grouping returns a permutation which rearranges its first
argument such that identical values are adjacent to each other. Also
returned as attributes are the group-wise partitioning and the maximum
- a sequence of numeric, character or logical vectors, all of the same length, or a classed R object.
The function partially sorts the elements so that identical values are
NA values come first. This is guaranteed to be
stable, so ties are preserved, and if the data are already
grouped/sorted, the grouping is unchanged. This is useful for
aggregation and is particularly fast for character vectors.
Under the covers, the
"radix" method of
used, and the same caveats apply, including reduced precision when
comparing real values, restrictions on character encodings and lack of
support for long vectors (those with $2^31$ or more elements).
order, for a classed R object the grouping is based on
the result of
An object of class "grouping", the representation of which
should be considered experimental and subject to change. It is an
integer vector with two attributes:
- subscripts in the result corresponding to the last member of each group
- the maximum group size
(ii <- grouping(x <- c(1, 1, 3:1, 1:4, 3), y <- c(9, 9:1), z <- c(2, 1:9))) ## 6 5 2 1 7 4 10 8 3 9 rbind(x, y, z)[, ii]