[R] Extracting list names within a looped function

[Gaj Stan (BIGCAT)]

Hello Berend,

Thanks for your quick response.

I am aware of the names() function, but this is not the answer to my question, since it will not work within the foo function. 

Let me try to explain it again! (:

I want to analyze several datasets in a similar fashion and stored all data as seperate list components in a (here: $var1, $var2, $var3). Each component of that list has a unique name and will be sent through the foo()-function using a for-loop. This foo() function would (in my case) generate several plots and output files where I would like to use the name of the component ($var1, $var2 or $var3) in the labels and output file names. I currently am unable to do so.

As such, my end-goal would be that the foo function returns the name of the list components and not [1] "a[[i]]", as deparse(substitute(x)) does. 

The output of foo() needs to be:

 [1] "var1"
 [1] "var2"
 [1] "var3"

But I'm also happy with
 [1] "a[[\"var1\"]]
 [1] "a[[\"var2\"]]
 [1] "a[[\"var3\"]]
(or something similar)

This leads to the question: how can I adjust my foo() so it gives me the output mentioned above?
 
I hope that this clears things up.

  -- Stan


-----Original Message-----
From: Berend Hasselman [mailto:bhh at xs4all.nl] 
Sent: 15 November 2012 11:27
To: Gaj Stan (BIGCAT)
Cc: r-help at r-project.org
Subject: Re: [R] Extracting list names within a looped function


On 15-11-2012, at 11:14, Gaj Stan (BIGCAT) wrote:

> Hello all,
> 
> I have the following problem:
> 
> 1) A list was defined as 'a'
> 
> a <- list("var1"=c(100:1), "var2"=c(1:100), "var3"=rnorm(100))
> 
> 2) a function 'foo' was defined that extracts the variable name assigned to x using the deparse(substitute()) functionality. This name will then be used within the function to generate specific output files, etc.
> 
> foo <- function(x) {
>  print( deparse(substitute(x)) )
> }
> 
> However, I am currently interested in looping through all list variables and extract the list variable name from within the function. The current loop (see below) will result in
> 
> for(i in 1:length(a)) {
>  foo(a[[i]])
> }
> [1] "a[[i]]"
> 
> which actually does what I expected of deparse(substitute(x)), but is not what I wanted. I would like to end up with something like
> 
> [1] "var1"
> [1] "var2"
> [1] "var3"
> 
> or
> 
> [1] "a[[\"var1\"]]"
> [1] "a[[\"var2\"]]"
> [1] "a[[\"var3\"]]"
> 
> Keep in mind that x has to be a matrix, and not a list. This to keep the function as general as possible.
> 
> Does anyone have any idea on how to tackle this? Is deparse(substitute(x)) here the best way to go? Are there alternatives?

I'm not sure if I understand what you want but will this give you what you seem to want:

names(a)

Berend