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.

Answer 2

I am sure that it is not the best solution, but that works.

dt[a%in% dt[, a[2%in%b], by=a][, a],]

Answer 3

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