Hi, Here an example what I want to get :
> dt <- data.table(c(1,2,3,4),c("a","b","b","a"))
> dt
V1 V2
1: 1 a
2: 2 b
3: 3 b
4: 4 a
What I want to get is a V3 column, containing an aggregated vector :
> dt
V1 V2 V3
1: 1 a c(1,4)
2: 2 b c(2,3)
That does not work :
dt[,V3 := as.list(c(V1)),by=V2]
Is there a way to do so ?
Thanks
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