%!demo
%! ## `double` returns the integer *category codes* — each value's 1-based index
%! ## into the category list.  `<undefined>` elements map to `NaN`.
%!
%! C = categorical ({'S'; 'M'; 'L'; ''; 'S'}, {'S', 'M', 'L'}, 'Ordinal', true)
%! categories (C)
%! double (C)
