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 · 2012-05-27 20:38:13Z

72

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, 2012 at 20:38

answered May 27, 2012 at 19:55

Dason

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5 Comments

joran Over a year ago

@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.

bright-star Over a year ago

If you create a list of duplicate lists like mylist <- list(list1, list1), is list1 copied or referenced twice?

Leonid Over a year ago

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.

Dason Over a year ago

@Leonid It sounds like you should post your own question but is there a reason you aren't using something like lapply?

Luke Over a year ago

@Leonid mylist = append(mylist, list(list3))

Nicolás Ozimica · Accepted Answer · 2016-01-28 20:34:41Z

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"

NelsonGon · Accepted Answer · 2019-06-01 11:19:54Z

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

Antoni · Accepted Answer · 2019-09-05 12:10:48Z

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)

U. Windl · Accepted Answer · 2017-06-01 13:14:21Z

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"

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