R Lists

What Makes a List Different from a Vector

A vector demands that every element share the same type, but a list has no such rule. A single list can hold a number, a character string, a logical value, and even another vector or list, all at once. You create one with the list() function.

student <- list(name = "Priya", age = 21, passed = TRUE, grades = c(88, 91, 79))
student

length(student)

Accessing List Elements: [ ] vs [[ ]]

Single brackets [ ] pull out a sub-list - the result is still a list, just a smaller one. Double brackets [[ ]] reach inside and return the actual element itself, stripped of its list wrapper. This distinction trips up a lot of beginners, so it's worth testing both on the same list to see the difference.

student <- list(name = "Priya", age = 21, passed = TRUE)

student[1]     # a list containing one element
student[[1]]   # the character value "Priya" itself

class(student[1])
class(student[[1]])

• student[1] returns a list of length one
• student[[1]] returns the raw value stored at that position
• student["name"] returns a list, matched by name
• student[["name"]] returns the raw value, matched by name

Named Lists and the $ Shortcut

When a list's elements have names, the $ operator gives you a shorter way to reach into it than typing the name inside double brackets. student$age and student[["age"]] return exactly the same thing - $ is just quicker to type and read.

student <- list(name = "Priya", age = 21, grades = c(88, 91, 79))

student$name
student$grades[2]

student$age <- 22
student$age