--- a/scripts/set/union.m
+++ b/scripts/set/union.m
@@ -1,5 +1,6 @@
 ## Copyright (C) 1994, 1996, 1997, 1999, 2000, 2003, 2004, 2005, 2006,
 ##               2007, 2008 John W. Eaton
+## Copyright (C) 2008 Jaroslav Hajek
 ##
 ## This file is part of Octave.
 ##
@@ -18,10 +19,10 @@
 ## <http://www.gnu.org/licenses/>.
 
 ## -*- texinfo -*-
-## @deftypefn {Function File} {} union (@var{x}, @var{y})
-## @deftypefnx{Function File} {} union (@var{x}, @var{y}, "rows")
-## Return the set of elements that are in either of the sets @var{x} and
-## @var{y}.  For example,
+## @deftypefn {Function File} {} union (@var{a}, @var{b})
+## @deftypefnx{Function File} {} union (@var{a}, @var{b}, "rows")
+## Return the set of elements that are in either of the sets @var{a} and
+## @var{b}.  For example,
 ##
 ## @example
 ## @group
@@ -31,7 +32,7 @@
 ## @end example
 ##
 ## If the optional third input argument is the string "rows" each row of
-## the matrices @var{x} and @var{y} will be considered an element of sets.
+## the matrices @var{a} and @var{b} will be considered an element of sets.
 ## For example,
 ## @example
 ## @group
@@ -41,12 +42,18 @@
 ##     3   4
 ## @end group
 ## @end example
+##
+## @deftypefnx {Function File} {[@var{c}, @var{ia}, @var{ib}] =} union (@var{a}, @var{b})
+##
+## Return index vectors @var{ia} and @var{ib} such that @code{a==c(ia)} and
+## @code{b==c(ib)}.
+## 
 ## @seealso{create_set, intersection, complement}
 ## @end deftypefn
 
 ## Author: jwe
 
-function y = union (a, b, varargin)
+function [y, ia, ib] = union (a, b, varargin)
 
   if (nargin < 2 || nargin > 3)
     print_usage ();
@@ -56,21 +63,30 @@
     error ("union: if a third input argument is present, it must be the string 'rows'");
   endif
 
-  if (isempty (a))
-    y = create_set (b, varargin{:});
-  elseif (isempty (b))
-    y = create_set (a, varargin{:});
-  elseif (nargin == 2)
-    y = create_set ([a(:); b(:)]);
+  if (nargin == 2)
+    y = [a(:); b(:)];
+    na = numel (a); nb = numel (b);
     if (size (a, 1) == 1 || size (b, 1) == 1)
-      y = y(:).';
-    else
-      y = y(:);
+      y = y.';
     endif
   elseif (ndims (a) == 2 && ndims (b) == 2 && columns (a) == columns (b))
-    y = create_set ([a; b], "rows");
+    y = [a; b];
+    na = rows (a); nb = rows (b);
   else
     error ("union: input arguments must contain the same number of columns when \"rows\" is specified");
   endif
 
+  if (nargout == 1)
+    y = unique (y, varargin{:});
+  else
+    [y, i] = unique (y, varargin{:});
+    ia = i(i <= na);
+    ib = i(i > na) - na;
+  endif
+
+%!test
+%! a = [3, 1, 4, 1, 5]; b = [1, 2, 3, 4];
+%! [y, ia, ib] = union (a, b.');
+%! assert(y, [1, 2, 3, 4, 5]);
+%! assert(y, sort([a(ia), b(ib)]));
 endfunction