Lists

A vector holds values of one type. A list can hold anything — numbers, text, vectors, even other lists — each element of any type and length.

Real-world analogy: a vector is an egg carton (every slot holds an egg); a list is a toolbox, with compartments holding whatever you like.

Creating lists

person <- list(name = "Ada", age = 36, languages = c("R", "Python"))
person

$name
[1] "Ada"

$age
[1] 36

$languages
[1] "R"      "Python"

Accessing elements

person <- list(name = "Ada", age = 36, languages = c("R", "Python"))
person$name
person[["age"]]
person$languages[2]
length(person)
names(person)

[1] "Ada"
[1] 36
[1] "Python"
[1] 3
[1] "name"      "age"       "languages"

• $name and [["name"]] get an element's contents.
• Single brackets, person["name"], return a smaller list containing that element — a common source of confusion.

Analogy: if a list is a train, [ ] gives you a shorter train with some of the carriages; [[ ]] gives you what's inside one carriage.

Changing lists

person <- list(name = "Ada", age = 36)
person$age <- 37
person$email <- "ada@example.com"
person$name <- NULL
str(person)

List of 2
 $ age  : num 37
 $ email: chr "ada@example.com"

• Assigning to a new name adds an element.
• Assigning NULL removes one.
• str() shows a compact overview of any object's structure — invaluable for exploring data.

Many R functions return lists — for example, statistical tests and models — and JSON data read into R becomes lists.

Common mistakes

• Using [ ] when you meant [[ ]], then being surprised that arithmetic fails on the result.
• Typos in names: person$nmae quietly returns NULL rather than an error.