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be careful: using attach in R functions

5 messages · Li Dongfeng, Marc Schwartz, Thomas Petzoldt +2 more

Li Dongfeng
# 
Hi there,

  I have just found that the ``attach'' function
can get you into trouble when called many times.
For example, you have a simulation routine called ``f()'',
in which you used ``attach'' and no corresponding ``detach''.
Then you call this function many times. You will find that
the system performance get slower and slower,
because you are making the R search path longer and longer.
So be careful when you use attach in a function!

  Below is a demonstration of this performance loss,
you will see a linear growth in CPU time usage.
Adding a ``detach()'' call at the end of ``f''
will get rid of this problem.

###############################
f <- function(){
  theta <- list(one=2.0, two=0.3, three=0.4)
  attach(theta)
  x  <- c(one, two, three)
  sample(x, 1)
}

test <- function(n=400){
  timeu <- numeric(n)
  for(i in seq(n)){
    timeu[i] <-
      system.time({
        resi <- f()
      })[3]
  }
  plot(timeu)
}
test()
##############################


¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡Li Dongfeng
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡ldf-nospacm at math.pku.edu.cn
¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡2004-02-24
Marc Schwartz
#
On Tue, 2004-02-24 at 09:32, li dongfeng wrote:
A better general and more efficient solution, without getting into the
details of the above functions, would be to use with() instead of
attach()/detach().

Change your function 'f' to:

f <- function(){
  theta <- list(one = 2.0, two = 0.3, three = 0.4)
  x  <- with(theta, c(one, two, three))
  sample(x, 1)
}

Now run test() using with() versus attach()/detach() and note the time
savings. Even with the detach() it is not "efficient".

For example using:

f <- function(){
  theta <- list(one = 2.0, two = 0.3, three = 0.4)
  attach(theta)
  x  <- c(one, two, three)
  sample(x, 1)
  detach(theta)
}
[1] 38.54  0.02 38.92  0.00  0.00

versus using:

f <- function(){
  theta <- list(one = 2.0, two = 0.3, three = 0.4)
  x  <- with(theta, c(one, two, three))
  sample(x, 1)
}
[1] 0.16 0.00 0.16 0.00 0.00
 

Those are on a 3.2 Ghz P4 Dell 5150 laptop with 2 GB of RAM for
comparison.

See ?with for more information. You will find relatively recent posts on
this in the list archives as a preferred approach.

HTH,

Marc Schwartz
Thomas Petzoldt
#
li dongfeng wrote:
[..]
Yes, this behaviour is well known (to me using attach for differential
equation models long ago). Detaching ist absolutely required. As an
alternative one can use the with() function.

Thomas P.
Uwe Ligges
#
li dongfeng wrote:
Well, attach() may be useful for convenient interactive data analysis.
I'd never use it in a simulation, since you can access all objects
without attaching anything. 
(Well, I suggest not to use it at all, at least to our students ... and
I'm using it only in very rare cases myself.)

Uwe Ligges
Roger D. Peng
# 
Using attach() in this context may not be wise.  I tend to only use 
attach() when working interactively.  It might be better to use with() 
in this situation, such as

f <- function() {
	theta <- list(one=2.0, two=0.3, three=0.4)
	x <- with(theta, c(one, two, three))
	sample(x, 1)
}

-roger
li dongfeng wrote: