########################################################################
##
## Copyright (C) 1993-2024 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
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########################################################################

## -*- texinfo -*-
## @deftypefn {} {@var{t} =} trace (@var{A})
## Compute the trace of @var{A}, the sum of the elements along the main
## diagonal.
##
## The implementation is straightforward: @code{sum (diag (@var{A}))}.
## @seealso{eig}
## @end deftypefn

function t = trace (A)

  if (nargin < 1)
    print_usage ();
  endif

  if (ndims (A) > 2)
    error ("trace: only valid on 2-D objects");
  elseif (isempty (A))
    t = 0;
  elseif (isvector (A))
    t = A(1);
  else
    t = sum (diag (A));
  endif

endfunction


%!assert (trace ([1, 2; 3, 4]), 5)
%!assert (trace ([1, 2; 3, 4; 5, 6]), 5)
%!assert (trace ([1, 3, 5; 2, 4, 6]), 5)
%!assert (trace ([]), 0)
%!assert (trace (rand (1,0)), 0)
%!assert (trace ([3:10]), 3)

%!error <Invalid call> trace ()
%!error <only valid on 2-D objects> trace (reshape (1:9,[1,3,3]))