interactive datatables for shiny

DT <- as.data.table(iris)
DT[, MONTHS := sample(month.abb, size = .N, replace = TRUE)]
DT[, LETTERS := sample(LETTERS[rep(1:5, 3)], size = .N, replace = TRUE)]
DT

grp1 <- c("Species", "LETTERS")
grp2 <- c("MONTHS")
resDT <- DT[, .(grpDT = {
   res <- list(as.data.table(.SD)[, .N, grp2])
   res
}), grp1]

## SUM OF COUNTS IN ALL SEPTEMBERS ACROSS LETTERS FOR THE SPECIES SETOSA
sum(unlist(lapply(resDT[Species == "setosa", grpDT], function(dt) dt[MONTHS == "Sep", N])))