get_id(), get_wave(), and get_periods() are extractor
functions that can be used to retrieve the names of the id and wave
variables or time periods of a panel_data frame.
Examples
data("WageData")
wages <- panel_data(WageData, id = id, wave = t)
get_wave(wages)
#> [1] "t"
get_id(wages)
#> [1] "id"
get_periods(wages)
#> [1] 1 2 3 4 5 6 7