fill
If set, missing values will be replaced with this value. Note
that there are two types of missingness in the input: explicit missing
values (i.e. NA
), and implicit missings, rows that simply aren't
present. Both types of missing value will be replaced by fill
.
drop
If FALSE
, will keep factor levels that don't appear in the
data, filling in missing combinations with fill
.
sep
If NULL
, the column names will be taken from the values of
key
variable. If non-NULL
, the column names will be given
by "".