Select multiple elements from a list

Question

I have a list in R some 10,000 elements long. Say I want to select only elements, 5, 7, and 9. I'm not sure how I would do that without a for loop.

I want to do something like mylist[[c(5,7,9]] but that doesn't work. I've also tried the lapply function but haven't been able to get that working either.

From the documentation found at ?"[[": The most important distinction between [, [[ and $ is that the [ can select more than one element whereas the other two select a single element. — A5C1D2H2I1M1N2O1R2T1
– A5C1D2H2I1M1N2O1R2T1, Commented Aug 25, 2012 at 4:05
Further to @mrdwab's comment, this being the vectorised world of R, "a single element" may be a vector! — seancarmody
– seancarmody, Commented Aug 25, 2012 at 4:37

Glen_b · Accepted Answer · 2020-02-15 05:19:48Z

183

mylist[c(5,7,9)] should do it.

You want the sublists returned as sublists of the result list; you don't use [[]] (or rather, the function is [[) for that -- as Dason mentions in comments, [[ grabs the element.

edited Feb 15, 2020 at 5:19

answered Aug 25, 2012 at 4:02

Glen_b

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6 Comments

Dason Over a year ago

Just to add some more detail - "[" will return a sublist so it makes sense that you could use multiple inputs with it. "[[" will actually grab the element itself so it can only take a single input since it isn't returning a list (unless the element itself is a list).

masfenix Over a year ago

Is this vectorized? I have two lists: b contains 10 million elements. filter_bins is a list where each element is an index. so to only get elements from b with the same index, I have lapply(filter_bins, function(x) b[x, ]) so b[x, ] only returns a subset of the rows (where each element is chosen by filter_bins).. I hope this makes sense. My question is that this is very slow

Glen_b Over a year ago

@masfenix you should generate a new question describing the details.

derelict Over a year ago

Can the above solution be done with something like dplyr::select() and contains('some text")?

Dason Over a year ago

@HermanToothrot You don't. At least not directly. You could lapply or sapply over the indices you want to extract and iteratively grab the corresponding element with [[. But if you want to extract multiple elements then really you're grabbing a sublist and you should just use [

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Mark · Accepted Answer · 2023-08-20 05:50:57Z

Glen_b's answer is the correct one. Below are some examples of how you can use it with pipes, functions like lapply and map(), and within a dataframe with list columns:

With pipes:

library(tidyverse)

x <- as.list(letters)
nums <- c(5, 7, 9)

x[nums] # Glen's answer

nums |> x[i = _] # using the native pipe

nums %>% {x[.]} # using magrittr's pipe
# nums %>% x[i = .] is equivalent

Using map(), lapply() etc.

y = list(as.list(letters), as.list(LETTERS))

map(y, \(x) x[nums]) # lambda function \(x) x[nums] is equivalent to ~ .x[nums] or function(x){x[nums]}
lapply(y, \(x) x[nums])

With list-type columns in dataframes:

df <- tibble(
    x = list(x)) # we need to put x within another list, as tibble() will unnest x otherwise

df |> 
  mutate(z = map(x, \(x) x[nums])) |> 
  pull(z)

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