groupid is an enhanced version of data.table::rleid for atomic vectors. It generates a run-length type group-id where consecutive identical values are assigned the same integer. It is a generalization as it can be applied to unordered vectors, generate group id's starting from an arbitrary value, and skip missing values.

groupid(x, o = NULL, start = 1L, na.skip = FALSE, check.o = TRUE)

## Arguments

x

an atomic vector of any type. Attributes are not considered.

o

an (optional) integer ordering vector specifying the order by which to pass through x.

start

integer. The starting value of the resulting group-id. Default is starting from 1.

na.skip

logical. Skip missing values i.e. if TRUE something like groupid(c("a", NA, "a")) gives c(1, NA, 1) whereas FALSE gives c(1, 2, 3).

check.o

logical. Programmers option: FALSE prevents checking that each element of o is in the range [1, length(x)], it only checks the length of o. This gives some extra speed, but will terminate R if any element of o is too large or too small.

## Value

An integer vector of class 'qG'. See qG.

## Examples

groupid(airquality$Month) #> [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 #> [39] 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 #> [ reached getOption("max.print") -- omitted 83 entries ] #> attr(,"N.groups") #> [1] 5 #> attr(,"class") #> [1] "qG" "na.included" groupid(airquality$Month, start = 0)
#>  [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1
#> [39] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2
#>  [ reached getOption("max.print") -- omitted 83 entries ]
#> attr(,"N.groups")
#> [1] 5
groupid(wlddev$country)[1:100] #> [1] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 #> [39] 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 #> [ reached getOption("max.print") -- omitted 30 entries ] ## Same thing since country is alphabetically ordered: (groupid is faster..) all.equal(groupid(wlddev$country), qG(wlddev$country, na.exclude = FALSE)) #> [1] TRUE ## When data is unordered, group-id can be generated through an ordering.. uo <- order(rnorm(fnrow(airquality))) monthuo <- airquality$Month[uo]
o <- order(monthuo)
groupid(monthuo, o)
#>  [1] 5 4 5 3 3 4 4 1 1 4 4 3 1 1 1 3 1 1 3 4 5 1 1 3 3 5 4 2 1 3 1 5 1 2 4 2 5 4
#> [39] 1 2 3 2 1 3 1 4 4 2 3 4 5 2 3 5 3 2 3 1 1 1 4 5 3 1 2 4 5 2 1 2
#>  [ reached getOption("max.print") -- omitted 83 entries ]
#> attr(,"N.groups")
#> [1] 5
#> attr(,"class")
#> [1] "qG"          "na.included"
identical(groupid(monthuo, o)[o], unattrib(groupid(airquality\$Month)))
#> [1] TRUE