Get elements from arbitrary nested list

Question

I have a nested list like this one:

nested_list = list(children = list(CV = "hello", 
                                   children = list(CV = "out", 
                                                   children = list(CV = "there", 
                                                                   TE = "blaa")), 
                                   TE = "hello",
                                   children = list(CV = "tom", 
                                                   TE = "lisa", 
                                                   children = list(TE = "bob"))), 
                   children = list(CV = "sam", 
                                   children = list(CV = "out", 
                                                   children = list(CV = "there", 
                                                                   TE = "blaa", 
                                                                   other = "other element"))))

and I need to extract all elements that are named with "CV" or "TE". There might also be elements with other names as CV or TE.

The elements with names "TE" or "CV" can be lists themselves, so I need to extract the lists. Here´s some input with lists as elements that need to be extracted.

nested_list2 = list(children = list(CV = "hello", 
                                   children = list(CV = list(subject = "sub", value = list(key = "1", var = "45")), 
                                                   children = list(CV = "there", 
                                                                   TE = list(subject = "sub2")))))

The problem is the unknown depth level of nested lists. so I need a function that I can use on any nested list.

This is the expected output for nested_list:

exp_output = list(CV = "hello", 
                  CV = "out", 
                  CV = "there", 
                  TE = "blaa", 
                  TE = "hello", 
                  CV = "tom", 
                  TE = "lisa", 
                  TE = "bob", 
                  CV = "sam",
                  CV = "out", 
                  CV = "there", 
                  TE = "blaa")

and this would be the expected output of nested_list2:

exp_output2 = list(CV = list("hello"),
                   CV = list(subject = "sub", value = list(key = "1", var = "45")),
                   CV = list("there"),
                   TE = list(subject = "sub2"))

I know that the list elements will not have unique names, but I need this structure. Is there such a function? I haven´t found anything.

Any help appreciated.

Question is unclear, did you mean something like unlist(nested_list)? — user2974951
– user2974951, Commented Oct 1, 2018 at 8:26
Sorry, I updated my question and added the expected output. There can be other named elements. — WinterMensch
– WinterMensch, Commented Oct 1, 2018 at 8:37

moodymudskipper · Accepted Answer · 2018-10-01 12:11:40Z

2

You can use a recursive function that appends a list :

res <- list()
fun <- function(x, nms){
  if(any(names(x) %in% nms)){
    res <<- c(res,x[names(x) %in% nms])
    x <- x[!names(x) %in% nms]
  }
  if (is.list(x)) lapply(x,fun,nms)
}
fun(nested_list2, c("CV","TE"))

res <- list()
fun(nested_list, c("CV","TE"))
str(res)
# List of 12
# $ CV: chr "hello"
# $ TE: chr "hello"
# $ CV: chr "out"
# $ CV: chr "there"
# $ TE: chr "blaa"
# $ CV: chr "tom"
# $ TE: chr "lisa"
# $ TE: chr "bob"
# $ CV: chr "sam"
# $ CV: chr "out"
# $ CV: chr "there"
# $ TE: chr "blaa"

res <- list()
fun(nested_list2, c("CV","TE"))
str(res)
# List of 4
# $ CV: chr "hello"
# $ CV:List of 2
# ..$ subject: chr "sub"
# ..$ value  :List of 2
# .. ..$ key: chr "1"
# .. ..$ var: chr "45"
# $ CV: chr "there"
# $ TE:List of 1
# ..$ subject: chr "sub2"

answered Oct 1, 2018 at 12:11

moodymudskipper

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1 Comment

user51462 Over a year ago

Hi Moody, thank you for this solution, it's helped me out. I was wondering if the function could be modified to be more self-contained by not using the global assign <<-?

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