library(data.table)
#> Warning: package 'data.table' was built under R version 4.0.5
options(datatable.verbose=TRUE)
d = as.data.table(iris)


d[, head(.SD,1L),by="Species"]
#> Finding groups using forderv ... forder.c received 150 rows and 1 columns
#> 0.000s elapsed (0.000s cpu) 
#> Finding group sizes from the positions (can be avoided to save RAM) ... 0.000s elapsed (0.000s cpu) 
#> lapply optimization changed j from 'head(.SD, 1L)' to 'list(head(Sepal.Length, 1L), head(Sepal.Width, 1L), head(Petal.Length, 1L), head(Petal.Width, 1L))'
#> GForce optimized j to 'list(ghead(Sepal.Length, 1L), ghead(Sepal.Width, 1L), ghead(Petal.Length, 1L), ghead(Petal.Width, 1L))'
#> Making each group and running j (GForce TRUE) ... gforce initial population of grp took 0.001
#> gforce assign high and low took 0.000
#> gforce eval took 0.000
#> 0.000s elapsed (0.000s cpu)
#>       Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1:     setosa          5.1         3.5          1.4         0.2
#> 2: versicolor          7.0         3.2          4.7         1.4
#> 3:  virginica          6.3         3.3          6.0         2.5
d[, head(.SD,2L),by="Species"]
#> Finding groups using forderv ... forder.c received 150 rows and 1 columns
#> 0.000s elapsed (0.000s cpu) 
#> Finding group sizes from the positions (can be avoided to save RAM) ... 0.000s elapsed (0.000s cpu) 
#> lapply optimization changed j from 'head(.SD, 2L)' to 'list(head(Sepal.Length, 2L), head(Sepal.Width, 2L), head(Petal.Length, 2L), head(Petal.Width, 2L))'
#> GForce is on, left j unchanged
#> Old mean optimization is on, left j unchanged.
#> Making each group and running j (GForce FALSE) ... 
#>   memcpy contiguous groups took 0.000s for 3 groups
#>   eval(j) took 0.000s for 3 calls
#> 0.000s elapsed (0.000s cpu)
#>       Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1:     setosa          5.1         3.5          1.4         0.2
#> 2:     setosa          4.9         3.0          1.4         0.2
#> 3: versicolor          7.0         3.2          4.7         1.4
#> 4: versicolor          6.4         3.2          4.5         1.5
#> 5:  virginica          6.3         3.3          6.0         2.5
#> 6:  virginica          5.8         2.7          5.1         1.9


d[, tail(.SD,1L),by="Species"] 
#> Finding groups using forderv ... forder.c received 150 rows and 1 columns
#> 0.000s elapsed (0.000s cpu) 
#> Finding group sizes from the positions (can be avoided to save RAM) ... 0.000s elapsed (0.000s cpu) 
#> lapply optimization changed j from 'tail(.SD, 1L)' to 'list(tail(Sepal.Length, 1L), tail(Sepal.Width, 1L), tail(Petal.Length, 1L), tail(Petal.Width, 1L))'
#> GForce optimized j to 'list(gtail(Sepal.Length, 1L), gtail(Sepal.Width, 1L), gtail(Petal.Length, 1L), gtail(Petal.Width, 1L))'
#> Making each group and running j (GForce TRUE) ... gforce initial population of grp took 0.000
#> gforce assign high and low took 0.000
#> gforce eval took 0.000
#> 0.000s elapsed (0.000s cpu)
#>       Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1:     setosa          5.0         3.3          1.4         0.2
#> 2: versicolor          5.7         2.8          4.1         1.3
#> 3:  virginica          5.9         3.0          5.1         1.8
d[, tail(.SD,2L),by="Species"]
#> Finding groups using forderv ... forder.c received 150 rows and 1 columns
#> 0.000s elapsed (0.000s cpu) 
#> Finding group sizes from the positions (can be avoided to save RAM) ... 0.000s elapsed (0.000s cpu) 
#> lapply optimization changed j from 'tail(.SD, 2L)' to 'list(tail(Sepal.Length, 2L), tail(Sepal.Width, 2L), tail(Petal.Length, 2L), tail(Petal.Width, 2L))'
#> GForce is on, left j unchanged
#> Old mean optimization is on, left j unchanged.
#> Making each group and running j (GForce FALSE) ... 
#>   memcpy contiguous groups took 0.000s for 3 groups
#>   eval(j) took 0.000s for 3 calls
#> 0.000s elapsed (0.000s cpu)
#>       Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1:     setosa          5.3         3.7          1.5         0.2
#> 2:     setosa          5.0         3.3          1.4         0.2
#> 3: versicolor          5.1         2.5          3.0         1.1
#> 4: versicolor          5.7         2.8          4.1         1.3
#> 5:  virginica          6.2         3.4          5.4         2.3
#> 6:  virginica          5.9         3.0          5.1         1.8