On Thu, 14 Apr 2005, Liaw, Andy wrote:
I suspect you've attach()'ed `DF' multiple times in your code (possibly inside a loop, or perhaps a function that was called several times). Note that if it were a `package', it would show up in search() as `package:DF' rather than just `DF'. Also, R Core folks took care to avoid attaching the same package multiple times:
library(MASS) search()[1] ".GlobalEnv" "package:MASS" "package:methods" "package:stats" [5] "package:graphics" "package:grDevices" "package:utils" "package:datasets" [9] "Autoloads" "package:base"library(MASS) search()[1] ".GlobalEnv" "package:MASS" "package:methods" "package:stats" [5] "package:graphics" "package:grDevices" "package:utils" "package:datasets" [9] "Autoloads" "package:base"
Notice how trying to load a package that's already on the search path has no effect.
This is not true for R objects, though.
When you attach a data frame, say, `DF', (or a list), it places a _copy_ on the search path, so you can access the variables in the data frame (or components of the list) directly. When you make modifications to the variables (such as x[i] <- something, rather than DF$x[i] <- something), the modifications are applied to the _copy_ on the search path, not the original.
Not quite. The correct description is in ?attach (and apart from mentioning attach was used, reading the help before posting is de rigeur).
Here is the version from 2.1.0 beta (which has been expanded):
The database is not actually attached. Rather, a new environment
is created on the search path and the elements of a list (including
columns of a dataframe) or objects in a save file are _copied_
into the new environment. If you use '<<-' or 'assign' to assign
to an attached database, you only alter the attached copy, not the
original object. (Normal assignment will place a modified version
in the user's workspace: see the examples.) For this reason
'attach' can lead to confusion.Examples:
summary(women$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) # unchangedNotice the difference between <- and <<- .
Assigning to an element follows the same rules, and in addition is an error unless an object exists that can be suitably subscripted.
-- Brian D. Ripley, [EMAIL PROTECTED] Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
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