\HeaderA{factor}{Factors}{factor}
\aliasA{addNA}{factor}{addNA}
\aliasA{as.factor}{factor}{as.factor}
\aliasA{as.ordered}{factor}{as.ordered}
\aliasA{is.factor}{factor}{is.factor}
\aliasA{is.na<\Rdash.factor}{factor}{is.na<.Rdash..factor}
\aliasA{is.ordered}{factor}{is.ordered}
\aliasA{Math.factor}{factor}{Math.factor}
\aliasA{Ops.factor}{factor}{Ops.factor}
\aliasA{Ops.ordered}{factor}{Ops.ordered}
\aliasA{ordered}{factor}{ordered}
\aliasA{Summary.factor}{factor}{Summary.factor}
\keyword{category}{factor}
\keyword{NA}{factor}
\begin{Description}\relax
The function \code{factor} is used to encode a vector as a factor (the
terms \sQuote{category} and \sQuote{enumerated type} are also used for
factors).  If \code{ordered} is \code{TRUE}, the factor levels are
assumed to be ordered.
For compatibility with S there is also a function \code{ordered}.

\code{is.factor}, \code{is.ordered}, \code{as.factor} and \code{as.ordered}
are the membership and coercion functions for these classes.
\end{Description}
\begin{Usage}
\begin{verbatim}
factor(x = character(),
       levels = sort(unique.default(x), na.last = TRUE),
       labels = levels, exclude = NA, ordered = is.ordered(x))

ordered(x, ...)

is.factor(x)
is.ordered(x)

as.factor(x)
as.ordered(x)

addNA(x, ifany=FALSE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] a vector of data, usually taking a small number of distinct
values.
\item[\code{levels}] an optional vector of the values that \code{x} might
have taken. The default is the set of values taken by \code{x},
sorted into increasing order.
\item[\code{labels}] \emph{either} an optional vector of labels for the
levels (in the same order as \code{levels} after removing those in
\code{exclude}), \emph{or} a character string of length 1.
\item[\code{exclude}] a vector of values to be excluded when forming the
set of levels. This should be of the same type as \code{x}, and
will be coerced if necessary.
\item[\code{ordered}] logical flag to determine if the levels should be regarded
as ordered (in the order given).
\item[\code{...}] (in \code{ordered(.)}): any of the above, apart from
\code{ordered} itself.
\item[\code{ifany}] (in \code{addNA}): Only add an \code{NA} level if it is
used, i.e. if \code{any(is.na(x))}.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The type of the vector \code{x} is not restricted.

Ordered factors differ from factors only in their class, but methods
and the model-fitting functions treat the two classes quite differently.

The encoding of the vector happens as follows. First all the values
in \code{exclude} are removed from \code{levels}. If \code{x[i]} equals
\code{levels[j]}, then the \code{i}-th element of the result is
\code{j}.  If no match is found for \code{x[i]} in \code{levels},
then the \code{i}-th element of the result is set to \code{\LinkA{NA}{NA}}.

Normally the \sQuote{levels} used as an attribute of the result are
the reduced set of levels after removing those in \code{exclude}, but
this can be altered by supplying \code{labels}. This should either
be a set of new labels for the levels, or a character string, in
which case the levels are that character string with a sequence
number appended.

\code{factor(x, exclude=NULL)} applied to a factor is a no-operation
unless there are unused levels: in that case, a factor with the
reduced level set is returned.  If \code{exclude} is used it should
also be a factor with the same level set as \code{x} or a set of codes
for the levels to be excluded.

The codes of a factor may contain \code{\LinkA{NA}{NA}}.  For a numeric
\code{x}, set \code{exclude=NULL} to make \code{\LinkA{NA}{NA}} an extra
level (prints as \code{<NA>}); by default, this is the last level.

If \code{NA} is a level, the way to set a code to be missing (as
opposed to the code of the missing level) is to
use \code{\LinkA{is.na}{is.na}} on the left-hand-side of an assignment (as in
\code{is.na(f)[i] <- TRUE}; indexing inside \code{is.na} does not work).
Under those circumstances missing values are currently printed as
\code{<NA>}, i.e., identical to entries of level \code{NA}.

\code{is.factor} is generic: you can write methods to handle
specific classes of objects, see \LinkA{InternalMethods}{InternalMethods}.
\end{Details}
\begin{Value}
\code{factor} returns an object of class \code{"factor"} which has a
set of integer codes the length of \code{x} with a \code{"levels"}
attribute of mode \code{\LinkA{character}{character}}.  If \code{ordered} is true
(or \code{ordered} is used) the result has class
\code{c("ordered", "factor")}.

Applying \code{factor} to an ordered or unordered factor returns a
factor (of the same type) with just the levels which occur: see also
\code{\LinkA{[.factor}{[.factor}} for a more transparent way to achieve this.

\code{is.factor} returns \code{TRUE} or \code{FALSE} depending on
whether its argument is of type factor or not.  Correspondingly,
\code{is.ordered} returns \code{TRUE} when its
argument is ordered and \code{FALSE} otherwise.

\code{as.factor} coerces its argument to a factor.
It is an abbreviated form of \code{factor}.

\code{as.ordered(x)} returns \code{x} if this is ordered, and
\code{ordered(x)} otherwise.

\code{addNA} modifies a factor by turning \code{NA} into an extra
level (so that \code{NA} values are counted in tables, for instance).
\end{Value}
\begin{Section}{Warning}
The interpretation of a factor depends on both the codes and the
\code{"levels"} attribute.  Be careful only to compare factors with
the same set of levels (in the same order).  In particular,
\code{as.numeric} applied to a factor is meaningless, and may
happen by implicit coercion.  To transform a factor \code{f} to
its original numeric values, \code{as.numeric(levels(f))[f]} is
recommended and slightly more efficient than
\code{as.numeric(as.character(f))}.

The levels of a factor are by default sorted, but the sort order
may well depend on the locale at the time of creation, and should
not be assumed to be ASCII.

There are some anomalies associated with factors that have
\code{NA} as a level.  It is suggested to use them sparingly, e.g.,
only for tabulation purposes.
\end{Section}
\begin{Section}{Comparison operators and group generic methods}
There are \code{"factor"} and \code{"ordered"} methods for the
\LinkA{group generic}{group generic} \code{\LinkA{Ops}{Ops}}, which
provide methods for the \LinkA{Comparison}{Comparison} operators.  (The rest of the
group and the \code{\LinkA{Math}{Math}} and
\code{\LinkA{Summary}{Summary}} groups generate an error as
they are not meaningful for factors.)

Only \code{==} and \code{!=} can be used for factors: a factor can
only be compared to another factor with an identical set of levels
(not necessarily in the same ordering) or to a character vector.
Ordered factors are compared in the same way, but the general dispatch
mechanism precludes comparing ordered and unordered factors.

All the comparison operators are available for ordered factors.
Sorting is done by the levels of the operands: if both operands are
ordered factors they must have the same level set.
\end{Section}
\begin{Note}\relax
In earlier versions of \R, storing character data as a factor was more
space efficient if there is even a small proportion of
repeats.  Since \R{} 2.6.0 identical character strings share storage, so
the difference is now small in most cases.  (Integer values are stored
in 4 bytes whereas each reference to a character string needs a
pointer of 4 or 8 bytes.)
\end{Note}
\begin{References}\relax
Chambers, J. M. and Hastie, T. J. (1992)
\emph{Statistical Models in S}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{[.factor}{[.factor}} for subsetting of factors.

\code{\LinkA{gl}{gl}} for construction of balanced factors and
\code{\LinkA{C}{C}} for factors with specified contrasts.
\code{\LinkA{levels}{levels}} and \code{\LinkA{nlevels}{nlevels}} for accessing the
levels, and \code{\LinkA{unclass}{unclass}} to get integer codes.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
(ff <- factor(substring("statistics", 1:10, 1:10), levels=letters))
as.integer(ff)  # the internal codes
factor(ff)      # drops the levels that do not occur
ff[, drop=TRUE] # the same, more transparently

factor(letters[1:20], labels="letter")

class(ordered(4:1)) # "ordered", inheriting from "factor"

## suppose you want "NA" as a level, and to allow missing values.
(x <- factor(c(1, 2, NA), exclude = NULL))
is.na(x)[2] <- TRUE
x  # [1] 1    <NA> <NA>
is.na(x)
# [1] FALSE  TRUE FALSE

## Using addNA()
Month <- airquality$Month
table(addNA(Month))
table(addNA(Month, ifany=TRUE))
\end{ExampleCode}
\end{Examples}

