Flag a firm's chronologically first occurrence of a 0/1 event — first_occurrence

Returns 1 only on the earliest time value (per id) at which x == 1 — not the first row of each subsequent episode, the single first occurrence ever. NA values in x are treated as "not an event" and do not break the chain (matching Stata's sum() treating missing as 0 in the cumulative-sum idiom the source uses for this same purpose — see Pseudocode/00_conventions.md, FIRST_OCCURRENCE_FLAG).

Usage

first_occurrence_flag(id, time, x)

Arguments

id: Panel unit identifier vector (e.g. idf).
time: Panel time vector (e.g. year).
x: A 0/1 (or NA) numeric vector, the same length as id/time, typically a high-growth-firm flag.

Value

An integer vector (0L/1L), the same length and order as x.