[R] How to use a variable after $ ($x) for subscripting a list, How to source from a character object (source(x))

[David Winsemius]

On Sep 13, 2011, at 4:11 AM, drflxms wrote:

> Dear R colleagues,
>
> as result of a function a huge list is generated. From this result  
> list
> I'd like to extract information.
>
> Think of the list i.e. as an object named "listResult" with the
> following form:
>
> [[a]]
> [1] [2] [3] [4] [5]
>
> [[b]]
> [1] [2] [3] [4] [5]
>
> [[c]]
> [1] [2] [3] [4] [5]
>
> where levels=c(a,b,c)
>
> I'd like to extract a data.frame like
>
> a [2]
> b [2]
> c [2]
>

Naming the last dog "cc" instead of "c":

 > as.data.frame(lapply(listResult, "[", 2) )
   a b cc
1 2 2  2



> What I tried, is a function like this:
>
> library(foreach)
> loopExtract <- function(input) {
> foreach(i=1:length(levels)) %do% {listResult[[i]]$input}
> ...
>
> where the name of the variable [2] is meant to be the input.

It would have been had you used this form:

> foreach(i=1:length(levels)) %do% {listResult[[i]][[input]]}


>
> Unfortunately it turned out, that the "input"-variable after the $ is
> not substituted as expected. Subscripting the list with a variable  
> after
> the $ seems not possible.

Correct. That is the main difference between "$" and "[[" and the  
reason "$" is far less useful in programming situations.

> The only workaround I found for that is a function like
>
> loopExtract <- function(input) {
> codetemplate <- as.character("result <- foreach(i=1:length(levels))  
> %do%
> {listResult[[i]]$input)}")
> require(stringr)
> write(str_replace_all(codetemplate, "input", input), file="tmp.r")
> source("tmp.r")
> return(result)
> }
>
> in other words I stored a template of the desired code as characters  
> in
> an object, substituted the "input" string in that character object,
> wrote the result to a file and sourced the code of that file.
>
> I stored the result in a file cause the expression  
> source(codetemplate)
> did not work. From the documentation I learned, that one can only  
> source
> "connections". And there seems no chance to establish a connection  
> to an
> object. (Probably no one else, except me, has such a strange idea.)
>
> Well, it works that way, but even though I am not a real programmer, I
> realize how dirty this solution is. There must be a much simpler and
> elegant way.
>
> To sum it all up, my problem can be reduced to the following two  
> questions
>
> (1) How to subscript a list with a variable after $
> (2) How to source from an object containing a code template
> or (probably the most promising)

I'm afraid you lost me after a while. If using "[[" with a character- 
valued argument which is intended to be interpreted is not what you  
needed,  then make a slightly less wordy posting that has an example  
of what you wanted.

> (3) How to avoid both ;)

Avoid .... what?

>
> Unfortunately I am not experienced enough to find the solution.  
> Please help!
>
> Greetings from sunny Munich, Felix
>

-- 

David Winsemius, MD
West Hartford, CT