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+++ b/scripts/control/is_digital.m
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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 DIGITAL = is_digital(sys)
+# function DIGITAL = is_digital(sys)
+# retrurn nonzero if system is digital
+# exits with an error of sys is a mixed (continuous and discrete) system
+
+# a s hodel July 1996
+# $Revision: 1.1.1.1 $
+# SYS_INTERNAL accesses members of system structure
+
+  # checked for sampled data system (mixed)
+  # discrete system
+  cont = sum(sys.yd == 0) + sys.n;
+  dig = sum(sys.yd != 0) + sys.nz + sys.tsam;
+  if( cont*dig != 0)
+   sysout(sys);
+   error("continuous/discrete system; use syscont, sysdisc, or c2d first");
+  else
+    DIGITAL = (sys.tsam > 0);
+  endif
+ 
+endfunction