AddLinearModel and CompactLinearModel class objects along with introduction of fitlm and stepwiselm

inst/LinearFormula.m

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+## Copyright (C) 2026 Jayant Chauhan <0001jayant@gmail.com>
+##
+## This file is part of the statistics package for GNU Octave.
+##
+## This program 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.
+##
+## This program 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
+## this program; if not, see <http://www.gnu.org/licenses/>.
+
+classdef LinearFormula
+## -*- texinfo -*-
+## @deftp {statistics} LinearFormula
+##
+## Linear formula encapsulation class.
+##
+## A @code{LinearFormula} object encapsulates the internal formula
+## representation for a @code{LinearModel} or @code{CompactLinearModel}.
+## When displayed in the console, it shows only the human-readable
+## formula string (e.g., @code{y ~ 1 + x1 + x2}).
+##
+## @strong{Properties}
+##
+## @multitable @columnfractions 0.30 0.70
+## @headitem Property @tab Description
+## @item @code{ResponseName} @tab Name of the response variable.
+## @item @code{LinearPredictor} @tab Human-readable formula string.
+## @item @code{Terms} @tab Binary terms matrix (n_terms x p).
+## @item @code{HasIntercept} @tab Logical: true if model has intercept.
+## @item @code{InModel} @tab Logical vector indicating which terms are
+##   active.
+## @item @code{CoefficientNames} @tab Cell array of coefficient names.
+## @item @code{PredictorNames} @tab Cell array of predictor names.
+## @item @code{VariableNames} @tab Cell array of all variable names.
+## @end multitable
+##
+## @seealso{fitlm, LinearModel, CompactLinearModel}
+## @end deftp
+
+  properties (SetAccess = protected)
+    ResponseName     = "";
+    LinearPredictor  = "";
+    Terms            = [];
+    HasIntercept     = true;
+    InModel          = [];
+    CoefficientNames = {};
+    PredictorNames   = {};
+    VariableNames    = {};
+  endproperties
+
+  methods
+
+    function obj = LinearFormula (s)
+      ## LinearFormula  Constructor.
+      ##
+      ## obj = LinearFormula (s)
+      ##
+      ## S is a struct with formula fields.  If S is already a
+      ## LinearFormula, it is returned as-is.
+
+      if (nargin < 1)
+        return;
+      endif
+
+      if (isa (s, "LinearFormula"))
+        obj = s;
+        return;
+      endif
+
+      if (! isstruct (s))
+        error ("LinearFormula: constructor argument must be a struct.");
+      endif
+
+      if (isfield (s, "ResponseName"))
+        obj.ResponseName = s.ResponseName;
+      endif
+      if (isfield (s, "LinearPredictor"))
+        obj.LinearPredictor = s.LinearPredictor;
+      endif
+      if (isfield (s, "Terms"))
+        obj.Terms = s.Terms;
+      endif
+      if (isfield (s, "HasIntercept"))
+        obj.HasIntercept = s.HasIntercept;
+      endif
+      if (isfield (s, "InModel"))
+        obj.InModel = s.InModel;
+      endif
+      if (isfield (s, "CoefficientNames"))
+        obj.CoefficientNames = s.CoefficientNames;
+      endif
+      if (isfield (s, "PredictorNames"))
+        obj.PredictorNames = s.PredictorNames;
+      endif
+      if (isfield (s, "VariableNames"))
+        obj.VariableNames = s.VariableNames;
+      endif
+    endfunction
+
+    function disp (obj)
+      ## disp  Display only the formula string.
+      ##
+      ## Matches MATLAB's LinearFormula display behavior: shows only
+      ## the human-readable formula (e.g., "y ~ 1 + x1 + x2").
+      fprintf ("    %s ~ %s\n", obj.ResponseName, obj.LinearPredictor);
+    endfunction
+
+    function s = char (obj)
+      ## char  Convert to character representation.
+      s = sprintf ("%s ~ %s", obj.ResponseName, obj.LinearPredictor);
+    endfunction
+
+  endmethods
+
+endclassdef