getGroups.data.frame: Extract Groups from a Data Frame
Description
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
## S3 method for class 'data.frame':
getGroups(object, form, level, data, sep)
Arguments
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.
References
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models
in S and S-PLUS", Springer, esp. pp. 100, 461.