FR: allow natural syntax for semi-anti-joins

[MichaelChirico]

I posed this as an SO Q a while back; I still think it'd be a useful feature to see implemented for clarity of code.

Example:

DT <- data.table(a = rep(LETTERS, each = 10), id = rep(1:10, 26), key = c("a", "id"))
#desired syntax
DT[.(!"A", !3:4)]
#equivalent current syntax
DT[CJ(setdiff(unique(a), "A"), setdiff(unique(id), 3:4))]

[jangorecki]

I think we first need to have != operator (and !(a%in%b)) for non-equi join, then it could be processed fast. Currently it may be better to use just i = a!="A" & !(id %in% 3:4) instead of proposed CJ syntax which won't scale in memory. Of course it depends on the data, but better to wait using vector scan than get a memory issues and swapping.

[arunsrinivasan]

Agree with Jan. This'll become more straightforward when != is added to non-equi joins. Have added to #1801.