Lists

List concept

A list can be thought of as zero or more data elements (of any type), kept in order.
The elements can be accessed by their position (or index): first element, second element, …
Typically the elements are also named and consequently can be also accessed by the names.

A list allows to keep together related information.
For example, functions often return lists when they calculate more than one value at once.

Create a list

Type the following code to manualy create a list and store it in a variable person:

person <- list(
  name = "Bob",
  age = 44,
  children = c( "Amy", "Dan", "Eve" )
)

Now, check how the person list is printed:

person

$name
[1] "Bob"

$age
[1] 44

$children
[1] "Amy" "Dan" "Eve"

Test the class of person:

class( person )

[1] "list"

Try the function str (structure) on person.
It gives a compact display of data stored in a variable and is useful for a list:

str( person )

List of 3
 $ name    : chr "Bob"
 $ age     : num 44
 $ children: chr [1:3] "Amy" "Dan" "Eve"

Length and elements by position

Use length to find the number of elements in a list:

length( person )

[1] 3

Elements of a list can be accessed by their position (first, second, …) with help of [[...]] (double bracket) operator.
Try the following to get the first and the last elements:

person[[1]]

[1] "Bob"

person[[ length(person) ]]

[1] "Amy" "Dan" "Eve"

You may indeed observe that the elements of the person list are of different classes:

class( person[[1]] )

[1] "character"

class( person[[2]] )

[1] "numeric"

Names and elements name

To get names of all elements of a list enter:

names( person )

[1] "name"     "age"      "children"

Each of these names can be used with the [[...]] operator to get/set an element by name:

person[[ 'name' ]]

[1] "Bob"

person[[ 'children' ]]

[1] "Amy" "Dan" "Eve"

Try to add a new element to the list using:

person[[ 'smoker' ]] <- TRUE
person

$name
[1] "Bob"

$age
[1] 44

$children
[1] "Amy" "Dan" "Eve"

$smoker
[1] TRUE

and next change the element:

person[[ 'smoker' ]] <- FALSE
person

$name
[1] "Bob"

$age
[1] 44

$children
[1] "Amy" "Dan" "Eve"

$smoker
[1] FALSE

and then remove that element from the list:

person[[ 'smoker' ]] <- NULL
person

$name
[1] "Bob"

$age
[1] 44

$children
[1] "Amy" "Dan" "Eve"

Note what happens when you try to access a not present element:

person[[ 'isMarried' ]]

NULL

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