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25
26## -*- texinfo -*-
27## @deftypefn {} {} trace (@var{A})
28## Compute the trace of @var{A}, the sum of the elements along the main
29## diagonal.
30##
31## The implementation is straightforward: @code{sum (diag (@var{A}))}.
32## @seealso{eig}
33## @end deftypefn
34
35function y = trace (A)
36
37  if (nargin != 1)
38    print_usage ();
39  endif
40
41  if (ndims (A) > 2)
42    error ("trace: only valid on 2-D objects");
43  elseif (isempty (A))
44    y = 0;
45  elseif (isvector (A))
46    y = A(1);
47  else
48    y = sum (diag (A));
49  endif
50
51endfunction
52
53
54%!assert (trace ([1, 2; 3, 4]), 5)
55%!assert (trace ([1, 2; 3, 4; 5, 6]), 5)
56%!assert (trace ([1, 3, 5; 2, 4, 6]), 5)
57%!assert (trace ([]), 0)
58%!assert (trace (rand (1,0)), 0)
59%!assert (trace ([3:10]), 3)
60
61%!error trace ()
62%!error trace (1, 2)
63%!error <only valid on 2-D objects> trace (reshape (1:9,[1,3,3]))
64