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.
‘database’. This can be a
integer specifying position in
name to use for the attached database. Names starting with
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
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
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
"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
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
environment is returned invisibly with a
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
In interactive use,
with is usually preferable to the
what is a
save()-produced file in which case
attach() is a (safety) wrapper for
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
changes the search path.
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.
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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)
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