Changing factor levels on a column with setattr is sensitive for how the column was created

Question

I want to change factor levels of a column using setattr. However, when the column is selected the standard data.table way (dt[ , col]), the levels are not updated. On the other hand, when selecting the column in an unorthodox way in a data.table setting—namely using $—it works.

library(data.table)

# Some data 
d <- data.table(x = factor(c("b", "a", "a", "b")), y = 1:4)
d
#    x y
# 1: b 1
# 2: a 2
# 3: a 3
# 4: b 4

# We want to change levels of 'x' using setattr
# New desired levels
lev <- c("a_new", "b_new")

# Select column in the standard data.table way 
setattr(x = d[ , x], name = "levels", value = lev)

# Levels are not updated
d
#    x y
# 1: b 1
# 2: a 2
# 3: a 3
# 4: b 4

# Select column in a non-standard data.table way using $
setattr(x = d$x, name = "levels", value = lev)

# Levels are updated
d
#        x y
# 1: b_new 1
# 2: a_new 2
# 3: a_new 3
# 4: b_new 4

# Just check if d[ , x] really is the same as d$x
d <- data.table(x = factor(c("b", "a", "a", "b")), y = 1:4)
identical(d[ , x], d$x)
# [1] TRUE
# Yes, it seems so

It feels like I'm missing some data.table (R?) basics here. Can anyone explain what's going on?

---

I have found two other post on setattr and levels:

setattr on levels preserving unwanted duplicates (R data.table)

How does one change the levels of a factor column in a data.table

Both of them used $ to select the column. Neither of them mentioned the [ , col] way.

Answer 1

It might help to understand if you look at the address from both expressions:

address(d$x)
# [1] "0x10e4ac4d8"
address(d$x)
# [1] "0x10e4ac4d8"


address(d[,x])
# [1] "0x105e0b520"
address(d[,x])
# [1] "0x105e0a600"

Note that the address from the first expression doesn't change when you call it multiple times, while the second expression does which indicates it is making a copy of the column due to the dynamic nature of the address, so setattr on it will have no effect on the original data.table.