R Arrays Are Not What You Think

When you're learning a programming language, it's often helpful to review the basics. I've been providing quick articles on the R programming language - some of the basic data structures and data types. Take a minute with me and review the array data structure.

Arrays in R are different than Arrays in Python

In every other language, an array is a collection of values. You access those values by index or name, so you'll typically see something like myArray[3] - which is the third item of a variable called "myArray." But that's not the most useful way to think of arrays in R.

Vector = 1 dimension, Matrix = 2 dimensions, Array = 3 or more dimensions

Think of arrays in context with vectors and matrices. So for example, here's a vector...

I.am.a.vector <- c("twas","brillig","and","the","slithey","toves","did","gyre","and","gimble","in","wabe")

> I.am.a.vector

 [1] "twas"    "brillig" "and"     "the"     "slithey" "toves"   "did"     "gyre"    "and"     "gimble"  "in"      "wabe" 
        

In R, a vector looks like a python array - or an array in most other languages. But it's a vector and has one dimension - width.

You can convert a vector to a matrix just by adding a number of rows (or columns)...

> I.am.a.matrix <- matrix(I.am.a.vector, nrow = 3)

> I.am.a.matrix

     [,1]      [,2]      [,3]   [,4]    
[1,] "twas"    "the"     "did"  "gimble"
[2,] "brillig" "slithey" "gyre" "in"    
[3,] "and"     "toves"   "and"  "wabe"  
        

Note that the number of columns in that matrix are automatically calculated. If you have twelve elements in a vector and you divide them into three rows, then you MUST have four columns. Matrices in R have two dimensions (width and height)

So...you can convert a vector to an array by adding rows, columns, and levels. Let's take I.a.an.array and divide it into two matrices, each with two rows and three columns...

> I.am.an.array <- array(I.am.a.vector,c(2,3,2) )

> I.am.an.array
, , 1

     [,1]      [,2]  [,3]     
[1,] "twas"    "and" "slithey"
[2,] "brillig" "the" "toves"  

, , 2

     [,1]   [,2]     [,3]  
[1,] "did"  "and"    "in"  
[2,] "gyre" "gimble" "wabe"
)        

Essentially, we've created two matrices and stacked them on top of each other. Want to get a number out of the array? Just use three indexes...

> I.am.an.array[2,3,2]
[1] "wabe"
        

Want to see more?

I publish a weekly video on R topics. Here's the video on Arrays...

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