PU
ntimes <- 100
test <- data.table(
group_id = rep(c('blue', 'green', 'red', 'black'), each = ntimes),
event = sample(c('tg1', 'tg2', 'tg3'), ntimes * 4, replace = TRUE),
ts = rep(seq(Sys.Date() - ntimes + 1, Sys.Date(), 1), 4))
test[, list(var = sum(event[ts %between% c('2017-09-01', '2017-09-15')] == 'tg1')), by = group_id]