2-D arrays: working with a grid

A grid of values

• A normal array is a single row of values.
• A 2-D array is a grid: it has rows and columns, like a table or a spreadsheet.
• We use it for game boards, images, tables of marks, and more.

Making a 2-D array

• int[][] g = new int[3][4]; makes a grid with 3 rows and 4 columns.
• Every cell starts at 0 (for int).
• You can also fill it directly with values inside {...}.

public class Main {
    public static void main(String[] args) {
        int[][] g = new int[3][4];   // 3 rows, 4 columns, all 0

        int[][] table = {
            {1, 2, 3},
            {4, 5, 6}
        };   // 2 rows, 3 columns
        System.out.println(table[0][2]);   // 3
        System.out.println(table[1][0]);   // 4
    }
}

Reading and writing one cell

• A cell needs two indexes: g[row][col]. Row first, then column.
• Both indexes start at 0.
• g[1][2] = 9; stores 9 in row 1, column 2.

public class Main {
    public static void main(String[] args) {
        int[][] g = new int[2][2];
        g[0][0] = 5;
        g[0][1] = 7;
        g[1][0] = 9;
        g[1][1] = 11;
        System.out.println(g[1][0]);   // 9
        g[1][0] = g[1][0] + 1;
        System.out.println(g[1][0]);   // 10
    }
}

How big is the grid?

• g.length — the number of rows.
• g[0].length — the number of columns in row 0.
• In AP CSA every row has the same length, so g[0].length is the column count for the whole grid.

public class Main {
    public static void main(String[] args) {
        int[][] g = {
            {1, 2, 3},
            {4, 5, 6}
        };
        System.out.println(g.length);      // 2 rows
        System.out.println(g[0].length);   // 3 columns
    }
}

Row-major traversal

• To visit every cell, use a loop inside a loop.
• The outer loop picks the row r. The inner loop picks the column c.
• This visits row 0 fully, then row 1, and so on. This order is called row-major.

public class Main {
    public static void main(String[] args) {
        int[][] g = {
            {1, 2, 3},
            {4, 5, 6}
        };
        int total = 0;
        for (int r = 0; r < g.length; r++) {
            for (int c = 0; c < g[0].length; c++) {
                total = total + g[r][c];
            }
        }
        System.out.println(total);   // 21
    }
}

Common mistakes

• Index a grid grid[row][col].
• grid.length is the number of rows; grid[0].length is the columns.

Now you try

• Each task pre-fills the class skeleton — write your code inside main, or complete the method shown.
• Press Run to compile and run, then Check answer.
• Your code compiles and runs on the server, so even the first run is fast.