how to 'get' an object that is part of a list

Thank you Gabor.  Very interesting solution.
If I get it right, the first argument in function f is just a 
placeholder to help extract the right element out of the list(...) 
that is passed to length.  Very smart trick.

Jim's solution appears a bit simpler, at least along the lines that I 
was
thinking:

my.length <- function(...) {
       names <- as.character(substitute(list(...)))[-1]
       sapply(names, function(x){y <- eval(parse(text=x)); length(y)}) 
}

-Christos

-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck at gmail.com]
Sent: Sunday, December 24, 2006 7:41 AM
To: jim holtman
Cc: christos at nuverabio.com;
Subject: Re: [R] how to 'get' an object that is part of a list

Is this what you are looking for:

my.length.2 <- function(...) {

+    f <- function(nm, val) length(val)
+    mapply(f, make.names(as.list(match.call()[-1])), list(...)) }

my.length.2(xx, xx$b)

 xx xx.b
  2    5

On 12/24/06, jim holtman <jholtman at gmail.com> wrote:

use 'eval' and 'parse'

xx

$a
[1] 1 2 3 4 5

$b
[1] "a" "b" "c" "d" "e"

eval(parse(text='xx$a'))

[1] 1 2 3 4 5

So that way you can pass in the character string and then 'parse' it.



On 12/24/06, Christos Hatzis <christos at nuverabio.com> wrote:

This might be an trivial thing but I am stuck.

Consider:

xx <- list(a=1:5, b=letters[1:5])

Although object xx is accessible through its name, how can object 
xx$b be accessed similarly through its name?

get("xx")

$a
[1] 1 2 3 4 5

$b
[1] "a" "b" "c" "d" "e"

get("xx$b")

Error in get(x, envir, mode, inherits) : variable "xx$b" was not 
found

get("xx")$b will not work in my case because it will probably 
require parsing to make it work within a function. E.g.

my.length <- function(...) {
       names <- as.character(substitute(list(...)))[-1]
       sapply(names, FUN=function(x){y <- get(x); length(y)}) }

my.length(xx)

xx
2

my.length(xx$a)

Error in get(x, envir, mode, inherits) : variable "xx$a" was not 
found

my.length(xx$a, xx$b)

Error in get(x, envir, mode, inherits) : variable "xx$a" was not 
found

Thank you.

Christos Hatzis, Ph.D.
Nuvera Biosciences, Inc.
400 West Cummings Park
Suite 5350
Woburn, MA 01801
Tel: 781-938-3830
www.nuverabio.com

--
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?