Setting column name in "group by" operation with data.table

data.table, r

Solution

DT[,list(z=sum(y)+3,a=mean(y*z)),by=x]
   x  z  a
1: a  6  9
2: b 15 60

Since you are new to `data.table`, I recommend that you also study the help page of the `setnames` function as well as `?data.table` and the `data.table` vignettes.

Problem

I am a new user of the data.table package in R. I am trying to give a name to the new column created by a "group by" command ``` > DT = data.table(x=rep(c("a","b"),c(2,3)),y=1:5) > DT x y 1: a 1 2: a 2 3: b 3 4: b 4 5: b 5 > DT[,{z=sum(y);z+3},by=x] x V1 1: a 6 2: b 15 ``` - I would like to name the V1 (default) column directly (not having to use colnames), is it possible? Additionally, is it possible to perform several group by operations in one command, that would result in something like: ``` x V1 V2 1: a 6 something 2: b 15 something ``` Thanks

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