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

You can use an if condition with .SD after grouping dt by column a:

dt[, if(2 %in% b) .SD, a]

#   a b
#1: 1 5
#2: 1 2
#3: 1 2
#4: 2 3
#5: 2 5
#6: 2 2

From ?.SD, .SD is a data.table containing the Subset of x's Data for each group. Combined with if condition, we return nothing if 2 is not in column b and drop the corresponding group.