an object from which to drop unused factor levels.
passed to factor(); factor levels which
should be excluded from the result even if present. Note that this
was implicitlyNA in R <= 3.3.1 which did drop
NA levels even when present in x, contrary to the
documentation. The current default is compatible with x[ , drop=TRUE].
further arguments passed to methods
indices of columns from which not to drop levels
droplevels returns an object of the same class as x
The method for class "factor" is currently equivalent to
factor(x, exclude=exclude). For the data frame method, you
should rarely specify exclude “globally” for all factor
columns; rather the default uses the same factor-specific
exclude as the factor method itself.
The except argument follow the usual indexing rules.
subset for subsetting data frames.
factor for definition of factors.
drop for dropping array dimensions.
drop1 for dropping terms from a model.
[.factor for subsetting of factors.