In R, a list is a group of elements of any type. In contrast to a vector, there is no limitation to the type of elements in a list. A list can contain vectors, data tables, lists, other objects, or any combination of them.

Combine elements in a list

To construct a list from its constituent elements, all we have to do is use the list function, with the desired elements in brackets. Below, we’ll create two vectors a and b and then put them, along with one more element, in a list named x:

a <- c(5,6,4)
b <- c("Jack","Jill")
x <- list(a,b,6)

Let’s have a look at our new list:

x

## [[1]]
## [1] 5 6 4
## 
## [[2]]
## [1] "Jack" "Jill"
## 
## [[3]]
## [1] 6

The index of each list element is shown in double square brackets (e.g. [[1]]). The [1] on the left side of each vector is the index of its own left-most element, as vectors are commonly displayed.

Another way to examine a list is the str function, displaying the list’s structure:

str(x)

## List of 3
##  $ : num [1:3] 5 6 4
##  $ : chr [1:2] "Jack" "Jill"
##  $ : num 6

As we see, x is a list of 3 elements:

  • a numeric vector of length 3

  • a character vector of length 2

  • a numeric vector of length 1

Accessing list elements

To get one or more elements from a list, write a numeric vector with their index numbers in square brackets. For example, to get the 2nd element of x:

x[2]

## [[1]]
## [1] "Jack" "Jill"

However, x[2] is not a vector and we cannot use it as one. This is because an index number in single square brackets returns a subset of the list - i.e. a list of one element. And a list of one element is not the same as purely that one element!

To get a pure single element instead of a list subset, write the index in double square brackets:

x[[2]]

## [1] "Jack" "Jill"