strange error with non-empty empty j

[MichaelChirico]

This is pretty minor, but worth noting/documenting/fixing if it's easy. I came across it when commenting out parts of my code to test the output of a join absent performing the j operation:

library(data.table)
DT = data.table(a = 1:5)
DT[ , {}]

Error in jsub[[2L]] : subscript out of bounds

Closest operation in base R I guess is:

with(DT, {})
# NULL

I guess I would expect this to just return DT since that's what DT[ , ] does.

[MichaelChirico]

Just at a glance, the problem is j is not missing (as in DT[ , ]), but length(jsub) == 1L. Around here [.data.table runs a few tests that look at jsub[[2L]]. So we could just put an escape when length(jsub) == 1L...