[Request] Filter by group criteria

[red-swan]

DT[ .N > 60, by = groupingID]
to quickly filter out any groups where you don't have 60 observations or
DT[ min(Date) > "1989-11-09" , by = groupingID]
to filter out a group that started before the Berlin Wall fell

I'm new to github and don't know how this all works. Forgive me if this is not proper protocol. Thank you.

[franknarf1]

I think this is roughly the same as #788

[red-swan]

If this is something that you've decided you'd like to implement, I'd love to help. I'm new to collaborating, though, so I would need help to be helpful.

[arunsrinivasan]

Yes, it's a dup. There's already a way to do it.. DT[, .SD[.N > 60], by=ID]. It just needs to be optimised better.. which I'll work on later for this release.

PS: of course we'd be happy to receive PRs. There are quite a few issues for you to choose from :-).