attr it is possible to set attributes on a
NULL object: it will first be coerced to an empty list.
Note that some attributes (namely
tsp) are treated specially and have restrictions on
the values which can be set. (Note that this is not true of
levels which should be set for factors via the
levels replacement function.)
Attributes are not stored internally as a list and should be thought
of as a set and not a vector, i.e, the order of the elements of
attributes() does not matter. This is also reflected by
identical()'s behaviour with the default argument
attrib.as.set = TRUE. Attributes must have unique names (and
NA is taken as
"NA", not a missing value).
Assigning attributes first removes all attributes, then sets any
dim attribute and then the remaining attributes in the order
given: this ensures that setting a
dim attribute always precedes
mostattributes assignment takes special care for the
attributes, and assigns them only when known to be valid whereas an
attributes assignment would give an error if any are not. It
is principally intended for arrays, and should be used with care on
classed objects. For example, it does not check that
row.names are assigned correctly for data frames.
The names of a pairlist are not stored as attributes, but are reported
as if they were (and can be set by the replacement form of
NULL objects cannot have attributes and attempts to
assign them will promote the object to an empty list.
Both assignment and replacement forms of