Multidimensional Arrays

A two-dimensional array is an array of arrays — perfect for grids, tables, game boards and matrices. Java's multidimensional arrays can even have rows of different lengths.

Creating a 2D array

JAVA

void main() {
    int[][] grid = {
        {1, 2, 3},
        {4, 5, 6}
    };
    System.out.println(grid[0][2]);
    System.out.println(grid[1][0]);
    System.out.println("Rows: " + grid.length + ", columns: " + grid[0].length);
}

Output

3
4
Rows: 2, columns: 3

grid[row][column]: the first index picks the row (an inner array), the second the element within it.

Looping over rows and columns

JAVA

void main() {
    int[][] seats = new int[3][4];
    seats[1][2] = 1;
    seats[2][0] = 1;
    for (int r = 0; r < seats.length; r++) {
        for (int c = 0; c < seats[r].length; c++) {
            System.out.print(seats[r][c] == 1 ? "X " : ". ");
        }
        System.out.println();
    }
}

Output

. . . .
. . X .
X . . .

new int[3][4] creates 3 rows of 4 zeros. Use seats[r].length for the inner loop so it works even when rows differ in length.

for-each over a 2D array

JAVA

void main() {
    int[][] scores = {
        {80, 92, 75},
        {66, 70, 88},
        {95, 99, 91}
    };
    int student = 1;
    for (int[] row : scores) {
        int sum = 0;
        for (int s : row) sum += s;
        System.out.println("Student " + student++ + " average: " + sum / row.length);
    }
}

Output

Student 1 average: 82
Student 2 average: 74
Student 3 average: 95

Jagged arrays

JAVA

import java.util.Arrays;

void main() {
    int[][] triangle = new int[5][];
    for (int i = 0; i < triangle.length; i++) {
        triangle[i] = new int[i + 1];
        triangle[i][0] = 1;
        triangle[i][i] = 1;
        for (int j = 1; j < i; j++) {
            triangle[i][j] = triangle[i - 1][j - 1] + triangle[i - 1][j];
        }
    }
    for (int[] row : triangle) {
        System.out.println(Arrays.toString(row));
    }
}

Output

[1]
[1, 1]
[1, 2, 1]
[1, 3, 3, 1]
[1, 4, 6, 4, 1]

Because a 2D array is an array of arrays, each row can have its own length. Here the rows of Pascal's triangle grow by one each time.

Printing with deepToString

JAVA

import java.util.Arrays;

void main() {
    char[][] board = {
        {'X', 'O', 'X'},
        {' ', 'X', 'O'},
        {'O', ' ', 'X'}
    };
    System.out.println(Arrays.deepToString(board));
}

Output

[[X, O, X], [ , X, O], [O,  , X]]

Matrix arithmetic

JAVA

import java.util.Arrays;

void main() {
    int[][] a = {{1, 2}, {3, 4}};
    int[][] b = {{5, 6}, {7, 8}};
    int[][] product = new int[2][2];
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 2; j++) {
            for (int k = 0; k < 2; k++) {
                product[i][j] += a[i][k] * b[k][j];
            }
        }
    }
    System.out.println(Arrays.deepToString(product));
}

Output

[[19, 22], [43, 50]]

Try it yourself

Check whether a tic-tac-toe board has a winning row.

Output

X wins on row 2

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