R in a Nutshell, 2nd Edition

Lists

An R list is an ordered collection of objects. Like vectors, you can refer to elements in a list by position:

> l <- list(1, 2, 3, 4, 5)
> l[1]
[[1]]
[1] 1

> l[[1]]
[1] 1

Additionally, each element in a list may be given a name and then be referred to by that name. For example, suppose we wanted to represent a few properties of a parcel (a real, physical parcel, to be sent through the mail). Suppose the parcel is destined for New York, has dimensions of 2 inches deep by 6 inches wide by 9 inches long, and costs $12.95 to mail. The three properties are all different data types in R: a character, a numeric vector of length 3, and a vector of length 1. We could combine the information into an object like this:

> parcel <- list(destination="New York", dimensions=c(2, 6, 9), price=12.95)

It is then possible to refer to each component individually using the $ notation. For example, if we wanted to get the price, we would use the following expression:

> parcel$price
[1] 12.95

Lists are a very important building block in R, because they allow the construction of heterogeneous structures. For example, data frames are built on lists.