array of lists in r

Question

Suppose if I want to have 10 element array each element is a list/map. I am doing this:

x = array(list(), 10)
x[1][[ "a" ]] = 1
Warning message:
In x[1][["a"]] = 1 :
  number of items to replace is not a multiple of replacement length
>

Is this the right approach? I want each element of the array to be a map.

Answer 1

What you're calling an "array" is usually just called a list in R. You're getting tripped up by the difference between [ and [[ for lists. See the section "Recursive (list-like) objects" in help("[").

x[[1]][["a"]] <- 1

UPDATE:
Note that the solution above creates a list of named vectors. In other words, something like

x[[1]][["a"]] <- 1
x[[1]][["b"]] <- 1:2

won't work because you can't assign multiple values to one element of a vector. If you want to be able to assign a vector to a name, you can use a list of lists.

x[[1]] <- as.list(x[[1]])
x[[1]][["b"]] <- 1:2

Answer 2

If you really want to do this, then, because the elements of the lists in each element of the array do not have names, you can't index by a character vector. In your example, there is no x[1][[ "a" ]]:

> x[1][[ "a" ]]
NULL

If there are no names then you need to index by a numeric:

> x[1][[ 1 ]] <- 1
[1] 1

It would seem more logical to have a list though than an array:

> y <- vector(mode = "list", length = 10)
> y
[[1]]
NULL

[[2]]
NULL

[[3]]
NULL

[[4]]
NULL

[[5]]
NULL
....