data.table

Produce frequency table by group

In R:	In Stata:
dt[, .(rowcount = .N), by=group]	tab group

Compute max of var per group

In R:	In Stata:
dt[, group_max := max(var), by=group]	by group: egen group_max = max(var)

Create row id within group

In R:	In Stata:
dt[, r_id:=seq_len(.N), by=group]	by group: g r_id = _n

lubridate (w/ data.table)

Create Date from string

dt[, date_var := as.Date(as.character(num_date), format=”%Y%m%d”)]

Convert Date to year-quarter

dt[, yq := quarter(date_var, with_year=TRUE)

Create Date from components (e.g., year-quarter from year and quarter)

yq = quarter(parse_date_time(paste0(year, “.”, quarter), orders=”Yq”)