documentation of ":=" (or maybe rather "[" actually)

[joseffrank]

After creation of a data.table

library(data.table)
a=fread("
nr,a
1,a
2,b
")
a

a new one is created the following way:

b <- a[]
b

NB: I did not just assign b <- a but b <- a[]. According to the documentation (see below) I'd expect b to be independent from a, and changes not reflect back to a. On a first glance that seems to be the case:

b[1, 1] <- 4
b
a
all(b==a)

However when using := to assign the new value that logic seems broken:

b <- a[]

b
b[1, nr:=4]
b
a
all(b==a)

What am I missing from the documentation, which says that the subsetting operation returns a new data.table, which then in turn is updated by reference using the := operator?

The effect is the same when creating b as b <- a[TRUE]. However when using b <- a[1:.N] then both objects seem to be separate entities as I'd expect.

See Note section of :=:

DT[a > 4, b := c] is different from DT[a > 4][, b := c]. The first expression updates (or adds) column b with the value c on those rows where a > 4 evaluates to TRUE. X is updated by reference, therefore no assignment needed.

The second expression on the other hand updates a new data.table that's returned by the subset operation. Since the subsetted data.table is ephemeral (it is not assigned to a symbol), the result would be lost; unless the result is assigned, for example, as follows: ans <- DT[a > 4][, b := c].

> sessionInfo()
R version 4.1.3 (2022-03-10)
Platform: i386-w64-mingw32/i386 (32-bit)
Running under: Windows 10 x64 (build 18363)

Matrix products: default

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252
[3] LC_MONETARY=German_Germany.1252 LC_NUMERIC=C
[5] LC_TIME=German_Germany.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] data.table_1.14.2

loaded via a namespace (and not attached):
[1] compiler_4.1.3

[jangorecki]

Thank you for reporting. Related issue #3215