rowid {data.table}R Documentation

Generate unique row ids within each group


Convenience functions for generating a unique row ids within each group. It accepts atomic vectors, lists, data.frames or data.tables as input.

rowid is intended for interactive use, particularly along with the function dcast to generate unique ids directly in the formula.

rowidv(DT, cols=c("x", "y")) is equivalent to column N in the code DT[, N := seq_len(.N), by=c("x", "y")].

See examples for more.


rowid(..., prefix=NULL)
rowidv(x, cols=seq_along(x), prefix=NULL)



A vector, list, data.frame or data.table.


A sequence of numeric, integer64, character or logical vectors, all of same length. For interactive use.


Only meaningful for lists, data.frames or data.tables. A character vector of column names (or numbers) of x.


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).


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, rleid


DT = data.table(x=c(20,10,10,30,30,20), y=c("a", "a", "a", "b", "b", "b"), z=1:6)

rowid(DT$x) # 1,1,2,1,2,2
rowidv(DT, cols="x") # same as above

rowid(DT$x, prefix="group") # prefixed with 'group'

rowid(DT$x, DT$y) # 1,1,2,1,2,1
rowidv(DT, cols=c("x","y")) # same as above
DT[, .(N=seq_len(.N)), by=.(x,y)]$N # same as above

# convenient usage with dcast
dcast(DT, x ~ rowid(x, prefix="group"), value.var="z")
#     x group1 group2
# 1: 10      2      3
# 2: 20      1      6
# 3: 30      4      5

[Package data.table version 1.15.99 Index]