| attach {base} | R Documentation |
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.
attach(what, pos = 2L, name = deparse(substitute(what)),
warn.conflicts = TRUE)
what |
‘database’. This can be a
|
pos |
integer specifying position in |
name |
name to use for the attached database. Names starting with
|
warn.conflicts |
logical. If |
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. See http://www.omegahat.net/RObjectTables/.
The environment is returned invisibly with a
"name" attribute.
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.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
library, detach, search,
objects, environment, with.
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)