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(\         


        

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.

Answer 2

For conciseness, you can now use .() instead of list()

DT[, .(z=sum(y)+3, a=mean(y*z)), by=x]