attributes() and attr()

The function attributes(object) gives a list of all the non-intrinsic attributes currently defined for that object. The function attr(object,name) can be used to select a specific attribute. These functions are rarely used, except in rather special circumstances when some new attribute is being created for some particular purpose, for example to associate a creation date or an operator with an . object. The concept, however, is very important.

Some care should be exercised when assigning or deleting attributes since they are an integral part of the object system used in ..

When it is used on the left hand side of an assignment it can be used either to associate a new attribute with object or to change an existing one. For example

attr(z,"dim") <- c(10,10)

allows . to treat z as if it were a $10\times10$ matrix.