If no other arguments are supplied it depends on the type of x. The first item
of a vector or list. The first row of a data.frame
or data.table
.
Otherwise, whatever xts::first
returns (if package xts has been loaded,
otherwise a helpful error).
If any argument is supplied in addition to x
(such as n
or
keep
in xts::first
), regardless of x
's type, then
xts::first
is called if xts has been loaded, otherwise a helpful error.