% File src/library/base/man/factor.Rd
% Part of the R package, https://www.R-project.org
% Copyright 1995-2016 R Core Team
% Distributed under GPL 2 or later

\name{droplevels}
\title{Drop Unused Levels from Factors}
\alias{droplevels}
\alias{droplevels.factor}
\alias{droplevels.data.frame}
\concept{categorical variable}
\concept{enumerated type}
\concept{category}
\description{
  The function \code{droplevels} is used to drop unused levels from a
  \code{\link{factor}} or, more commonly, from factors in a data frame.
}
\usage{
droplevels(x, \dots)
\S3method{droplevels}{factor}(x, exclude = if(anyNA(levels(x))) NULL else NA, \dots)
\S3method{droplevels}{data.frame}(x, except, exclude, \dots)
}
\arguments{
  \item{x}{an object from which to drop unused factor levels.}
  \item{exclude}{passed to \code{\link{factor}()}; factor levels which
    should be excluded from the result even if present.  Note that this
    was \emph{implicitly} \code{NA} in \R <= 3.3.1 which did drop
    \code{NA} levels even when present in \code{x}, contrary to the
    documentation.  The current default is compatible with \code{x[ , drop=TRUE]}.}
  \item{\dots}{further arguments passed to methods.}
  \item{except}{indices of columns from which \emph{not} to drop levels.}
}
\value{
  \code{droplevels} returns an object of the same class as \code{x}}
\details{
  The method for class \code{"factor"} is currently equivalent to
  \code{factor(x, exclude=exclude)}.  For the data frame method, you
  should rarely specify \code{exclude} \dQuote{globally} for all factor
  columns; rather the default uses the same factor-specific
  \code{exclude} as the factor method itself.

  The \code{except} argument follows the usual indexing rules.
}

\note{ 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 \emph{not} in general drop
  unused levels).  By default, levels are dropped from all factors in a
  data frame, but the \code{except} argument allows you to specify
  columns for which this is not wanted.
}

\seealso{
  \code{\link{subset}} for subsetting data frames.
  \code{\link{factor}} for definition of factors.
  \code{\link{drop}} for dropping array dimensions.
  \code{\link{drop1}} for dropping terms from a model.
  \code{\link{[.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)
}
\keyword{category}
\keyword{NA}
