fun(object, ...)
The functions of the
Summary
group
(
sum()
,
prod()
,
any()
, etc) are those
functions that return a single value summarizing their input.
This method always produces an error, since all the Summary Group functions
require a numerical or logical object as an argument.
Indeed, the method exists
becauseit is necessary to prevent interpretation of factors or ordered factors
as numeric objects.
Use
codes()
if you want a numeric representation of the equivalence
relation defined by the factor.