apply a Positioning Method to every group. works like ddply from
plyr package, but the grouping column is always called groups, and
the Positioning Method is not necessarily a function (but can be).
gapply(d, method, ...,
groups = "groups")data frame with column groups.
Positioning Method to apply to every group separately.
additional arguments, passed to Positioning Methods.
can also be useful for piece column.
data frame of results after applying FUN to each group in d.