Each variable named in the expression after the `|' operator on
the right hand side of
form is evaluated in
object. If
more than one variable is indicated in
level they are combined
into a data frame; else the selected variable is returned as a vector.
When multiple grouping levels are defined in
form and
`level > 1', the levels of the returned factor are obtained by
pasting together the levels of the grouping factors of level greater
or equal to
level, to ensure their uniqueness.
USAGE:
getGroups(object, form, level)
REQUIRED ARGUMENTS:
object
an object inheriting from class
data.frame.
OPTIONAL ARGUMENTS:
form
an optional formula with a conditioning expression on its
right hand side (i.e. an expression involving the `|'
operator). Defaults to
formula(object).
level
a positive integer vector with the level(s) of grouping to
be used when multiple nested levels of grouping are present. Defaults
to all levels of nesting.
VALUE:
either a data frame with columns given by the grouping factors
indicated in
level, from outer to inner, or, when a single
level is requested, a factor representing the selected grouping
factor.