[R] Extracting elements out of list in list in list

[Bert Gunter]

Chee Hee's approach is both simpler and almost surely more efficient,
but I wanted to show another that walks the tree (i.e. the list)
directly using recursion at the R level to pull out the desired
components. This is in keeping with R's "functional" programming
paradigm and avoids the use of regular expressions to extract the
desired components from the unlist() version.

extr <- function(x,nm){
  if(is.recursive(x)){
    wh <- names(x) %in% nm
    c(x[wh],lapply(x[!wh],extr,nm=nm) )
  } else NULL
}

## The return value contains a bunch of NULLs; so use unlist() to remove them

> unlist(extr(x,"A"))
f1.x1.A f1.x2.A f2.x3.A f2.x4.A
     11      12      13      14


I would welcome any possibly "slicker" versions of the above.

Cheers,
Bert

Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll




On Fri, Jan 16, 2015 at 7:23 AM, Chel Hee Lee <chl948 at mail.usask.ca> wrote:
> This approach may not be fancy as what you are looking for.
>
>> xl <- unlist(x)
>> xl[grep("A", names(xl))]
> f1.x1.A f1.x2.A f2.x3.A f2.x4.A
>      11      12      13      14
>>
>
> I hope this helps.
>
> Chel Hee Lee
>
> On 01/16/2015 04:40 AM, Rainer M Krug wrote:
>>
>> Hi
>>
>> Consider the following variable:
>>
>> --8<---------------cut here---------------start------------->8---
>> x1 <- list(
>>    A = 11,
>>    B = 21,
>>    C = 31
>> )
>>
>> x2 <- list(
>>    A = 12,
>>    B = 22,
>>    C = 32
>> )
>>
>> x3 <- list(
>>    A = 13,
>>    B = 23,
>>    C = 33
>> )
>>
>> x4 <- list(
>>    A = 14,
>>    B = 24,
>>    C = 34
>> )
>>
>> y1 <- list(
>>    x1 = x1,
>>    x2 = x2
>> )
>>
>> y2 <- list(
>>    x3 = x3,
>>    x4 = x4
>> )
>>
>> x <- list(
>>    f1 = y1,
>>    f2 = y2
>> )
>> --8<---------------cut here---------------end--------------->8---
>>
>>
>> To extract all fields named "A" from y1, I can do
>>
>> ,----
>> | > sapply(y1, "[[", "A")
>> | x1 x2
>> | 11 12
>> `----
>>
>> But how can I do the same for x?
>>
>> I could put an sapply into an sapply, but this would be less then
>> elegant.
>>
>> Is there an easier way of doing this?
>>
>> Thanks,
>>
>> Rainer
>>
>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.