Lists are very useful as they are heterogeneous i.e. they can contain different data types. If you remember, vectors and matrices are homogeneous i.e. they can contain only one type of data. If you include different data types, they will all be coerced to the same type. With lists, it is different. In this chapter, we will delve deeper into lists.
To create a list, we use the list() function. Let us create a simple list to demonstrate how they can contain different data types.
# numeric vector
vect1 <- 1:10
# character vector
vect2 <- c('Jack', 'John', 'Jill')
# logical vector
vect3 <- c(TRUE, FALSE)
# matrix
mat <- matrix(data = 1:9, nrow = 3)
# list
l <- list(vect1, vect2, vect3, mat)
l[[1]]
[1] 1 2 3 4 5 6 7 8 9 10
[[2]]
[1] "Jack" "John" "Jill"
[[3]]
[1] TRUE FALSE
[[4]]
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9If you observe the output, all the elements of the list retain their data types. Now let us learn how to access the elements of the list.
You can access the elements of a list using the following operators:
[[[$Let us try them one by one.
# using [[
l[[1]] [1] 1 2 3 4 5 6 7 8 9 10# using [
l[1][[1]]
[1] 1 2 3 4 5 6 7 8 9 10[[ extracts the element itself (which may be a vector, matrix, data frame, or nested list), while [ returns a new list containing the selected element(s). The $ operator can be used only when we have named elements in the list. Let us add names to the elements. Use the names() function to add names to the list.
# named elements
names(l) <- c('vect1', 'vect2', 'vect3', 'mat')
l$vect1
[1] 1 2 3 4 5 6 7 8 9 10
$vect2
[1] "Jack" "John" "Jill"
$vect3
[1] TRUE FALSE
$mat
[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9# use $
l$vect1 [1] 1 2 3 4 5 6 7 8 9 10# use [[
l[['vect1']] [1] 1 2 3 4 5 6 7 8 9 10# use [
l['vect1']$vect1
[1] 1 2 3 4 5 6 7 8 9 10You can coerce other objects to list using as.list().
# vector
vect1 <- 1:10
as.list(vect1)[[1]]
[1] 1
[[2]]
[1] 2
[[3]]
[1] 3
[[4]]
[1] 4
[[5]]
[1] 5
[[6]]
[1] 6
[[7]]
[1] 7
[[8]]
[1] 8
[[9]]
[1] 9
[[10]]
[1] 10# matrix
mat <- matrix(data = 1:9, nrow = 3)
as.list(mat)[[1]]
[1] 1
[[2]]
[1] 2
[[3]]
[1] 3
[[4]]
[1] 4
[[5]]
[1] 5
[[6]]
[1] 6
[[7]]
[1] 7
[[8]]
[1] 8
[[9]]
[1] 9Use unlist() to coerce a list to a vector.
# unlist
unlist(l) vect11 vect12 vect13 vect14 vect15 vect16 vect17 vect18 vect19 vect110
"1" "2" "3" "4" "5" "6" "7" "8" "9" "10"
vect21 vect22 vect23 vect31 vect32 mat1 mat2 mat3 mat4 mat5
"Jack" "John" "Jill" "TRUE" "FALSE" "1" "2" "3" "4" "5"
mat6 mat7 mat8 mat9
"6" "7" "8" "9" # remove names
unlist(l, use.names = FALSE) [1] "1" "2" "3" "4" "5" "6" "7" "8" "9"
[10] "10" "Jack" "John" "Jill" "TRUE" "FALSE" "1" "2" "3"
[19] "4" "5" "6" "7" "8" "9"