Creating an Empty List in R

An empty list means it does not contain any elements. An empty list and NULL have a key difference. An empty list means there is a list object with 0 elements, and a NULL list means NULL; it does not have a class “list”.

Here are three ways to create an empty list in R:

1. Using list()
2. Using vector()
3. Assigning NULL to empty an existing list

Why would you create an empty list?

The most frequent use is to store elements that you will generate in the future when you start accumulating. It provides a flexible way to create a data structure dynamically.

You can use an empty list as an empty container, a placeholder, or a starting point for more complex data structures.

Method 1: Using list()

The list() function creates a list object with 0 elements. Don’t pass any arguments to this function.

Syntax

empty_data = list()

Visual representation

If you find the length of the list, it will be 0.

Example

empty_list <- list() 

print(empty_list)

print(length(empty_list))

Output

list()
[1] 0

When you want to pre-allocate a list of specific lengths, use this approach. It also makes it more efficient because you know in advance how many elements a list will hold eventually.

Creating a list of empty lists

While working on a project, you might come across a requirement when you want a single container to hold multiple independent lists.

num_lists <- 5 # Number of empty lists you want
list_of_lists <- replicate(num_lists, list())

# Verify:

length(list_of_lists) # Output: 5

class(list_of_lists) # Output: "list"

class(list_of_lists[[1]]) # Output: "list" (each element is a list)

Method 2: Using vector()

The vector(“list“, length = 0) function is a more flexible approach to creating an empty list with a specific length. For example, you can have a list with five NULL values, which is itself empty.

Visual representation

Here, the length is 5. Even though it is empty.

Syntax

vector(mode = 'list', length)

Parameters

Example

len <- 5

empty_list <- vector(mode = "list", length = len)

empty_list

class(empty_list)

print(length(empty_list))

Output

[[1]]
NULL

[[2]]
NULL

[[3]]
NULL

[[4]]
NULL

[[5]]
NULL

[1] "list"

[1] 5

Method 3: Assigning NULL to an existing list

If you assign NULL to an existing list, it will become NULL. It’s crucial to know that after assigning NULL, the class of the object becomes “NULL”, not “list”.

So, here, the class itself is changing from “list” to NULL. If you want to keep the class as a “list”, then refer to the list() approach.

Example

rv <- c(1, 2, 3, 4)

lst <- list(rv)
lst
typeof(lst)
lst <<- NULL
lst
typeof(lst)

Output

[[1]]
[1] 1 2 3 4

[1] "list"
NULL
[1] "NULL"

That’s it!