R help - Apr 2004 - attaching data.frame/list within a function

I'm trying to find a good way of attaching a list within a function such 
that the attached variables (the list's members) precede the global 
environment (.GlobalEnv) in the search list.  Here is a non-working example 
using attach(), which hopefully explains better what I'm trying to do:

 > foo <- function(x=0, input=list(a=10)) {
+   attach(input)
+   on.exit(detach(input))
+   print(search())
+   print(a)
+ }
 >
 > a
Error: Object "a" not found
 > foo()  ## this prints out a = 10
[1] ".GlobalEnv"       "input"           
"package:methods"  "package:stats"
[5] "package:graphics" "package:utils"   
"Autoloads"        "package:base"
[1] 10
 >
 > a <- 0
 > foo()  ## this prints out a = 0, not a = 10
[1] ".GlobalEnv"       "input"           
"package:methods"  "package:stats"
[5] "package:graphics" "package:utils"   
"Autoloads"        "package:base"
[1] 0
 >


Thanks,

                 Gardar


___________________________________________________________________
Gardar Johannesson
Lawrence Livermore National Laboratory
7000 East Avenue, L-229
Livermore, CA 94550

johannesson1@llnl.gov
Tel: (925) 422-3901,  Fax: (925) 422-4141
	[[alternative HTML version deleted]]

Dear Gardar,

You could use with(); to modify your example slightly,
> a <- "a"
> b <- 5
> foo <- function(x=0, input=list(a=10)) {
+             with(input, {
+                 print(c(a, b, x))
+                 })
+             }
> foo()
[1] 10  5  0
> 
I hope that this helps,
 John 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Gardar 
> Johannesson
> Sent: Wednesday, April 14, 2004 1:14 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] attaching data.frame/list within a function
> 
> I'm trying to find a good way of attaching a list within a 
> function such that the attached variables (the list's 
> members) precede the global environment (.GlobalEnv) in the 
> search list.  Here is a non-working example using attach(), 
> which hopefully explains better what I'm trying to do:
> 
>  > foo <- function(x=0, input=list(a=10)) {
> +   attach(input)
> +   on.exit(detach(input))
> +   print(search())
> +   print(a)
> + }
>  >
>  > a
> Error: Object "a" not found
>  > foo()  ## this prints out a = 10
> [1] ".GlobalEnv"       "input"           
"package:methods"
> "package:stats"
> [5] "package:graphics" "package:utils"   
"Autoloads"
> "package:base"
> [1] 10
>  >
>  > a <- 0
>  > foo()  ## this prints out a = 0, not a = 10
> [1] ".GlobalEnv"       "input"           
"package:methods"
> "package:stats"
> [5] "package:graphics" "package:utils"   
"Autoloads"
> "package:base"
> [1] 0
>  >
> 
> 
> Thanks,
> 
>                  Gardar
>

What you are asking for doesn't even work at the R prompt, let alone inside
a function.  The _really_ ugly kludge I manage to come up with is to
manually assign the components of the list (or variables in the data frame)
into the function's environment:

f <- function(mylist=list(a=1:5)) {
    nm <- names(mylist)
    for (i in seq(along=nm)) {
        assign(nm[i], mget(nm[i], envir=NULL, ifnotfound=mylist[i]))
    }
    sapply(names(mylist), function(x) print(get(x)))
    ls()
}

Here are a couple of tests:
> f()
$a
[1] 1 2 3 4 5

[1] "a"      "i"      "mylist" "nm"   
> f(list(b=5:1, z="ha!"))
$b
[1] 5 4 3 2 1

$z
[1] "ha!"

[1] "b"      "i"      "mylist" "nm"    
"z"

Andy
> From: Gardar Johannesson
> 
> I'm trying to find a good way of attaching a list within a 
> function such 
> that the attached variables (the list's members) precede the global 
> environment (.GlobalEnv) in the search list.  Here is a 
> non-working example 
> using attach(), which hopefully explains better what I'm trying to do:
> 
>  > foo <- function(x=0, input=list(a=10)) {
> +   attach(input)
> +   on.exit(detach(input))
> +   print(search())
> +   print(a)
> + }
>  >
>  > a
> Error: Object "a" not found
>  > foo()  ## this prints out a = 10
> [1] ".GlobalEnv"       "input"           
"package:methods"
> "package:stats"
> [5] "package:graphics" "package:utils"   
"Autoloads"
> "package:base"
> [1] 10
>  >
>  > a <- 0
>  > foo()  ## this prints out a = 0, not a = 10
> [1] ".GlobalEnv"       "input"           
"package:methods"
> "package:stats"
> [5] "package:graphics" "package:utils"   
"Autoloads"
> "package:base"
> [1] 0
>  >
> 
> 
> Thanks,
> 
>                  Gardar
> 
> 
> ___________________________________________________________________
> Gardar Johannesson
> Lawrence Livermore National Laboratory
> 7000 East Avenue, L-229
> Livermore, CA 94550
> 
> johannesson1 at llnl.gov
> Tel: (925) 422-3901,  Fax: (925) 422-4141
> 	[[alternative HTML version deleted]]
> 
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