\HeaderA{unlist}{Flatten Lists}{unlist}
\keyword{list}{unlist}
\keyword{manip}{unlist}
\begin{Description}\relax
Given a list structure \code{x}, \code{unlist} simplifies it to
produce a vector which contains all the atomic components
which occur in \code{x}.
\end{Description}
\begin{Usage}
\begin{verbatim}
unlist(x, recursive = TRUE, use.names = TRUE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{x}] An \R{} object, typically a list or vector.
\item[\code{recursive}] logical.  Should unlisting be applied to list
components of \code{x}?
\item[\code{use.names}] logical.  Should names be preserved?
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
\code{unlist} is generic: you can write methods to handle
specific classes of objects, see \LinkA{InternalMethods}{InternalMethods}.

If \code{x} has length zero the result is \code{NULL}.

If \code{recursive = FALSE}, the function will not recurse beyond the
first level items in \code{x}.

Factors are treated specially.  If all non-list elements of \code{x}
are factors (or ordered factors) then the result will be a factor with
levels the union of the level sets of the elements, in the order the
levels occur in the level sets of the elements (which means that if
all the elements have the same level set, that is the level set of the
result).

\code{x} can be an atomic vector, but then \code{unlist} does nothing useful,
not even drop names.

By default, \code{unlist} tries to retain the naming
information present in \code{x}.  If \code{use.names = FALSE} all
naming information is dropped.

Where possible the list elements are coerced to a common mode during
the unlisting, and so the result often ends up as a character
vector.  Vectors will be coerced to the highest type of the components
in the hierarchy NULL < raw < logical < integer < real < complex < character
< list < expression: pairlists are treated as lists.

A list is a (generic) vector, and the simplified vector might still be
a list (and might be unchanged).  Non-vector elements of the list
(for example language elements such as names, formulas and calls)
are not coerced, and so a list containing one or more of these remains a
list.  (The effect of unlisting an \code{\LinkA{lm}{lm}} fit is a list which
has individual residuals as components.)
\end{Details}
\begin{Value}
\code{NULL} or an expression or a vector of an appropriate mode to
hold the list components.

The output type is determined from the highest type
of the components in the hierarchy raw < logical < integer < real <
complex < character < list < expression, after coercion of pairlists
to lists.
\end{Value}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{c}{c}}, \code{\LinkA{as.list}{as.list}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
unlist(options())
unlist(options(), use.names=FALSE)

l.ex <- list(a = list(1:5, LETTERS[1:5]), b = "Z", c = NA)
unlist(l.ex, recursive = FALSE)
unlist(l.ex, recursive = TRUE)

l1 <- list(a="a", b=2, c=pi+2i)
unlist(l1) # a character vector
l2 <- list(a="a", b=as.name("b"), c=pi+2i)
unlist(l2) # remains a list
\end{ExampleCode}
\end{Examples}

