Aggregating sub totals and grand totals with data.table

Question

I\'ve got a data.table in R:

library(data.table)
set.seed(1)
DT = data.table(
  group=sample(letters[1:2],100,replace=TRUE), 
  year=sample(2010         


        

Answer 1

I'm not aware of a simple way. Here's a first stab at an implementation. I don't know margins=TRUE in plyr, is this what that does?

crossby = function(DT, j, by) {
    j = substitute(j)
    ans = rbind(
        DT[,eval(j),by],
        DT[,list("Total",eval(j)),by=by[1]],
        cbind("Total",DT[,eval(j),by=by[2]]),
        list("Total","Total",DT[,eval(j)]),
        use.names=FALSE
        # 'use.names' argument added in data.table v1.8.0
    )
    setkeyv(ans,by)
    ans
}

crossby(DT, mean(v), c("group","year"))

      group  year        V1
 [1,]     a  2010 0.2926945
 [2,]     a  2011 0.4176346
 [3,]     a  2012 0.4227796
 [4,]     a Total 0.3901875
 [5,]     b  2010 0.5231845
 [6,]     b  2011 0.4997119
 [7,]     b  2012 0.4306871
 [8,]     b Total 0.4835788
 [9,] Total  2010 0.4278093
[10,] Total  2011 0.4463616
[11,] Total  2012 0.4271160
[12,] Total Total 0.4350153