> n  <-  4; k  <- 2
> mm <- data.table( a=  rep(1:k,n), b = seq_len(n*k) , d= rep(1:n,k))
> mm[ a == 2, e := sum(b),d][]
   a b d  e
1: 1 1 1 NA
2: 2 2 2  8
3: 1 3 3 NA
4: 2 4 4 12
5: 1 5 1 NA
6: 2 6 2  8
7: 1 7 3 NA
8: 2 8 4 12
> packageVersion("data.table")
[1] '1.14.2'

> n  <-  4 ; k  <- 2
> mm <- data.table( a=  rep(1:k,n), b = seq_len(n*k) , d= rep(1:n,k))
> mm[ a == 2, e := sum(b),d][]
Index: <a>
       a     b     d     e
   <int> <int> <int> <int>
1:     1     1     1     8
2:     2     2     2    12
3:     1     3     3     8
4:     2     4     4    12
5:     1     5     1    NA
6:     2     6     2    NA
7:     1     7     3    NA
8:     2     8     4    NA
> packageVersion("data.table")
[1] ‘1.14.3’