| 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 = 2, name = deparse(substitute(what)),
warn.conflicts = TRUE)
what |
“database”. This may currently 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 |
alternative way to specify the database to be attached. |
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.
|
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$height).
By default the database is attached in position 2 in the search path,
immediately after the user's workspace and before all previously
loaded packages and previously attached databases. This can be altered
to attach later in the search path with the pos option, but you
cannot attach at pos=1.
The database is not actually attached. Rather, a new environment is
created on the search path and the elements of a list (including
columns of a data frame) or objects in a save file or an environment
are copied into the new environment. If you use
<<- or assign to assign to an attached
database, you only alter the attached copy, not the original object.
(Normal assignment will place a modified version in the user's
workspace: see the examples.)
For this reason attach can lead to confusion.
One useful ‘trick’ is to use what = NULL (or equivalently a
length-zero list) to create a new environment on the search path into
which objects can be assigned by assign or
load or sys.source.
There are hooks to attach user-defined table objects of class
"UserDefinedDatabase", supported by the Omegahat package
RObjectTables. See http://www.omegahat.org/RObjectTables/.
The environment is returned invisibly with a
"name" attribute.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
library, detach, search,
objects, environment, with.
summary(women$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)