\HeaderA{attach}{Attach Set of R Objects to Search Path}{attach}
\keyword{data}{attach}
\begin{Description}\relax
The database is attached to the \R{} search path.  This means that the
database is searched by \R{} when evaluating a variable, so objects in
the database can be accessed by simply giving their names.
\end{Description}
\begin{Usage}
\begin{verbatim}
attach(what, pos = 2, name = deparse(substitute(what)),
       warn.conflicts = TRUE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{what}] \sQuote{database}.  This can be a
\code{data.frame} or a \code{list} or a \R{} data file created with
\code{\LinkA{save}{save}} or \code{NULL} or an environment.  See also
\sQuote{Details}.
\item[\code{pos}] integer specifying position in \code{\LinkA{search}{search}()} where
to attach.
\item[\code{name}] name to use for the attached database.
\item[\code{warn.conflicts}] logical.  If \code{TRUE}, warnings are
printed about \code{\LinkA{conflicts}{conflicts}} from attaching the database,
unless that database contains an object \code{.conflicts.OK}.  A
conflict is a function masking a function, or a non-function masking
a non-function.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
When evaluating a variable or function name \R{} searches for
that name in the databases listed by \code{\LinkA{search}{search}}.  The first
name of the appropriate type is used.

By attaching a data frame (or list) to the search path it is possible
to refer to the variables in the data frame by their names alone,
rather than as components of the data frame (e.g. in the example below,
\code{height} rather than \code{women\$height}).

By default the database is attached in position 2 in the search path,
immediately after the user's workspace and before all previously
loaded packages and previously attached databases.  This can be altered
to attach later in the search path with the \code{pos} option, but you
cannot attach at \code{pos=1}.

The database is not actually attached.  Rather, a new environment is
created on the search path and the elements of a list (including
columns of a data frame) or objects in a save file or an environment
are \emph{copied} into the new environment.  If you use
\code{\LinkA{<{}<\Rdash}{<.Rlbrace..Rrbrace.<.Rdash.}} or \code{\LinkA{assign}{assign}} to assign to an attached
database, you only alter the attached copy, not the original object.
(Normal assignment will place a modified version in the user's
workspace: see the examples.)
For this reason \code{attach} can lead to confusion.

One useful \sQuote{trick} is to use \code{what = NULL} (or equivalently a
length-zero list) to create a new environment on the search path into
which objects can be assigned by \code{\LinkA{assign}{assign}} or
\code{\LinkA{load}{load}} or \code{\LinkA{sys.source}{sys.source}}.

Names starting \code{"package:"} are reserved for
\code{\LinkA{library}{library}} and should not be used by end users.  The name
given for the attached environment will be used by \code{\LinkA{search}{search}}
and can be used as the argument to \code{\LinkA{as.environment}{as.environment}}.

There are hooks to attach user-defined table objects of class
\code{"UserDefinedDatabase"}, supported by the Omegahat package
\pkg{RObjectTables}.  See \url{http://www.omegahat.org/RObjectTables/}.
\end{Details}
\begin{Value}
The \code{\LinkA{environment}{environment}} is returned invisibly with a
\code{"name"} attribute.
\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{library}{library}}, \code{\LinkA{detach}{detach}}, \code{\LinkA{search}{search}},
\code{\LinkA{objects}{objects}}, \code{\LinkA{environment}{environment}}, \code{\LinkA{with}{with}}.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
require(utils)

summary(women$height)   # refers to variable 'height' in the data frame
attach(women)
summary(height)         # The same variable now available by name
height <- height*2.54   # Don't do this. It creates a new variable
                        # in the user's workspace
find("height")
summary(height)         # The new variable in the workspace
rm(height)
summary(height)         # The original variable.
height <<- height*25.4  # Change the copy in the attached environment
find("height")
summary(height)         # The changed copy
detach("women")
summary(women$height)   # unchanged

## Not run: 
## create an environment on the search path and populate it
sys.source("myfuns.R", envir=attach(NULL, name="myfuns"))
## End(Not run)\end{ExampleCode}
\end{Examples}

