Lists in R - VRCBuzz

In this tutorial, you will learn about what is lists in R?, how to create list in R?, and how to access variable(s) and/or observation(s ) from a list?

What is list in R?

Lists are the more general class of data storage. Lists are generic vectors where each element can be any type of object; e.g., a vector (of any mode), a matrix, a data frame or a function. Because of this flexibility, lists are the basis for most complex objects in R.

How to create list in R?

In R, list can be created using the list() function.

Creating an empty list in R

An empty list can be created using list() function without any arguments.

List_01 <- list()
List_01

list()

Another way to create an empty list with specific length is to use vector() function with list as first argument and length as second argument.

List_02 <- vector("list", length = 4)
List_02

[[1]]
NULL

[[2]]
NULL

[[3]]
NULL

[[4]]
NULL

Like vectors, lists group the data into a one-dimension set. In vector we can store similar kind of objects, but in list we can store all kind of objects.

Creating a list in R

The list in R can be created using list() function by specifying the different components (either named or unnamed) of list.

The function to create list in R is

list(name_1 = object_1,
     name_2 = object_2,
     ...,
     name_m = object_m)

If names are omitted while creating a list, the component of lists are numbered only.

list1 <- list(1.34, c("Bob", "John"), TRUE, 1 + 2i)
list1

[[1]]
[1] 1.34

[[2]]
[1] "Bob"  "John"

[[3]]
[1] TRUE

[[4]]
[1] 1+2i

Every component of a list is a vector. In the above example, the double bracket [[1]] is the first component of the list having one element and [[2]] is the second component of a list having two elements.

Note that the list function works like concatenation function c(). The c() function combine the objects to create a vector. The list() function combines the objects to create a list.

# display the no. of top level components
length(list1)

[1] 4

# display the mode of list1
mode(list1)

[1] "list"

is.list(list1)

[1] TRUE

Creating Named List in R

Named list can be created using list() function by specifying the names to each component of a list.

list2 <- list(value = 1.34, 
              names = c("Bob", "John"), TRUE, 1 + 2i)
list2

$value
[1] 1.34

$names
[1] "Bob"  "John"

[[3]]
[1] TRUE

[[4]]
[1] 1+2i

Combining two or more lists

Two or more lists can be combined using concatenation function c().

List01 <- list(
  A = 1:4,
  B = c("Male", "Female"),
  C = matrix(1:4, nrow = 2)
)
List02 <- list(P = 1:5, Q = c(TRUE, FALSE))
New_List01 <- c(List01, List02)
New_List01

$A
[1] 1 2 3 4

$B
[1] "Male"   "Female"

$C
     [,1] [,2]
[1,]    1    3
[2,]    2    4

$P
[1] 1 2 3 4 5

$Q
[1]  TRUE FALSE

Two or more lists can also be combined using append() function. The append() function is used to add elements to a vector but it can also be used for list too.

New_List02 <- append(List01, List02)
New_List02

$A
[1] 1 2 3 4

$B
[1] "Male"   "Female"

$C
     [,1] [,2]
[1,]    1    3
[2,]    2    4

$P
[1] 1 2 3 4 5

$Q
[1]  TRUE FALSE

Accessing elements of list

Accessing Components of a list

The components in a list are given numbers or given names. Hence the components may be referred by its number or name.

The component of a list is referred by

its number like list2[[2]] or
its name like list2$name or
its name like list2[["name"]]

The element of component is referred by

list2[[2]][1] or
list2$name[1].

list2[2]

$names
[1] "Bob"  "John"

list2[[2]][1]

[1] "Bob"

list2[["value"]]

[1] 1.34

# display component whose name is 'names'
list2$names

[1] "Bob"  "John"

# display 1st element of component whose name is 'names'
list2$names[1]

[1] "Bob"

list2[c(1, 3)]

$value
[1] 1.34

[[2]]
[1] TRUE

list2[-1]

$names
[1] "Bob"  "John"

[[2]]
[1] TRUE

[[3]]
[1] 1+2i

# display 1st element from 2nd component of list2
list2[[2]][1]

[1] "Bob"

# display all elements of 2nd component
list2[2]

$names
[1] "Bob"  "John"

# display all elements of 2nd component
list2[[2]]

[1] "Bob"  "John"

Membership and Coercion function for list

The membership and coercion functions for lists are as follows:

is.list(x) : Check whether x is lists
as.list(x) : Convert x to lists

Earlier we have created a list New_List01. To check whether it is a list or not, we can use is.list() function.

is.list(New_List01)

[1] TRUE

Suppose we have a numeric vector x. To convert a numeric vector to list, we can use as.list() function.

x <- c(10, 20, 25)
x

[1] 10 20 25

# check whether x is list
is.list(x)

[1] FALSE

# convert x to list
y <- as.list(x)
y

[[1]]
[1] 10

[[2]]
[1] 20

[[3]]
[1] 25

# check whether y is list
is.list(y)

[1] TRUE

Endnote

In this tutorial you learned about what is list in R, how to create list in R and how to access elements of list using different methods.

Learn more about data structures in R refer to the following tutorials:

Hope you enjoyed learning list in R. The content is more than sufficient to understand list in R.