Getting Started with R - Part 9: Lists and the [[]] operator

Lists allows us to keep a collection of various objects together

I am posting this tutorial as I learn R. I will respond to feedback for errata in the comments.

What is a list?

A list object allows us to maintain a a list objects of various types. We can mix different values of different types in a 1 dimensional list structure similar to what you find in a vecotor but without the restrictions of holding a single type.

Constructing a data frame

A list is constructed with list() and we pass in the values we want to keep in that list. We can pass matrices, vectors, simple values and even dataframes to be held in a list

city_name <- c( "Columbus", "Cleveland", "Cincinnati")
alist <- list( city_name, 12.4, TRUE, matrix(1:9, nrow=3))
alist

returns the list and its content

[[1]]
[1] "Columbus"   "Cleveland"  "Cincinnati"

[[2]]
[1] 12.4

[[3]]
[1] TRUE

[[4]]
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

The [[]] indexer

The [[]] indexer is slightly different from the [] indexer in that it returns a single element and drops names. If we index our list using [], you will notice that elements are returned with a [[listindex]]. For instance

alist[1]

returns

[[1]]
[1] "Columbus"   "Cleveland"  "Cincinnati"

and

alist[[1]]

returns

[1] "Columbus"   "Cleveland"  "Cincinnati"

The difference is not immediately obious until you try to use the result. Try these command

alist[[1]][1]
alist[[1]][2]

alist[1][1]
alist[1][2]

returns

[1] "Columbus"

[1] "Cleveland"

[[1]]
[1] "Columbus"   "Cleveland"  "Cincinnati"

[[1]]
NULL

So what is going on here? If you query the classes of these types you can get more clarity

class(alist[2])
class(alist[[2]])

Reveals what is going on behind the scenes

[1] "list"
[1] "numeric"

The list holds its elements inside a object of class list and our data is the first element in that object. Let us dig deeper into this by giving our list elements names

alist_named <- list (cities=city_name, magic_number=12.4, TRUE, matrix_3x3=matrix(1:9, nrow=3))
alist_named

Shows the same list, but now we have named values

$cities
[1] "Columbus"   "Cleveland"  "Cincinnati"

$magic_number
[1] 12.4

[[3]]
[1] TRUE

$matrix_3x3
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9

This new naming allows us to also use the $ operator to extract values like this: alist_named$magic_number. That is easy to read, yet shorter than typing alist_named[["magic_number"]]

If we re-run our earlier queries with [] and [[]] you will notice the one includes the name with the value, the other just the value at that index position

Adding to a list

Adding to a list is simple. We use the c() combine function.

alist_named_with_extra <- c(alist_named, extra="Hello")
alist_named_with_extra

You will see in the results that the new alist_named_with_extra$extra value was added