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## -*- texinfo -*-
## @deftypefn  {} {} flip (@var{x})
## @deftypefnx {} {} flip (@var{x}, @var{dim})
## Return a copy of array @var{x} flipped across dimension @var{dim}.
##
## If @var{dim} is unspecified it defaults to the first non-singleton
## dimension.
##
## Examples:
##
## @example
## ## row vector
## flip ([1  2  3  4])
##       @result{}  4  3  2  1
##
## ## column vector
## flip ([1; 2; 3; 4])
##       @result{}  4
##           3
##           2
##           1
##
## ## 2-D matrix along dimension 1
## flip ([1 2; 3 4])
##       @result{}  3  4
##           1  2
##
## ## 2-D matrix along dimension 2
## flip ([1 2; 3 4], 2)
##       @result{}  2  1
##           4  3
## @end example
##
## @seealso{fliplr, flipud, rot90, rotdim, permute, transpose}
## @end deftypefn

function y = flip (x, dim)

  if (nargin < 1 || nargin > 2)
    print_usage ();
  endif

  nd = ndims (x);
  sz = size (x);
  if (nargin == 1)
    ## Find the first non-singleton dimension.
    (dim = find (sz > 1, 1)) || (dim = 1);
  elseif (! (isscalar (dim) && isindex (dim)))
    error ("flip: DIM must be a positive integer");
  endif

  idx(1:max(nd, dim)) = {':'};
  idx{dim} = size (x, dim):-1:1;
  y = x(idx{:});

endfunction


%!assert (flip ([1 2; 3 4], 2), [2 1; 4 3])
%!assert (flip ([1 2; 3 4], 3), [1 2; 3 4])

## Test defaults
%!assert (flip ([1 2 3 4]), [4 3 2 1])
%!assert (flip ([1 2 3 4].'), [4 3 2 1].')
%!assert (flip ([1 2; 3 4]), flip ([1 2 ; 3 4], 1))

## Test NDArrays
%!test
%! a(1:2,1:2,1) = [1 2; 3 4];
%! a(1:2,1:2,2) = [5 6; 7 8];
%! b(1:2,1:2,1) = [5 6; 7 8];
%! b(1:2,1:2,2) = [1 2; 3 4];
%! assert (flip (a, 3), b);

%!test
%! a = b = zeros (2, 2, 1, 2);
%! a(1:2,1:2,:,1) = [1 2; 3 4];
%! a(1:2,1:2,:,2) = [5 6; 7 8];
%! b(1:2,1:2,:,1) = [5 6; 7 8];
%! b(1:2,1:2,:,2) = [1 2; 3 4];
%! assert (flip (a, 3), a);
%! assert (flip (a, 4), b);
%! assert (flip (a, 5), a);

%!error flip ()
%!error flip (1, 2, 3)
%!error <DIM must be a positive integer> flip (magic (3), -1)