Lesson 4d - Lists

Lists are another type of container that R has to offer.

Lesson Objectives

• Use lists to create containers with elements of multiple data types

What is a List?

A list is a container that can contain elements of different data types, unlike vectors.

Creating a List

The format to create a list is the following:

myList = list(value1, value2, value3, ...)

Storing Values by Name

Just like with vectors, we can give values keys so that we can reference them by name later on.

myList = list(a = "apple", b = 2, c = TRUE)

Accessing Items in a List

Lists are strange when it comes to indexing.

myList <- list(2, 2, 3, "a string", TRUE)

# if we were to access the first index like normal, we would get a list of 1 element
myList[1]

[[1]]
[1] 2

To get the actual value, we need to use double square brackets [[ ]].

myList <- list(2, 2, 3, "a string", TRUE)

myList[[1]]

[1] 2

This makes a huge difference when you’re using the values given to you.

myList <- list(2, 2, 3, "a string", TRUE)

myList[[1]] + 1     # Correct
myList[1] + 1       # Incorrect

[1] 3
Error in myList[1] + 1 : non-numeric argument to binary operator

Accessing Values by Name

myList = list(a = "apple", b = 2, c = TRUE)
myList[["a"]]

[1] "apple"

Accessing Multiple Items

myList = list(2, 2, 3, "a string", TRUE)
myList[c(1, 2, 4)] # Creates a new list with only items #1, #2, and #4

[[1]]
[1] 2

[[2]]
[1] 2

[[3]]
[1] "a string"

Modifying Values in a List

The process to modify values in a list is identical to how matrices and vectors have their values modified. Index the item you want to change and assign it to a new value.

myList = list(2, 2, 3, "a string", TRUE)
myList[[3]] <- "another string"
myList

[[1]]
[1] 2

[[2]]
[1] 2

[[3]]
[1] "another string"

[[4]]
[1] "a string"

[[5]]
[1] TRUE

Adding Elements to a List

There are several ways to add elements to a list. If we simply want to add a new element at the end of a list, we can use the append() function.

myList = list(2, 3)
myList <- append(myList, 4)
myList

[[1]]
[1] 2

[[2]]
[1] 3

[[3]]
[1] 4

You can also use the append function to insert a new element at a specific position by setting after to your desired index.

myList = list(2, 3)
myList <- append(myList, 4, after=1)
myList

[[1]]
[1] 2

[[2]]
[1] 4

[[3]]
[1] 3

Finally, you can also add a new element to a list by assigning a value to its named index directly.

myList = list(a = "apple", b = "banana")
myList["c"] <- "cookie"
myList

$a
[1] "apple"

$b
[1] "banana"

$c
[1] "cookie"

Key Points / Summary

• Lists should be used when you need to store data of multiple data types