Generate unique row ids within each group
rowid.Rd
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.
Usage
rowid(..., prefix=NULL)
rowidv(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).
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.
Examples
DT = data.table(x=c(20,10,10,30,30,20), y=c("a", "a", "a", "b", "b", "b"), z=1:6)
rowid(DT$x) # 1,1,2,1,2,2
#> [1] 1 1 2 1 2 2
rowidv(DT, cols="x") # same as above
#> [1] 1 1 2 1 2 2
rowid(DT$x, prefix="group") # prefixed with 'group'
#> [1] "group1" "group1" "group2" "group1" "group2" "group2"
rowid(DT$x, DT$y) # 1,1,2,1,2,1
#> [1] 1 1 2 1 2 1
rowidv(DT, cols=c("x","y")) # same as above
#> [1] 1 1 2 1 2 1
DT[, .(N=seq_len(.N)), by=.(x,y)]$N # same as above
#> [1] 1 1 2 1 2 1
# convenient usage with dcast
dcast(DT, x ~ rowid(x, prefix="group"), value.var="z")
#> Key: <x>
#> x group1 group2
#> <num> <int> <int>
#> 1: 10 2 3
#> 2: 20 1 6
#> 3: 30 4 5
# x group1 group2
# 1: 10 2 3
# 2: 20 1 6
# 3: 30 4 5