function [yout,x,n] = dstep(a,b,c,d,iu,n)
%DSTEP  Step response of discrete-time linear systems.
%   DSTEP(A,B,C,D,IU)  plots the response of the discrete system:
%
%       x[n+1] = Ax[n] + Bu[n]
%       y[n]   = Cx[n] + Du[n]
%
%   to a step applied to the single input IU.  The number of points is
%   determined automatically.
%
%   DSTEP(NUM,DEN)  plots the step response of the polynomial transfer
%   function  G(z) = NUM(z)/DEN(z)  where NUM and DEN contain the 
%   polynomial coefficients in descending powers of z.
%
%   DSTEP(A,B,C,D,IU,N) or DSTEP(NUM,DEN,N) uses the user-supplied 
%   number of points, N.  When invoked with left hand arguments,
%       [Y,X] = DSTEP(A,B,C,D,...)
%       [Y,X] = DSTEP(NUM,DEN,...)
%   returns the output and state time history in the matrices Y and X.
%   No plot is drawn on the screen.  Y has as many columns as there 
%   are outputs and X has as many columns as there are states.
%
%   See also: STEP, IMPULSE, INITIAL, LSIM.

%   J.N. Little 4-21-85
%   Revised JNL 7-18-88, CMT 7-31-90, ACWG 6-21-92
%   Revised A. Potvin 10-1-94
%   Copyright 1986-2000 The MathWorks, Inc. 
%   $Revision: 1.13 $  $Date: 2000/06/14 17:15:34 $

%warning(['This calling syntax for ' mfilename ...
%   ' will not be supported in the future: use STEP(SYS,...) instead.'])

ni = nargin;
no = nargout;
if ni==0,
   eval('exresp(''dstep'')')
   return
end
error(nargchk(2,6,ni));
Ts = -1;

% Determine which syntax is being used
switch ni
case 2
   if size(a,1)>1,
      % SIMO syntax
      a = num2cell(a,2);
      den = b;
      b = cell(size(a,1),1);
      b(:) = {den};
   end
   sys = tf(a,b,Ts);
   n = [];

case 3
   % Transfer function form with time vector
   if size(a,1)>1,
      % SIMO syntax
      a = num2cell(a,2);
      den = b;
      b = cell(size(a,1),1);
      b(:) = {den};
   end
   sys = tf(a,b,Ts);
   n = c;

case 4
   % State space system without iu or time vector
   sys = ss(a,b,c,d,Ts);
   n = [];

otherwise
   % State space system, with iu but w/o time vector
   if min(size(iu))>1,
      error('IU must be a vector.');
   elseif isempty(iu),
      iu = 1:size(d,2);
   end
   sys = ss(a,b(:,iu),c,d(:,iu),Ts);
   if ni<6, 
      n = [];
   end
end

if no==1,
   yout = step(sys,n);
   yout = yout(:,:);
elseif no>1,
   [yout,t,x] = step(sys,n);
   yout = yout(:,:);
   x = x(:,:);
   n = length(t);
else
   step(sys,n);
end

% end dstep
