Extract Groups from a Data Frame
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.
## S3 method for class 'data.frame' getGroups(object, form, level, data, sep)
object |
an object inheriting from class |
form |
an optional formula with a conditioning expression on its
right hand side (i.e. an expression involving the |
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. |
data |
unused |
sep |
character, the separator to use between group levels when
multiple levels are collapsed. The default is |
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.
José Pinheiro and Douglas Bates bates@stat.wisc.edu
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer, esp. pp. 100, 461.
getGroups(Pixel) getGroups(Pixel, level = 2)
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