Exactly one column, of any name, holding the position of each row
within its temporal context. It is declared separately from
variables_when, which holds the context itself — session, trial,
observation — and which, with variables_what, is what the frame is
grouped by. The index is never a grouping variable.
Details
An anievent() has none: a bout spans an interval rather than sitting
at a point, so it is delimited by start and stop, which are
declared temporal columns. Its variables_index is NA, and asking
for it here is an error rather than a guess.
See also
set_index() to change it, get_variables_when() for the
full set of temporal columns.
Examples
af <- example_aniframe(n_obs = 3, n_individuals = 1, n_keypoints = 1)
get_index(af)
#> [1] "time"