attributes: Object Attribute Lists

Description

These functions access an object's attributes. The first form below returns the object's attribute list. The replacement forms uses the list on the right-hand side of the assignment as the object's attributes (if appropriate).

Usage

attributes(obj)
attributes(obj) <- value
mostattributes(obj) <- value

Arguments

obj

an object

value

an appropriate named list of attributes, or NULL.

Details

Unlike 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 class, comment, dim, dimnames, names, row.names and 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 the dimnames attribute.

The mostattributes assignment takes special care for the dim, names and dimnames 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 attributes).

NULL objects cannot have attributes and attempts to assign them will promote the object to an empty list.

Both assignment and replacement forms of attributes are primitive functions.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

Examples

Run this code

# NOT RUN {
x <- cbind(a = 1:3, pi = pi) # simple matrix with dimnames
attributes(x)

## strip an object's attributes:
attributes(x) <- NULL
x # now just a vector of length 6

mostattributes(x) <- list(mycomment = "really special", dim = 3:2,
   dimnames = list(LETTERS[1:3], letters[1:5]), names = paste(1:6))
x # dim(), but not {dim}names
# }