attach() inside function

Question

I'd like to give a params argument to a function and then attach it so that I can use a instead of params$a everytime I refer to the list element a.

run.simulation<-function(model,params){
attach(params)
#
# Use elements of params as parameters in a simulation
detach(params)
}

Is there a problem with this? If I have defined a global variable named c and have also defined an element named c of the list "params" , whose value would be used after the attach command?

score 25 · Accepted Answer · answered Apr 27 '11 at 07:29

25

Noah has already pointed out that using attach is a bad idea, even though you see it in some examples and books. There is a way around. You can use "local attach" that's called with. In Noah's dummy example, this would look like

with(params, print(a))

which will yield identical result, but is tidier.

answered Apr 27 '11 at 07:29

Roman Luštrik

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+1 Worth explaining what `with()` and `within()` do. In the example given, `with()` creates an environment from `params` and then evaluates the expression `print(a)` _inside_ that environment. Hence the components of `params` are visible when the expression is evaluated, without needing to `attach()` them. – Gavin Simpson Apr 27 '11 at 09:51
Thank you very much for the lucid answers. – statistician_in_training Apr 27 '11 at 18:47
So what worked in the end was 'myFun <-function(model,params){with(params),{...}}' where ... is the function body – statistician_in_training Apr 30 '11 at 19:00

score 11 · Answer 2 · edited Jul 16 '12 at 14:03

11

Another possibility is:

run.simulation <- function(model, params){
    # Assume params is a list of parameters from 
    # "params <- list(name1=value1, name2=value2, etc.)"
    for (v in 1:length(params)) assign(names(params)[v], params[[v]])
    # Use elements of params as parameters in a simulation
}

edited Jul 16 '12 at 14:03

Aaron left Stack Overflow

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answered Jul 16 '12 at 13:41

Jean-Luc Jannink

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score 8 · Answer 3 · edited Aug 09 '16 at 20:23

Easiest way to solve scope problems like this is usually to try something simple out:

a = 1
params = c()
params$a = 2
myfun <- function(params) {
  attach(params)
  print(a)
  detach(params)
}
myfun(params)

The following object(s) are masked _by_ .GlobalEnv:

a

# [1] 1

As you can see, R is picking up the global attribute a here.

It's almost always a good idea to avoid using attach and detach wherever possible -- scope ends up being tricky to handle (incidentally, it's also best to avoid naming variables c -- R will often figure out what you're referring to, but there are so many other letters out there, why risk it?). In addition, I find code using attach/detach almost impossible to decipher.

score 0 · Answer 4 · answered Feb 06 '19 at 17:40

0

Jean-Luc's answer helped me immensely for a case that I had a data.frame Dat instead of the list as specified in the OP:

for (v in 1:ncol(Dat)) assign(names(Dat)[v], Dat[,v])

answered Feb 06 '19 at 17:40

swihart

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attach() inside function

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