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## -*- texinfo -*-
## @deftypefn {} {} cosd (@var{x})
## Compute the cosine for each element of @var{x} in degrees.
##
## The function is more accurate than @code{cos} for large values of @var{x}
## and for multiples of 90 degrees (@code{@var{x} = 90 + 180*n} with n an
## integer) where @code{cosd} returns 0 rather than a small value on the order
## of eps.
## @seealso{acosd, cos}
## @end deftypefn

function y = cosd (x)

  if (nargin < 1)
    print_usage ();
  endif

  if (! isnumeric (x))
    error ("cosd: X must be numeric");
  endif

  ## Advance phase by 90 degrees to transform sin to cos and use sind().
  y = sind (x + 90);

endfunction


%!assert (cosd (10:20:360), cos ([10:20:360] * pi/180), 5*eps)
%!assert (cosd ([-270, -90, 90, 270]) == 0)
%!assert (cosd ([-360, -180, 0, 180, 360]), [1, -1, 1, -1, 1])
%!assert (cosd ([-Inf, NaN, +Inf, 0]), [NaN, NaN, NaN, 1])
%!assert (cosd (+23) == cosd (-23))
%!assert (cosd (1e6), 0.17364817766693033, 5*eps)
%!assert (cosd (90 + 180i), -i*sinh (pi))
%!assert (cosd (1e6 + 180i), 2.01292156189451577 + 11.3732880565446539i, 5*eps)

%!error <Invalid call> cosd ()
%!error <X must be numeric> cosd ("abc")