Remove unnecessary lock on data.table

[jangorecki]

unique(.SD) makes the lock on data.table:

library(data.table)
data.table(a=c(1,1,2))[, unique(.SD)][, b := 5]
# Error in `[.data.table`(data.table(a = c(1, 1, 2))[, unique(.SD)], , `:=`(b,  : 
#   .SD is locked. Using := in .SD's j is reserved for possible future use; a tortuously flexible way to modify by group. Use := in j directly to modify by group by reference.

While this working

data.table(a=c(1,1,2))[, unique(.SD)][, copy(.SD)][, b := 5]
#    a b
#1: 1 5
#2: 2 5

As the unique(.SD) is not a by reference operation, such := should work IMO.

[franknarf1]

Not sure if this is any less wasteful than a copy, but c(unique(.SD)) is unlocked:

data.table(a=c(1,1,2))[, c(unique(.SD))][, b := 5]