The Art of R Programming

[1] 5

[[1]]$v

[1] 12

[[2]]

[[2]]$w

[1] 13

> length(a)

[1] 2

This code makesainto a two-component list, with each component itself

also being a list.

The concatenate functionc()has an optional argumentrecursive, which

controls whetherflatteningoccurs 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

1259

In the first case, we accepted the default value ofrecursive, which is

FALSE, and obtained a recursive list, with theccomponent of the main list

itself being another list. In the second call, withrecursiveset toTRUE,wegot

a single list as a result; only the names look recursive. (It’s odd that setting

recursivetoTRUEgives anonrecursivelist.)

Recall that our first example of lists consisted of an employee database.

I mentioned that since each employee was represented as a list, the entire

database would be a list of lists. That is a concrete example of recursive lists.

100 Chapter 4