-pole
+    if (sys.sys(1) == 1)
+      [sys.num,sys.den] = zp2tf(sys.zer,sys.pol,sys.k);
+      sys.sys(2) = 1;
+    # if original system is state-space
+    elseif(sys.sys(1) == 2)
+      [sys.num,sys.den] = ss2tf(sys.a,sys.b,sys.c,sys.d);
+      sys.sys(2) = 1; 
+    endif
+  endif
+
+
+  # update zero-pole if desired
+  if ( (strcmp(opt, "zp") + strcmp(opt,"all")) && (! sys.sys(3)) )
+    # check to make sure the system is not discrete and continuous
+    is_digital(sys);
+
+    # original system is transfer function
+    if (sys.sys(1) == 0)
+      [sys.zer,sys.pol,sys.k] = tf2zp(sys.num,sys.den);
+      sys.sys(3) = 1;
+    # original system is state-space
+
+    elseif(sys.sys(1) == 2)
+      [sys.zer,sys.pol,sys.k] = ss2zp(sys.a,sys.b,sys.c,sys.d);
+      sys.sys(3) = 1; 
+    endif
+
+  endif
+
+  # update state-space if desired
+  if ( (strcmp(opt, "ss") + strcmp(opt,"all")) && (! sys.sys(4)) )
+    # original system is transfer function
+    if (sys.sys(1) == 0)
+      [sys.a,sys.b,sys.c,sys.d] = tf2ss(sys.num,sys.den);
+      sys.sys(4) = 1;
+    # original system is zero-pole
+    elseif(sys.sys(1) == 1)
+      [sys.a,sys.b,sys.c,sys.d] = zp2ss(sys.zer,sys.pol,sys.k);
+      sys.sys(4) = 1; 
+    endif
+
+    # create new state names
+    sys.stname = sysdefstname(sys.n, sys.nz);
+  endif
+  
+
+endfunction