Although the R vector is a list of items, it suffers from the constraint that all elements in a vector must be the same data type. The list data type is a one dimensional list of items, where each item can be a different data type. Items in a list can be anything, including vectors, matrices and even other lists. A list can be created using the list() function:
> x = list(42, T, "wibble", matrix(1:10,5,2))
> x
[[1]]
[1] 42
[[2]]
[1] TRUE
[[3]]
[1] "wibble"
[[4]]
[,1] [,2]
[1,] 1 6
[2,] 2 7
[3,] 3 8
[4,] 4 9
[5,] 5 10
The list x contains integer, logical, character and matrix data types. To refer to an element in the list, we need to use the double-bracket notation, so x[[1]] is the first element in the list, which is a vector with a single element (42). The element x[[4]] is the 5×2 matrix shown.
Note the difference between the following two objects:
> intList = list(1:10) > intVector = 1:10 > intList [[1]] [1] 1 2 3 4 5 6 7 8 9 10 > intVector [1] 1 2 3 4 5 6 7 8 9 10
The object intList is a list containing a single element which is the vector of integers from 1 to 10. The object intVector is a vector with 10 elements. To get the number 4 out of intList, we’d need to say intList[[1]][4] while for intVector we say intVector[4].
The typeof() function returns ‘list’ when applied to a list, no matter what that list contains. As ‘list’ isn’t a numeric type, we can’t use any of the mathematical operations on a list.
We can name the elements of a list by using the names() function. For our list x above we could say:
> names(x) = c("number", "logical", "string", "matrix")
> x
$number
[1] 42
$logical
[1] TRUE
$string
[1] "wibble"
$matrix
[,1] [,2]
[1,] 1 6
[2,] 2 7
[3,] 3 8
[4,] 4 9
[5,] 5 10
Notice that the name of each element is prefixed by a $. We can use the $ notation to refer to list elements:
> x$matrix
[,1] [,2]
[1,] 1 6
[2,] 2 7
[3,] 3 8
[4,] 4 9
[5,] 5 10
We can also use the notation x[["matrix"]] to get the same element. (Vector elements can also be named, but we can’t use the $ notation to refer to vector elements; we must use vec["name"].)
Apart from that, there’s not a lot that can be done with lists at the top level. Their main use is as a storage container for other objects, and as the basis for the data frame, which is a much more commonly used data type.
