Generate run-length type group id

A convenience function for generating a run-length type id column to be used in grouping operations. It accepts atomic vectors, lists, data.frames or data.tables as input.

rleid(..., prefix=NULL)
rleidv(x, cols=seq_along(x), prefix=NULL)

Arguments

x	A vector, list, data.frame or data.table.
...	A sequence of numeric, integer64, character or logical vectors, all of same length. For interactive use.
cols	Only meaningful for lists, data.frames or data.tables. A character vector of column names (or numbers) of x.
prefix	Either NULL (default) or a character vector of length=1 which is prefixed to the row ids, returning a character vector (instead of an integer vector).

Details

At times aggregation (or grouping) operations need to be performed where consecutive runs of identical values should belong to the same group (See rle). The use for such a function has come up repeatedly on StackOverflow, see the See Also section. This function allows to generate "run-length" groups directly.

rleid is designed for interactive use and accepts a sequence of vectors as arguments. For programming, rleidv might be more useful.

Value

When prefix = NULL, an integer vector with same length as NROW(x), else a character vector with the value in prefix prefixed to the ids obtained.

See also

data.table, rowid, http://stackoverflow.com/q/21421047/559784

Examples

DT = data.table(grp=rep(c("A", "B", "C", "A", "B"), c(2,2,3,1,2)), value=1:10)
rleid(DT$grp) # get run-length ids
#>  [1] 1 1 2 2 3 3 3 4 5 5
rleidv(DT, "grp") # same as above
#>  [1] 1 1 2 2 3 3 3 4 5 5

rleid(DT$grp, prefix="grp") # prefix with 'grp'
#>  [1] "grp1" "grp1" "grp2" "grp2" "grp3" "grp3" "grp3" "grp4" "grp5" "grp5"

# get sum of value over run-length groups
DT[, sum(value), by=.(grp, rleid(grp))]
#>    grp rleid V1
#> 1:   A     1  3
#> 2:   B     2  7
#> 3:   C     3 18
#> 4:   A     4  8
#> 5:   B     5 19
DT[, sum(value), by=.(grp, rleid(grp, prefix="grp"))]
#>    grp rleid V1
#> 1:   A  grp1  3
#> 2:   B  grp2  7
#> 3:   C  grp3 18
#> 4:   A  grp4  8
#> 5:   B  grp5 19