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## -*- texinfo -*-
## @deftypefn {} {@var{tf} =} isdiag (@var{A})
## Return true if @var{A} is a diagonal numeric matrix which is defined as a
## 2-D array where all elements above and below the main diagonal are zero.
## @seealso{isbanded, istril, istriu, diag, bandwidth}
## @end deftypefn

function tf = isdiag (A)

  if (nargin < 1)
    print_usage ();
  endif

  if (strfind (typeinfo (A), "diagonal matrix"))
    tf = true;
  elseif ((isnumeric (A) || islogical (A)) && ndims (A) == 2)
    [i, j] = find (A);
    tf = all (i == j);
  else
    tf = false;
  endif

endfunction


%!assert (isdiag ("string"), false)
%!assert (isdiag (zeros (2,2,2)), false)
%!assert (isdiag (zeros (2)))
%!assert (isdiag ([]))
%!assert (isdiag (1))
%!assert (isdiag ([1, 1]), false)
%!assert (isdiag ([1; 1]), false)
%!assert (isdiag (eye (10)))
%!assert (isdiag (single (eye (10))))
%!assert (isdiag (logical (eye (10))))
%!assert (isdiag (speye (1e2)))
%!assert (isdiag (diag (1:10)))

## Test input validation
%!error <Invalid call> isdiag ()