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## later version. 
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## for more details.
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## -*- texinfo -*-
## @deftypefn {Function File } { @var{out} =} freqresp (@var{sys},@var{USEW}@{,@var{w}@});
##  Frequency response function - used internally by @code{bode}, @code{nyquist}.
##  minimal argument checking; "do not attempt to do this at home"
## 
## @strong{Inputs}
## @table @var
## @item sys
## system data structure
## @item USEW
## returned by @code{freqchkw}
## @item optional
##  must be present if @var{USEW} is true (nonzero)
## @end table
## @strong{Outputs}
## @table @var
## @item @var{out} 
## vector of finite @math{G(j*w)} entries (or @math{||G(j*w)||} for MIMO)
## @item w 
## vector of corresponding frequencies 
## @end table
## @end deftypefn

function [ff,w] = freqresp(sys,USEW,w);

  ## Written by: R. Bruce Tenison July 11, 1994
  ## SYS_INTERNAL accesses members of system data structure

  save_val = empty_list_elements_ok;
  empty_list_elements_ok = 1;

  ## Check Args
  if( (nargin < 2) || (nargin > 4) )
    usage ("[ff,w] = freqresp(sys,USEW{,w})");
  elseif( USEW & (nargin < 3) )
    error("USEW=1 but w was not passed.");
  elseif( USEW & isempty(w))
    warning("USEW=1 but w is empty; setting USEW=0");
    USEW=0;
  endif

  DIGITAL = is_digital(sys);

  ## compute default w if needed
  if(!USEW)
    if(is_siso(sys))
      sys = sysupdate(sys,"zp");
      [zer,pol] = sys2zp(sys);
    else
      zer = tzero(sys);
      pol = eig(sys2ss(sys));
    endif

    ## get default frequency range
    [wmin,wmax] = bode_bounds(zer,pol,DIGITAL,sysgettsam(sys));
    w = logspace(wmin,wmax,50);
  else
    w = reshape(w,1,length(w)); 	# make sure it's a row vector
  endif

  ## now get complex values of s or z
  if(DIGITAL)
    jw = exp(i*w*sysgettsam(sys));
  else
    jw = i*w;
  endif

  [nn,nz,mm,pp] = sysdimensions(sys);

  ## now compute the frequency response - divide by zero yields a warning
  if (strcmp(sysgettype(sys),"zp"))
    ## zero-pole form (preferred)
    [zer,pol,sysk] = sys2zp(sys);
    ff = ones(size(jw));
    l1 = min(length(zer)*(1-isempty(zer)),length(pol)*(1-isempty(pol)));
    for ii=1:l1
      ff = ff .* (jw - zer(ii)) ./ (jw - pol(ii));
    endfor

    ## require proper  transfer function, so now just get poles.
    for ii=(l1+1):length(pol)
      ff = ff ./ (jw - pol(ii));
    endfor
    ff = ff*sysk;

  elseif (strcmp(sysgettype(sys),"tf"))
    ## transfer function form 
    [num,den] = sys2tf(sys);
    ff = polyval(num,jw)./polyval(den,jw);
  elseif (mm==pp)
    ## The system is square; do state-space form bode plot
    [sysa,sysb,sysc,sysd,tsam,sysn,sysnz] = sys2ss(sys);
    n = sysn + sysnz;
    for ii=1:length(jw);
      ff(ii) = det(sysc*((jw(ii).*eye(n)-sysa)\sysb)+sysd);
    endfor;
  else
    ## Must be state space... bode                            
    [sysa,sysb,sysc,sysd,tsam,sysn,sysnz] = sys2ss(sys);
    n = sysn + sysnz;
    for ii=1:length(jw);
      ff(ii) = norm(sysc*((jw(ii)*eye(n)-sysa)\sysb)+sysd);
    endfor
    
  endif

  w = reshape(w,1,length(w));
  ff = reshape(ff,1,length(ff));

  ## restore global variable
  empty_list_elements_ok = save_val;
endfunction