Choose groups to keep/drop in data.table

Question

How can I drop/keep groups according to a condition in data.table? Is there a better method than adding a new column, then filtering on that column and removing it?

Answer 1

Here is another method that uses .I to return the row indices of selected groups and then subsets on the row:

dt[dt[, .I[2 %in% b], a]$V1]
   a b
1: 1 5
2: 1 2
3: 1 2
4: 2 3
5: 2 5
6: 2 2