[R] be careful: using attach in R functions

[Marc Schwartz]

On Tue, 2004-02-24 at 09:32, li dongfeng wrote:
> 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()
> ##############################


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)
}

> system.time(test())
[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)
}

> system.time(test())
[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