array(data, dim)

The array function is used to create multidimensional arrays. For instance, a 3-D array is like a rectangular cube of numbers. A 4-D array could be thought of a matrix of matrices, where each of those sub-matrices has the same number of dimensions.

- data – Data to fill the array, often a vector of values. Or if the vector is to be filled in later, perhaps simply NA.
- dim – A vector describing the dimensions of the array. The length of the vector describes the number of array dimensions, e.g. an array with two dimensions is a regular matrix.

Example. Below we create an array with three dimensions. We show how the data is filled into the array by counting through the first 5 entries, then we print out the entire array.

> a <- array(1:(2*2*3), c(2,2,3)) > a[1,1,1] [1] 1 > a[2,1,1] [1] 2 > a[1,2,1] [1] 3 > a[2,2,1] [1] 4 > a[1,1,2] [1] 5 > a , , 1 [,1] [,2] [1,] 1 3 [2,] 2 4 , , 2 [,1] [,2] [1,] 5 7 [2,] 6 8 , , 3 [,1] [,2] [1,] 9 11 [2,] 10 12

Notice that the entries in the array are accessed in a similar way to that of a matrix. We could also access sections of the array by leaving out the number in the brackets in one or more dimensions, much like we do with matrices:

```
> a[,,1]
[,1] [,2]
[1,] 1 3
[2,] 2 4
```

Tip. Arrays are the generalized form of vectors and matrices. Pay very close attention to which dimensions correspond to what. It can be helpful to store a lot of data in an array

*if*it it fits the array structure and the meanings of each dimension are clear.