Example: List Basics

Lists are one of the most flexible data structures in R.

Creation of a list

A list is created with the list function. Here are some examples:

my_list_1 <- list(c(1,2,3,4,5))
my_list_1

## [[1]]
## [1] 1 2 3 4 5

my_list_2 <- list(x=c(2.5, 3.5), y=c(5, 10))
my_list_2

## $x
## [1] 2.5 3.5
## 
## $y
## [1]  5 10

my_list_3 <- list(name=c("A", "B", "C"), my_list_2)
my_list_3

## $name
## [1] "A" "B" "C"
## 
## [[2]]
## [[2]]$x
## [1] 2.5 3.5
## 
## [[2]]$y
## [1]  5 10

Lists can also be structured (i.e. lists in lists) which looks like that:

my_list_4 <- list(c(1,2,3,4,5), c("A", "B", "C"))
my_list_4

## [[1]]
## [1] 1 2 3 4 5
## 
## [[2]]
## [1] "A" "B" "C"

Structure of a list

To get the length of a list, use the length function as for vectors. For information on the structure, use the str function:

length(my_list_3)

## [1] 2

str(my_list_3)

## List of 2
##  $ name: chr [1:3] "A" "B" "C"
##  $     :List of 2
##   ..$ x: num [1:2] 2.5 3.5
##   ..$ y: num [1:2] 5 10

In case of structured lists, the length function returns the dimension of the outer list:

length(my_list_4)

## [1] 2

str(my_list_4)

## List of 2
##  $ : num [1:5] 1 2 3 4 5
##  $ : chr [1:3] "A" "B" "C"

Displaying and accessing the content of a list

In order access elements of a list, you have to supply the position of the element in the vector within double square brackets (not in simple brackets as it is the case for vectors). Think about it as if you have to access the list and then the vector within the list element which contains the values. To finally access an element of the vector, one bracket pair is enough.

my_list_2[[1]]

## [1] 2.5 3.5

my_list_2[[1]][2]

## [1] 3.5

my_list_3[[2]][[1]][1]

## [1] 2.5

Changing, adding or deleting an element of a list

To add an element, you have to add a new vector to the list at a specified position. To overwrite an element of a list, you have to get it using the logic of accessing the value(s) of a list presented above:

# add an element to a list
my_list <- list(c(1,2,3,4,5))
my_list

## [[1]]
## [1] 1 2 3 4 5

my_list[[2]] <- c("A", "B")
my_list

## [[1]]
## [1] 1 2 3 4 5
## 
## [[2]]
## [1] "A" "B"

# overwrite a list element
my_list[[2]] <- c("G", "H")
my_list

## [[1]]
## [1] 1 2 3 4 5
## 
## [[2]]
## [1] "G" "H"

To actually delete an element, it has to be set to NULL.

my_list[[2]] <- NULL
my_list

## [[1]]
## [1] 1 2 3 4 5

For more information have a look at e.g. the respective data type site at Quick R. There you will also find an overview of how to get information about an object. Of course, looking into the package documentation or search the web is always a good idea, too.