[R] Extracting elements out of list in list in list
Chel Hee Lee
chl948 at mail.usask.ca
Tue Jan 20 07:07:53 CET 2015
Aha! I haven't thought about it. I really like the approach presented
by Bert Gunter in the previous post. It is a good lesson.
I made my previous code a little bit better by building a function that
pulls out only the desired component. At this time, the names of
sublists are changed as below (i.e. the names of 'A', 'B', 'C' in the
example given by Rainer M Krug are changed to 'A', 'Aha', 'alpha' here):
> x1 <- list(A = 11, Aha = 21, alpha = 31)
> x2 <- list(A = 12, Aha = 22, alpha = 32)
> x3 <- list(A = 13, Aha = 23, alpha = 33)
> x4 <- list(A = 14, Aha = 24, alpha = 34)
> y1 <- list(x1 = x1, x2 = x2)
> y2 <- list(x3 = x3, x4 = x4)
> x <- list(f1 = y1, f2 = y2)
>
> extr.1 <- function(x, name){
+ xl <- unlist(x)
+ depth <- sum(unlist(strsplit(names(xl)[1], split="")) == ".") + 1
+ xl[grep(paste("^",name,"$", sep=""), unlist(strsplit(names(xl), ".",
fixed=TRUE)))/depth]
+ }
> extr.1(x=x, name="alpha")
f1.x1.alpha f1.x2.alpha f2.x3.alpha f2.x4.alpha
31 32 33 34
> extr.1(x=x, name="A")
f1.x1.A f1.x2.A f2.x3.A f2.x4.A
11 12 13 14
> extr.1(x=x, name="a")
named numeric(0)
> extr.1(x=x, name="Aha")
f1.x1.Aha f1.x2.Aha f2.x3.Aha f2.x4.Aha
21 22 23 24
>
Hm.... this function 'extr.1()' seems to be (much) slower than the
function 'extr()'.
Chel Hee Lee
On 01/16/2015 11:34 AM, Bert Gunter wrote:
> 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.
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