Add DT[a==1][, b:=1] vs DT[a==1, b:=1] to ?":="

[matthieugomez]

I could not find anything in the documentation about how

DT <- data.table (a = 1)
DT[a==1][, b:=1]

is actually different from

DT <- data.table (a = 1)
DT1 <- DT[a==1]
DT1[,  b := 1]

Maybe the distinction should be written somewhere. This behavior surprised me.

[mattdowle]

They are the same in fact, iiuc. DT[a==1] returns a new data.table. In the first example the compound [,b:=1] is applied to that new data.table and the result hasn't been assigned to a symbol so that's lost. DT1 <- DT[a==1][, b:=1] was probably intended. And that's identical to the the DT1 that the 2nd example creates.

Agreed on documentation. Maybe it should state that := assigns by reference to whatever object is "passed" immediately to the left of the square bracket. Grateful for any suggestions of what exactly to write and where. (I find that when I ask, the response it often different to what I would have added!) I just had a look and ?":=" seems to be lacking on compound examples.

[matthieugomez]

Sorry - your explanation is clear, now I don't know even why I thought the two should give the same result.

[mattdowle]

No worries. Still, a good addition to ?":=". Will do.

[arunsrinivasan]

Added as Note to ?":=".