The Art of R Programming

Recursive Lists

Lists can be recursive, meaning that you can have lists within lists. Here’s an example:

> b <- list(u = 5, v = 12)
> c <- list(w = 13)
> a <- list(b,c)
> a
[[1]]
[[1]]$u
[1] 5

[[1]]$v
[1] 12


[[2]]
[[2]]$w
[1] 13


> length(a)
[1] 2

This code makes a into a two-component list, with each component itself also being a list.

The concatenate function c() has an optional argument recursive, which controls whether flattening occurs when recursive lists are combined.

> c(list(a=1,b=2,c=list(d=5,e=9)))
$a
[1] 1

$b
[1] 2

$c
$c$d
[1] 5

$c$e
[1] 9
> c(list(a=1,b=2,c=list(d=5,e=9)),recursive=T)
  a   b c.d c.e
  1   2   5   9

In the first case, we accepted the default value of recursive, which is FALSE, and obtained a recursive list, with the c component ...