MitchellThompkins · MitchellThompkins · Jun 7, 2026 · Jul 2, 2026 · Jul 4, 2026 · Jul 5, 2026
diff --git a/.gitignore b/.gitignore
@@ -54,4 +54,4 @@ TAGS$
 semantic.cache
 # Other text editors often create these
 ~.*
-
+*.swp
diff --git a/DESCRIPTION b/DESCRIPTION
@@ -1,5 +1,5 @@
 Name: control
-Version: 4.2.2+
+Version: 4.2.99+
 Date: XXXX-XX-XX
 Author: Lukas Reichlin <lukas.reichlin@gmail.com>
 Maintainer: Doug Stewart <doug.dastew@gmail.com>, Torsten Lilge <ttl-octave@mailbox.org>

diff --git a/NEWS b/NEWS
@@ -1,6 +1,12 @@
 
 Summary of important user-visible changes for releases of the control package
 
+===============================================================================
+control-4.2.99+  Upcoming 4.3.0, not yet released 
+===============================================================================
+
+**
+
 ===============================================================================
 control-4.2.2+  Upcoming 4.2.3, not yet released 
 ===============================================================================

diff --git a/inst/@tf/__c2d__.m b/inst/@tf/__c2d__.m
@@ -35,8 +35,7 @@
     sys=imp_invar(sys,1/tsam);
 
   elseif (strncmpi (method, "m", 1))    # "matched"
-    ## TODO: move this code to  @zpk/__c2d__.m  once ZPK models are implemented
-
+
     if (! issiso (sys))
       error ("tf: c2d: require SISO system for matched pole/zero method");
     endif
@@ -68,7 +67,7 @@
     w_d = exp (1j * w_c * tsam);
     k_d = real (k_c * prod (1j*w_c - z_c) / prod (1j*w_c - p_c) * prod (w_d - p_d) / prod (w_d - z_d));
 
-    tmp = zpk (z_d, p_d, k_d, tsam);
+    tmp = tf (zpk (z_d, p_d, k_d, tsam));
     sys.num = tmp.num;
     sys.den = tmp.den;
 

diff --git a/inst/@tf/__d2c__.m b/inst/@tf/__d2c__.m
@@ -25,7 +25,6 @@
 function sys = __d2c__ (sys, tsam, method = "zoh", w0 = 0)
 
   if (strncmpi (method, "m", 1))    # "matched"
-    ## TODO: move this code to  @zpk/__d2c__.m  once ZPK models are implemented
 
     if (! issiso (sys))
       error ("tf: d2c: require SISO system for matched pole/zero method");
@@ -48,11 +47,11 @@
     tol = sqrt (eps);
     while (any (abs ([p_d; z_d_orig] - w_d) < tol))
       w_c += 0.1 / tsam;
+      w_d = exp (1j * w_c * tsam);
     endwhile
-    w_d = exp (w_c * tsam);
-    k_c = real (k_d * prod (w_d - z_d_orig) / prod (w_d - p_d) * prod (w_c - p_c) / prod (w_c - z_c));
+    k_c = real (k_d * prod (w_d - z_d_orig) / prod (w_d - p_d) * prod (1j*w_c - p_c) / prod (1j*w_c - z_c));
 
-    tmp = zpk (z_c, p_c, k_c);
+    tmp = tf (zpk (z_c, p_c, k_c));
     sys.num = tmp.num;
     sys.den = tmp.den;
 

diff --git a/inst/@zpk/__c2d__.m b/inst/@zpk/__c2d__.m
@@ -0,0 +1,101 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Convert the continuous ZPK model into its discrete-time equivalent.
+## The matched method maps each stored pole and zero directly via exp (s*tsam),
+## avoiding the ill-conditioned polynomial round-trip of the TF representation.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: June 2026
+## Version: 0.1
+
+function sys = __c2d__ (sys, tsam, method = "zoh", w0 = 0)
+
+  if (strncmpi (method, "m", 1))    # "matched"
+
+    if (! issiso (sys))
+      error ("zpk: c2d: require SISO system for matched pole/zero method");
+    endif
+
+    z_c = sys.z{1};
+    p_c = sys.p{1};
+    k_c = sys.k;
+
+    p_d = exp (p_c * tsam);
+    z_d = exp (z_c * tsam);
+
+    if (any (! isfinite (p_d)) || any (! isfinite (z_d)))
+      error ("zpk: c2d: discrete-time poles and zeros are not finite");
+    endif
+
+    ## continuous-time zeros at infinity are mapped to -1 in discrete-time
+    ## except for one.  for non-proper transfer functions, no zeros at -1 are added.
+    np = length (p_c);              # number of poles
+    nz = length (z_c);              # number of finite zeros, np-nz number of infinite zeros
+    z_d = vertcat (z_d, repmat (-1, np-nz-1, 1));
+
+    ## the discrete-time gain k_d is matched at frequency w_c to continuous-time
+    ## gain k_c.  dc gain is taken (w_c=0) unless there are continuous-time
+    ## poles/zeros near the imaginary axis at j*w_c.  gain is evaluated on the
+    ## imaginary axis (s=j*w_c) and unit circle (z=exp(j*w_c*tsam)) so that
+    ## |H_d(exp(j*w_c*tsam))| = |H_c(j*w_c)| holds in the frequency domain.
+    w_c = 0;                        # start at dc
+    tol = sqrt (eps);               # poles/zeros within tol of j*w_c are avoided
+    while (any (abs ([p_c; z_c] - 1j*w_c) < tol))
+      w_c += 0.1 / tsam;
+    endwhile
+    w_d = exp (1j * w_c * tsam);
+    k_d = real (k_c * prod (1j*w_c - z_c) / prod (1j*w_c - p_c) * prod (w_d - p_d) / prod (w_d - z_d));
+
+    sys.z{1} = z_d;
+    sys.p{1} = p_d;
+    sys.k = k_d;
+
+  else
+    ## zoh/foh/tustin/prewarp/impulse are not per-root maps. their natural
+    ## representation is state-space.  convert back to zpk for type consistency.
+    sys = zpk (__c2d__ (ss (sys), tsam, method, w0));
+  endif
+
+endfunction
+
+
+%!test
+%! ## single pole: p_d = exp(p_c * Ts)
+%! sys = zpk ([], [-1], 1);
+%! sys_d = c2d (sys, 0.1, 'matched');
+%! assert (isa (sys_d, 'zpk'));
+%! [~, p_d] = zpkdata (sys_d, 'v');
+%! assert (p_d, exp (-0.1), 1e-14);
+
+%!test
+%! ## 25 poles clustered near the imaginary axis: matched c2d keeps them
+%! ## stable and maps each one exactly to exp(p*Ts) with no rounding error
+%! N = 25; Ts = 1/1000;
+%! p_s = (-0.001 + 1i * linspace (1, 25, N))' * 2*pi*4;
+%! sys_d = c2d (zpk ([], p_s, 1), Ts, 'matched');
+%! [~, p_d] = zpkdata (sys_d, 'v');
+%! assert (all (abs (p_d) < 1));
+%! assert (max (abs (p_d - exp (p_s * Ts))), 0, 1e-12);
+
+%!test
+%! ## non-matched methods still work on zpk and return zpk
+%! sys_d = c2d (zpk ([], [-1], 1), 0.1, 'zoh');
+%! assert (isa (sys_d, 'zpk'));
+%! assert (get (sys_d, 'tsam'), 0.1);
diff --git a/inst/@zpk/__ctranspose__.m b/inst/@zpk/__ctranspose__.m
@@ -0,0 +1,28 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Complex-conjugate transpose ZPK model by delegating to TF.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: July 2026
+## Version: 0.2
+
+function sys = __ctranspose__ (sys, ct)
+  sys = zpk (__ctranspose__ (tf (sys), ct));
+endfunction
diff --git a/inst/@zpk/__d2c__.m b/inst/@zpk/__d2c__.m
@@ -0,0 +1,76 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Convert the discrete ZPK model into its continuous-time equivalent.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: June 2026
+## Version: 0.1
+
+function sys = __d2c__ (sys, tsam, method = "zoh", w0 = 0)
+
+  if (strncmpi (method, "m", 1))    # "matched"
+
+    if (! issiso (sys))
+      error ("zpk: d2c: require SISO system for matched pole/zero method");
+    endif
+
+    z_d = sys.z{1};
+    p_d = sys.p{1};
+    k_d = sys.k;
+
+    if (any (abs (p_d) < eps) || any (abs (z_d) < eps))
+      error ("zpk: d2c: discrete-time poles and zeros at 0 not supported because log(0) is -Inf");
+    endif
+
+    z_d_orig = z_d;
+    z_d(abs (z_d+1) < sqrt (eps)) = [];    # remove zeros added at -1 by c2d
+
+    p_c = log (p_d) / tsam;
+    z_c = log (z_d) / tsam;
+
+    w_c = 0;
+    w_d = 1;
+    tol = sqrt (eps);
+    while (any (abs ([p_d; z_d_orig] - w_d) < tol))
+      w_c += 0.1 / tsam;
+      w_d = exp (1j * w_c * tsam);
+    endwhile
+    k_c = real (k_d * prod (w_d - z_d_orig) / prod (w_d - p_d) * prod (1j*w_c - p_c) / prod (1j*w_c - z_c));
+
+    sys.z{1} = z_c;
+    sys.p{1} = p_c;
+    sys.k = k_c;
+
+  else
+    sys = zpk (__d2c__ (ss (sys), tsam, method, w0));
+  endif
+
+endfunction
+
+
+%!test
+%! ## matched round trip is the identity on poles/zeros
+%! sys = zpk ([-3], [-1; -2], 4);
+%! sysd = c2d (sys, 0.1, 'matched');
+%! sysc = d2c (sysd, 'matched');
+%! [z, p, k] = zpkdata (sysc, 'v');
+%! assert (sort (real (p)), [-2; -1], 1e-10);
+%! assert (real (z), -3, 1e-10);
+%! assert (k, 4, 1e-10);
diff --git a/inst/@zpk/__freqresp__.m b/inst/@zpk/__freqresp__.m
@@ -0,0 +1,28 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Compute frequency response of ZPK model by delegating to TF.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: June 2026
+## Version: 0.1
+
+function H = __freqresp__ (sys, w, cellflag = false)
+  H = __freqresp__ (tf (sys), w, cellflag);
+endfunction
diff --git a/inst/@zpk/__get__.m b/inst/@zpk/__get__.m
@@ -0,0 +1,39 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Access key values of ZPK objects.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: June 2026
+## Version: 0.1
+
+function val = __get__ (sys, key)
+
+  switch (key)
+    case {"z", "zeros"}
+      val = sys.z;
+    case {"p", "poles"}
+      val = sys.p;
+    case {"k", "gain"}
+      val = sys.k;
+    otherwise
+      error ("zpk: get: invalid key name '%s'", key);
+  endswitch
+
+endfunction
diff --git a/inst/@zpk/__minreal__.m b/inst/@zpk/__minreal__.m
@@ -0,0 +1,28 @@
+## Copyright (C) 2026  Mitchell Thompkins <mitchell.thompkins@pm.me>
+##
+## This file is part of the control package for GNU 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 3 of the License, 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,
+## see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## Return minimal realization of ZPK model by delegating to TF.
+
+## Author: Mitchell Thompkins <mitchell.thompkins@pm.me>
+## Created: July 2026
+## Version: 0.2
+
+function sys = __minreal__ (sys, tol)
+  sys = zpk (__minreal__ (tf (sys), tol));
+endfunction
-Original file line number
+Diff line change
@@ Expand Up / @@ -54,4 +54,4 @@ TAGS$ @@
     semantic.cache
     # Other text editors often create these
     ~.*
+    *.swp