if (require("hflights")) {
hflights_dt <- tbl_dt(hflights)
group_size(group_by(hflights_dt, Year, Month, DayofMonth))
group_size(group_by(hflights_dt, Dest))
monthly <- group_by(hflights_dt, Month)
summarise(monthly, n = n(), delay = mean(ArrDelay))
}
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