--- a/NEWS	Wed Jul 10 17:25:39 2019 -0700
+++ b/NEWS	Wed Jul 10 17:32:52 2019 -0700
@@ -53,6 +53,10 @@
   behavior, or Matlab behavior from releases prior to R2012b, can be
   obtained by using the `"legacy"` flag.
 
+- The function `intersect` now accepts a `"legacy"` flag which changes
+  the index values (second and third outputs) as well as the orientation
+  of the outputs to match Matlab releases prior to R2012b.
+
 - Complex RESTful web services can now be accessed by the `webread` and
   `webwrite` functions alongside with the `weboptions` structure.  One
   major feature is the support for cookies to enable RESTful

--- a/scripts/set/intersect.m	Wed Jul 10 17:25:39 2019 -0700
+++ b/scripts/set/intersect.m	Wed Jul 10 17:32:52 2019 -0700
@@ -20,6 +20,7 @@
 ## -*- texinfo -*-
 ## @deftypefn  {} {@var{c} =} intersect (@var{a}, @var{b})
 ## @deftypefnx {} {@var{c} =} intersect (@var{a}, @var{b}, "rows")
+## @deftypefnx {} {@var{c} =} intersect (@dots{}, "legacy")
 ## @deftypefnx {} {[@var{c}, @var{ia}, @var{ib}] =} intersect (@dots{})
 ##
 ## Return the unique elements common to both @var{a} and @var{b} sorted in
@@ -32,15 +33,19 @@
 ## If the optional input @qcode{"rows"} is given then return the common rows of
 ## @var{a} and @var{b}.  The inputs must be 2-D matrices to use this option.
 ##
-## If requested, return index vectors @var{ia} and @var{ib} such that
+## If requested, return column index vectors @var{ia} and @var{ib} such that
 ## @code{@var{c} = @var{a}(@var{ia})} and @code{@var{c} = @var{b}(@var{ib})}.
 ##
+## Programming Note: The input flag @qcode{"legacy"} changes the shape of the
+## outputs (@var{c}, @var{i}, @var{j} to row vectors whenever at least one of
+## the inputs is a row vector.
+##
 ## @seealso{unique, union, setdiff, setxor, ismember}
 ## @end deftypefn
 
 function [c, ia, ib] = intersect (a, b, varargin)
 
-  if (nargin < 2 || nargin > 3)
+  if (nargin < 2 || nargin > 4)
     print_usage ();
   endif
 
@@ -58,13 +63,21 @@
     endif
     ia = ib = [];
   else
-    by_rows = nargin == 3;
-    isrowvec = isrow (a) && isrow (b);
+    by_rows = any (strcmp ("rows", varargin));
+    optlegacy = any (strcmp ("legacy", varargin));
+
+    if (optlegacy)
+      isrowvec = ! iscolumn (a) || ! iscolumn (b);
+    else
+      isrowvec = isrow (a) && isrow (b);
+    endif
 
     ## Form A and B into sets
     if (nargout > 1)
       [a, ja] = unique (a, varargin{:});
+      ja = ja(:);
       [b, jb] = unique (b, varargin{:});
+      jb = jb(:);
     else
       a = unique (a, varargin{:});
       b = unique (b, varargin{:});
@@ -72,13 +85,21 @@
 
     if (by_rows)
       c = [a; b];
-      [c, ic] = sortrows (c);
+      if (nargout > 1)
+        [c, ic] = sortrows (c);
+      else
+        c = sortrows (c);
+      endif
       ii = find (all (c(1:end-1,:) == c(2:end,:), 2));
       c = c(ii,:);
       len_a = rows (a);
     else
       c = [a(:); b(:)];
-      [c, ic] = sort (c);         # [a(:);b(:)](ic) == c
+      if (nargout > 1)
+        [c, ic] = sort (c);         # [a(:);b(:)](ic) == c
+      else
+        c = sort (c);
+      endif
       if (iscellstr (c))
         ii = find (strcmp (c(1:end-1), c(2:end)));
       else
@@ -88,15 +109,20 @@
       len_a = length (a);
     endif
 
+    ## Adjust output orientation for Matlab compatibility
+    if (isrowvec)
+      c = c.';
+    endif
+
     if (nargout > 1)
       ia = ja(ic(ii));            # a(ia) == c
       ib = jb(ic(ii+1) - len_a);  # b(ib) == c
+      if (optlegacy && isrowvec)
+        ia = ia.';
+        ib = ib.';
+      endif
     endif
 
-    ## Adjust output orientation for Matlab compatibility
-    if (! by_rows && isrowvec)
-      c = c.';
-    endif
   endif
 
 endfunction
@@ -107,10 +133,17 @@
 %! a = 1:4;
 %! b = 2:5;
 
-%!assert (size (intersect (a, b)), [1 3])
-%!assert (size (intersect (a', b)), [3 1])
-%!assert (size (intersect (a, b')), [3 1])
-%!assert (size (intersect (a', b')), [3 1])
+%!assert (size (intersect (a, b)), [1, 3])
+%!assert (size (intersect (a', b)), [3, 1])
+%!assert (size (intersect (a, b')), [3, 1])
+%!assert (size (intersect (a', b')), [3, 1])
+%!assert (size (intersect (a, b, "legacy")), [1, 3])
+%!assert (size (intersect (a', b, "legacy")), [1, 3])
+%!assert (size (intersect (a, b', "legacy")), [1, 3])
+%!assert (size (intersect (a', b', "legacy")), [3, 1])
+
+## Clear shared variables
+%!shared
 
 ## Test multi-dimensional arrays
 %!test
@@ -150,6 +183,19 @@
 %! assert (ia, [1:3]');
 %! assert (ib, [1:3]');
 
+## Test "legacy" argument
+%!test
+%! a = [7 1 7 7 4]; 
+%! b = [7 0 4 4 0];
+%! [c, ia, ib] = intersect (a, b);
+%! assert (c, [4, 7]);
+%! assert (ia, [5; 1]);
+%! assert (ib, [3; 1]);
+%! [c, ia, ib] = intersect (a, b, "legacy");
+%! assert (c, [4, 7]);
+%! assert (ia, [5, 4]);
+%! assert (ib, [4, 1]);
+
 ## Test return type of empty intersections
 %!assert (intersect (['a', 'b'], {}), {})
 %!assert (intersect ([], {'a', 'b'}), {})

--- a/scripts/set/private/validsetargs.m	Wed Jul 10 17:25:39 2019 -0700
+++ b/scripts/set/private/validsetargs.m	Wed Jul 10 17:32:52 2019 -0700
@@ -19,7 +19,7 @@
 
 ## Validate arguments for binary set operation.
 
-function [x, y] = validsetargs (caller, x, y, byrows_arg)
+function [x, y] = validsetargs (caller, x, y, varargin)
 
   isallowedarraytype = @(x) isnumeric (x) || ischar (x) || islogical (x);
 
@@ -42,21 +42,29 @@
       error ("%s: A and B must be arrays or cell arrays of strings", caller);
     endif
   elseif (nargin == 4)
-    if (! strcmpi (byrows_arg, "rows"))
-      error ("%s: invalid option: %s", caller, byrows_arg);
-    endif
+    for arg = varargin
+      switch (arg{1})
+        case "legacy"
+          ## Accepted option, do nothing.
 
-    if (iscell (x) || iscell (y))
-      error ('%s: cells not supported with "rows"', caller);
-    elseif (! (isallowedarraytype (x) && isallowedarraytype (y)))
-      error ("%s: A and B must be arrays or cell arrays of strings", caller);
-    else
-      if (ndims (x) > 2 || ndims (y) > 2)
-        error ('%s: A and B must be 2-dimensional matrices for "rows"', caller);
-      elseif (columns (x) != columns (y) && ! (isempty (x) || isempty (y)))
-        error ("%s: number of columns in A and B must match", caller);
-      endif
-    endif
+        case "rows"
+          if (iscell (x) || iscell (y))
+            error ('%s: cells not supported with "rows"', caller);
+          elseif (! (isallowedarraytype (x) && isallowedarraytype (y)))
+            error ("%s: A and B must be arrays or cell arrays of strings", caller);
+          else
+            if (ndims (x) > 2 || ndims (y) > 2)
+              error ('%s: A and B must be 2-dimensional matrices for "rows"', caller);
+            elseif (columns (x) != columns (y) && ! (isempty (x) || isempty (y)))
+              error ("%s: number of columns in A and B must match", caller);
+            endif
+          endif
+
+        otherwise
+          error ("%s: invalid option: %s", caller, byrows_arg);
+
+      endswitch
+    endfor
   endif
 
 endfunction