R data.table weird value/reference semantics

Question

(This is a follow up question to this.)

Check this toy code:

> x <- data.frame(a = 1:2)
> foo <- function(z) { setDT(z) ; z[, b:=3:4] ; z } 
&g         


        

Answer 1

library(data.table)

x <- data.frame(a = 1:2)
y <- x                #y is a reference to x
address(x)
#[1] "0x55e07e31a1e8"
address(y)
#[1] "0x55e07e31a1e8"
setDT(y)              #Add data.table to attr of y AND x, create a copy of it and let y point to it and make y a DT
address(x)
#[1] "0x55e07e31a1e8"
address(y)
#[1] "0x55e07e7b1300"
class(x)
#[1] "data.table" "data.frame"

x[, b:=3:4]
#Warnmeldung:
#In `[.data.table`(x, , `:=`(b, 3:4)) :
#  Invalid .internal.selfref detected and fixed by taking a (shallow) copy of the data.table so that := can add this new column by reference. At an earlier point, this data.table has been copied by R (or was created manually using structure() or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?setattr. If this message doesn't help, please report your use case to the data.table issue tracker so the root cause can be fixed or this message improved.

z <- data.frame(a = 1:2)
class(z) <- c("data.table", "data.frame")
z[, b:=3:4]
#Warnmeldung:
#In `[.data.table`(x, , `:=`(b, 3:4)) :
#  Invalid .internal.selfref detected and fixed by taking a (shallow) copy of the data.table so that := can add this new column by reference. At an earlier point, this data.table has been copied by R (or was created manually using structure() or similar). Avoid names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?setattr. If this message doesn't help, please report your use case to the data.table issue tracker so the root cause can be fixed or this message improved.