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

Question

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

1. I would like to name the V1 (default) column directly (not having to use colnames), is it possible?

2. 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

Answer 1

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.