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.