R Arrays
An array extends the idea of a matrix beyond two dimensions, letting you store data in three or more dimensions at once.
What Is an Array?
Where a vector is one-dimensional and a matrix is two-dimensional, an array can have any number of dimensions. A common use is stacking several matrices of the same shape into a single three-dimensional block, such as tracking a grid of values across several days.
Example
arr <- array(1:24, dim = c(2, 3, 4))
arr
dim(arr)Reading an Array's Shape
The dim argument passed to array() lists the size of each dimension in order: rows, then columns, then however many "layers" (sometimes called slices) you want stacked behind them. array(1:24, dim = c(2, 3, 4)) creates 4 layers, each a 2-row by 3-column matrix.
Inspecting Dimensions
arr <- array(1:24, dim = c(2, 3, 4))
nrow(arr)
ncol(arr)
dim(arr)[3]- A vector has one dimension: length only
- A matrix has two dimensions: rows and columns
- An array has two or more dimensions: rows, columns, and one or more additional layers
Accessing Array Elements
Indexing an array follows the same bracket pattern as a matrix, just with one extra index per extra dimension: array[row, column, layer]. Leaving any position blank selects everything along that dimension.
Slicing an Array
arr <- array(1:24, dim = c(2, 3, 4))
arr[1, 2, 3]
arr[, , 1]
arr[1, , ]Exercise: R Arrays
How does an R array fundamentally differ from an R matrix?
