Invalid .internal.selfref in data.table

Question

I needed to assign a \"second\" id to group some values inside my original id. this is my sample data:

dt<-structure(list(id = c(\"aaaa\", \"         


        

Answer 1

You could "shallow copy" while creating the list, so that 1) you don't do full memory copy (speed isn't affected) and 2) you don't get internal ref error (thanks to @mnel for this trick).

Creating data:

set.seed(45)
ss <- function() {
    tt <- sample(1:10, 1e6, replace=TRUE)
}
tt <- replicate(100, ss(), simplify=FALSE)
tt <- as.data.table(tt)

How you should go about creating the list (shallow copy):

system.time( {
    ll <- list(d1 = { # shallow copy here...
        data.table:::settruelength(tt, 0)
        invisible(alloc.col(tt))
    }, "a")
})
user  system elapsed
   0       0       0
> system.time(tt[, bla := 2])
   user  system elapsed
  0.012   0.000   0.013
> system.time(ll[[1]][, bla :=2 ])
   user  system elapsed
  0.008   0.000   0.010

So you don't compromise in speed and you don't get a warning followed by a full copy. Hope this helps.