Extract elements from a named list while keeping the name in R

user_10134235 0
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I'm trying to extract a single element from a list in R and store the element with its vector name as a new list.

Let's assume that I have a list called mylist that has an arbitrary amount of named vectors within and the length of each vector can vary, e.g.

mylist <- list("var1" = c(1,2,3), "var2" = c(10,8,4,2), "var3" = c(6,3), ...)

I know I can get an individual element by using either the name or the index of the vector, e.g.

mylist[["var1"]][1]
[1] 1
mylist[[2]][3]
[1] 4

However, these are only the values. What I want is to create a new list from this extracted value and use the original name of the vector where it was extracted from, something like

element1
$var1
[1] 1

element2
$var2
[1] 4

where element1 and element2 are the new lists. Is there an efficient way of doing this?

extract r list
3 Answers

You can use combination of pluck and list as shown below.

library(purrr)
mylist <- list("var1" = c(1,2,3), "var2" = c(10,8,4,2), "var3" = c(6,3))
element1 <- mylist %>% pluck("var1") %>% .[1] %>% list("var1" = .)
element2 <- mylist %>% pluck("var2") %>% .[3] %>% list("var2" = .)
user_20954110

Try this. You can use mapply() defining a function with two elements: a list x and a vector n which contains the desired positions of elements to extract. With that you can wrap the function using indexing in order to obtain a list L close to what you expect. Here the code:

#Data
mylist <- list("var1" = c(1,2,3), "var2" = c(10,8,4,2))
#List
L <- mapply(function(x,n) x[n],x=mylist,n=c(1,3),SIMPLIFY = F)

Output:

L
$var1
[1] 1

$var2
[1] 4
user_20808480

We can use map2 from purrr

library(purrr)
map2(mylist, c(1, 3), `[[`)

-output

#$var1
#[1] 1

#$var2
#[1] 4

or using pluck

map2(mylist, c(1, 3), pluck)

Or with Map from base R

Map(`[[`, mylist, c(1, 3))

-output

#$var1
#[1] 1

#$var2
#[1] 4

data

mylist <- list("var1" = c(1,2,3), "var2" = c(10,8,4,2))
user_37322710
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