help charlie

dsetin.R

library(data.table)

dt = data.table(i=rep(1:5, each=10), t=rep(1:5, times=2), x=runif(50), key=c('i', 't'))

myfunc = function(x) {
    return(data.frame(sum=sum(x), mean=mean(x)))
}

dt[, myfunc(x), by=.(i, t)]

myfunc2 = function(df) {
    setDF(df)
    return(data.frame(sum=sum(df$x), mean=mean(df$x)))
}

dt[, myfunc2(.SD), by=.(i, t)]

myfunc3 = function(dt) {
    df = data.frame(dt)
    return(data.frame(sum=sum(df$x), mean=mean(df$x)))
}

dt[, myfunc3(.SD), by=.(i, t)]