Processing should define Array just like it has Vector

p5.js Development
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The advantage of using the double [][] brackets is clear during assignment and is a simple extension of the 1D array assignment e.g.

arr[x][y] = 3.14159; // OK works fine
println(arr[x][y]); // outputs  3.14149

if the 2D array is implemented using a class, like PVector then the equivalent might be thought of as

arr(x, y) = 3.14159;  // illegal syntax will not compile

you would need something like

arr(x, y, 3.14159);

The actual syntax would be determined by the class implementation.

I assume that your students are using standard 1D array syntax [] so using [][] for assigning and retrieving values should be a logical step in their learning. The real difficulty is understanding how to create the array in the first place

So expanding on @GoToLoop’s idea this function will create a 2D array of floats

float[][] makeArray2D(int nbrCols, int nbrRows){
  float[][] a = new float[nbrCols][];
  for(int i = 0; i < nbrCols; i++)
    a[i] = new float[nbrRows];
  return a;
}

// and using it
float [][] array;

void setup(){
 array = makeArray2D(9,3);
}

so the indices are [col_nbr][row_nbr] which is probably what you want for graphics but it is simple enough to change the implementation to get [row_nbr][col_nbr] indices

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