CMUQ 15-121 Two Dimensional Arrays

• 1. Introduction
• 2. Allocating a 2D Array
• 3. Example with a 2D Array
• 4. Some API Functions

1. Introduction

In these notes we’ll talk about two dimensional arrays. The truth is that two dimensional arrays are not commonly used: There is almost certainly a better way to represent your data than a two dimensional array. However, there are some cases where they make intuitive sense and should be used. Here are some examples:

• A game board. (Think of games like chess, checkers, tic-tac-toe, etc.)
• A matrix for mathematics.

2. Allocating a 2D Array

Allocating a 2D array is fairly straight-forward. Consider the following line that allocates a 2D array for a chess board of chess pieces:

Piece[][] chessBoard = new Piece[8][8];

Or this for a simple tic-tac-toe board:

char[][] ticTacToe = new char[3][3];

When you allocate a 2D array you are actually just allocating an array of arrays.

3. Example with a 2D Array

Consider the following code to allocated a simple 2D array and fill it with numbers:

public class TwoDimensionalArrayDemo {
    int[][] arr;

    public TwoDimensionalArrayDemo() {
        // Allocate the 2D Array
        arr = new int[3][6];

        // Print some basic information
        System.out.println("The number of rows is: " + arr.length);
        System.out.println("The number of cols is: " + arr[0].length);

        // Fill the array with some numbers:
        for(int i = 0; i < arr.length; i++) {
            for(int j = 0; j < arr[i].length; j++) {
                arr[i][j] = i+j;
            }
        }
    }

    public void printArray() {
        // Print all the items in the array
        for(int i = 0; i < arr.length; i++) {
            for(int j = 0; j < arr[i].length; j++) {
                System.out.print(arr[i][j] + " ");
            }
            System.out.print("\n");
        }
    }

    public static void main(String[] args) {
        TwoDimensionalArrayDemo demo = new TwoDimensionalArrayDemo();
        demo.printArray();
    }

}

4. Some API Functions

Remember the Arrays class provided by Java that gave us nice things like Arrays.toString? It has some additional methods useful for 2D Arrays:

If you need to check equality or generate a String from a 2D array, these methods are the best ones to use.