droplevels is used to drop unused levels from a factor or, more commonly, from factors in a data frame.
"droplevels"(x, ...) "droplevels"(x, except, ...)
- an object from which to drop unused factor levels.
- further arguments passed to methods
- indices of columns from which not to drop levels
The method for class
"factor" is essentially equivalent to
except argument follow the usual indexing rules.
droplevelsreturns an object of the same class as
This function was introduced in R 2.12.0. It is primarily
intended for cases where one or more factors in a data frame
contains only elements from a reduced level set after
subsetting. (Notice that subsetting does not in general drop
unused levels). By default, levels are dropped from all factors in a
data frame, but the
except argument allows you to specify
columns for which this is not wanted.
aq <- transform(airquality, Month = factor(Month, labels = month.abb[5:9])) aq <- subset(aq, Month != "Jul") table(aq$Month) table(droplevels(aq)$Month)