## Copyright (C) 2012 Jordi Gutiérrez Hermoso
##
## This file is part of 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 -*-
## @deftypefn  {Function File} {} nthargout (@var{n}, @var{func}, @dots{})
## @deftypefnx {Function File} {} nthargout (@var{n}, @var{ntot}, @var{func}, @dots{})
## Return the @var{n}th output argument of function given by the
## function handle or string @var{func}.  Any arguments after @var{func}
## are passed to @var{func}.  The total number of arguments to call
## @var{func} with can be passed in @var{ntot}; by default @var{ntot}
## is @var{n}.  The input @var{n} can also be a vector of indices of the
## output, in which case the output will be a cell array of the
## requested output arguments.
##
## The intended use @code{nthargout} is to avoid intermediate variables.
## For example, when finding the indices of the maximum entry of a
## matrix, the following two compositions of nthargout
##
## @example
## @group
## @var{m} = magic (5);
## cell2mat (nthargout ([1, 2], @@ind2sub, size(@var{m}),
##                      nthargout (2, @@max, @var{m}(:))))
## @result{} 5   3
## @end group
## @end example
##
## @noindent
## are completely equivalent to the following lines:
##
## @example
## @group
## @var{m} = magic(5);
## [~, idx] = max (@var{M}(:));
## [i, j] = ind2sub (size (@var{m}), idx);
## [i, j]
## @result{} 5   3
## @end group
## @end example
##
## It can also be helpful to have all output arguments in a single cell
## in the following manner:
##
## @example
## @var{USV} = nthargout ([1:3], @@svd, hilb (5));
## @end example
##
## @seealso{nargin, nargout, varargin, varargout, isargout}
## @end deftypefn

## Author: Jordi Gutiérrez Hermoso

function out = nthargout (n, varargin)
  if (nargin < 2)
    print_usage ();
  endif

  if (isa (varargin{1}, "function_handle") || ischar (varargin{1}))
    ntot = max (n(:));
    func = varargin{1};
    args = varargin(2:end);
  elseif (isnumeric (varargin{1})
          && (isa (varargin{2}, "function_handle") || ischar (varargin{2})))
    ntot = varargin{1};
    func = varargin{2};
    args = varargin(3:end);
  else
    print_usage ();
  endif

  if (any (n != fix (n))  || ntot != fix (ntot) || any (n <= 0) || ntot <= 0)
    error ("nthargout: N and NTOT must consist of positive integers")
  endif

  outargs = cell (1, ntot);

  try
    [outargs{:}] = feval (func, args{:});
    if (numel (n) > 1)
      out = outargs(n);
    else
      out = outargs{n};
    endif
  catch
    err = lasterr ();
    if (strfind ("some elements undefined in return list", err))
      error ("nthargout: Too many output arguments: %d", ntot);
    else
      error (err);
    endif
  end_try_catch

endfunction


%!shared m
%! m = magic (5);
%!assert (nthargout ([1,2], @ind2sub, size (m), nthargout (2, @max, m(:))), {5,3})
%!assert (nthargout (3, @find, m(m>20)), [23, 24, 25, 21, 22]')