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)
- ‘database’. This can be a
listor a R data file created with
NULLor an environment. See also ‘Details’.
- integer specifying position in
search()where to attach.
- name to use for the attached database. Names starting with
package:are reserved for
- logical. If
TRUE, warnings are printed about
conflictsfrom 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
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
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.
"package:" are reserved for
library and should not be used by end users. Attached files are by default given the name
name argument given for the attached environment will be used by
search and can be used as the argument to
There are hooks to attach user-defined table objects of class
"UserDefinedDatabase", supported by the Omegahat package RObjectTables. See http://www.omegahat.org/RObjectTables/.
environment is returned invisibly with a
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
- To immediately follow the
attachcall by an
detachusing 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.
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)
Documentation reproduced from R 3.0.2. License: GPL-2.