How can I make a list of lists in R?

Question

I don't know how to make a list of lists in R. I have several lists, I want to store them in one data structure to make accessing them easier. However, it looks like you cannot use a list of lists in R, so if I get list l1 from another list, say, l2 then I cannot access elements l1. How can I implement it?

EDIT- I will show an example of what does not work for me:

list1 <- list()
list1[1] = 1
list1[2] = 2
list2 <- list()
list2[1] = 'a'
list2[2] = 'b'
list_all <- list(list1, list2)
a = list_all[1]
a[2]
#[[1]]
#NULL

but a should be a list!

Dason · Accepted Answer · 2012-05-27T20:38:13.627

69

You can easily make lists of lists

list1 <- list(a = 2, b = 3)
list2 <- list(c = "a", d = "b")
mylist <- list(list1, list2)

mylist is now a list that contains two lists. To access list1 you can use mylist[[1]]. If you want to be able to something like mylist$list1 then you need to do somethingl like

mylist <- list(list1 = list1, list2 = list2)
# Now you can do the following
mylist$list1

Edit: To reply to your edit. Just use double bracket indexing

a <- list_all[[1]]
a[[1]]
#[1] 1
a[[2]]
#[1] 2

edited May 27 '12 at 20:38

answered May 27 '12 at 19:55

Dason

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@capoluca To expand on Dason's explanation, `list_all[1]` does not select the first _element_ of `list_all`. For lists, `[` retrieves _sublists_ while `[[` retrieves _elements_. – joran May 27 '12 at 21:16
If you create a list of duplicate lists like `mylist <- list(list1, list1)`, is `list1` copied or referenced twice? – bright-star Nov 13 '14 at 04:37
What if I am creating a list of lists in a loop and so I do not have the individual sub-lists computed and stored in variables. Rather, I wish to add them to the list of lists one-by-one as I compute them. – Leonid Apr 25 '17 at 21:57
@Leonid It sounds like you should post your own question but is there a reason you aren't using something like `lapply`? – Dason Apr 26 '17 at 12:07
1

@Leonid mylist = append(mylist, list(list3)) – Luke Oct 26 '20 at 19:39

score 11 · Answer 2 · edited Jan 28 '16 at 20:34

11

Using your example::

list1 <- list()
list1[1] = 1
list1[2] = 2
list2 <- list()
list2[1] = 'a'
list2[2] = 'b'
list_all <- list(list1, list2)

Use '[[' to retrieve an element of a list:

b = list_all[[1]]
 b
[[1]]
[1] 1

[[2]]
[1] 2

class(b)
[1] "list"

edited Jan 28 '16 at 20:34

Nicolás Ozimica

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answered May 27 '12 at 19:54

Matthew Lundberg

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score 8 · Answer 3 · edited Jun 01 '19 at 11:19

8

If you are trying to keep a list of lists (similar to python's list.append()) then this might work:

a <- list(1,2,3)
b <- list(4,5,6)
c <- append(list(a), list(b))

> c
[[1]]
[[1]][[1]]
[1] 1

[[1]][[2]]
[1] 2

[[1]][[3]]
[1] 3


[[2]]
[[2]][[1]]
[1] 4

[[2]][[2]]
[1] 5

[[2]][[3]]
[1] 6

edited Jun 01 '19 at 11:19

NelsonGon

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answered Jul 18 '16 at 20:55

mgcdanny

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score 3 · Answer 4 · answered Sep 05 '19 at 12:10

The example creates a list of named lists in a loop.

  MyList <- list()
  for (aName in c("name1", "name2")){
    MyList[[aName]] <- list(aName)
  }
  MyList[["name1"]]
  MyList[["name2"]]

To add another list named "name3" do write:

  MyList$name3 <- list(1, 2, 3)

score 0 · Answer 5 · answered Jun 01 '17 at 13:14

As other answers pointed out in a more complicated way already, you did already create a list of lists! It's just the odd output of R that confuses (everybody?). Try this:

> str(list_all)
List of 2
 $ :List of 2
  ..$ : num 1
  ..$ : num 2
 $ :List of 2
  ..$ : chr "a"
  ..$ : chr "b"

And the most simple construction would be this:

> str(list(list(1, 2), list("a", "b")))
List of 2
 $ :List of 2
  ..$ : num 1
  ..$ : num 2
 $ :List of 2
  ..$ : chr "a"
  ..$ : chr "b"

How can I make a list of lists in R?

5 Answers5

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