attach Attach Set of R Objects to Search Path

Description

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.

Usage

attach(what, pos = 2L, name = deparse1(substitute(what), backtick=FALSE),
       warn.conflicts = TRUE)

Arguments

what	‘database’. This can be a data.frame or a list or a R data file created with save or NULL or an environment. See also ‘Details’.
pos	integer specifying position in search() where to attach.
name	name to use for the attached database. Names starting with package: are reserved for library.
warn.conflicts	logical. If TRUE, warnings are printed about conflicts from attaching the database, unless that database contains an object .conflicts.OK. A conflict is a function masking a function, or a non-function masking a non-function.

Details

When evaluating a variable or function name R searches for that name in the databases listed by 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, height rather than 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 attached packages and previously attached databases. This can be altered to attach later in the search path with the pos option, but you cannot attach at 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 copied into the new environment. If you use <<- or 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 attach can lead to confusion.

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

Names starting "package:" are reserved for library and should not be used by end users. Attached files are by default given the name file:what. The name argument given for the attached environment will be used by search and can be used as the argument to as.environment.

There are hooks to attach user-defined table objects of class "UserDefinedDatabase", supported by the Omegahat package RObjectTables.

Value

The environment is returned invisibly with a "name" attribute.

Good practice

attach has the side effect of altering the search path and this can easily lead to the wrong object of a particular name being found. People do often forget to detach databases.

In interactive use, with is usually preferable to the use of attach/detach, unless what is a save()-produced file in which case attach() is a (safety) wrapper for load().

In programming, functions should not change the search path unless that is their purpose. Often with can be used within a function. If not, good practice is to

• Always use a distinctive name argument, and

• To immediately follow the attach call by an on.exit call to detach using the distinctive name.

This ensures that the search path is left unchanged even if the function is interrupted or if code after the attach call changes the search path.

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

library, detach, search, objects, environment, with.

Examples

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)