base (version 3.4.0)

droplevels: Drop Unused Levels from Factors

Description

The function droplevels is used to drop unused levels from a factor or, more commonly, from factors in a data frame.

Usage

# S3 method for factor
droplevels(x, exclude = if(anyNA(levels(x))) NULL else NA, …)
# S3 method for data.frame
droplevels(x, except, exclude, …)

Arguments

x

an object from which to drop unused factor levels.

exclude

passed to factor(); factor levels which should be excluded from the result even if present. Note that this was implicitly NA 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

except

indices of columns from which not to drop levels

Value

droplevels returns an object of the same class as x

Details

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.

See Also

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.

Examples

aq <- transform(airquality, Month = factor(Month, labels = month.abb[5:9]))
aq <- subset(aq, Month != "Jul")
table(           aq $Month)
table(droplevels(aq)$Month)