Multidimensional Arrays

A cinema hall's seats are a grid: rows A, B, C, and seats 1, 2, 3, 4 in each row. A class's marks are a table: one row per student, one column per subject. When data has rows and columns, Java stores it in a two-dimensional (2D) array, and you reach each value with two indexes: [row][column].

A Marks Table

public class MarksTable {
    public static void main(String[] args) {
        int[][] marks = {
            {78, 92, 85},
            {88, 79, 94},
            {65, 70, 72}
        };
        System.out.printf("Rows: %d, columns: %d%n",
                marks.length, marks[0].length);
        System.out.println("Row 1, subject 2: " + marks[1][2]);
    }
}

Rows: 3, columns: 3
Row 1, subject 2: 94

How it works

1. Lines 3 to 7 create a 3 × 3 table: each inner {...} is one row.
2. marks.length is the number of rows; marks[0].length is the number of columns in row 0.
3. Line 10: marks[1][2] means row 1, column 2, both counted from 0, so it's the second student's third subject: 94.

Try it yourself: print marks[2][0]. Which student and which subject is that?

Rows, Columns, and an Array of Arrays

In Java, a 2D array is really an array of arrays: marks holds 3 references, and each points to a separate row array. That's why marks[1] is itself an array, with its own length, and why rows can even have different lengths (see "Jagged Arrays").

Creating an Empty Grid

public class Seats {
    public static void main(String[] args) {
        boolean[][] booked = new boolean[3][4];
        booked[1][2] = true;
        for (int row = 0; row < booked.length; row++) {
            for (int seat = 0; seat < booked[row].length; seat++) {
                System.out.print(booked[row][seat] ? "X " : "- ");
            }
            System.out.println();
        }
    }
}

- - - -
- - X -
- - - -

new boolean[3][4] makes 3 rows of 4 seats, every one false by default. The nested loops (see "Nested Loops") visit every seat: the outer loop picks the row, the inner loop goes along it.

Row Totals

public class RowTotals {
    public static void main(String[] args) {
        String[] names = {"Navika", "Srihitha", "Advitha"};
        int[][] marks = {{78, 92, 85}, {88, 79, 94}, {65, 70, 72}};
        for (int r = 0; r < marks.length; r++) {
            int total = 0;
            for (int mark : marks[r]) {
                total += mark;
            }
            System.out.println(names[r] + ": " + total);
        }
    }
}

Navika: 255
Srihitha: 261
Advitha: 207

The inner loop is a for-each over one row: marks[r] is an int[], so it works like any other array.

Common Mistakes

Swapping row and column. marks[2][1] and marks[1][2] are different cells. Decide once that the first index is the row.

Using marks.length for the columns. That's the number of rows. Use marks[row].length.

More than two dimensions without need. Java allows int[][][], but three-level loops are hard to follow. Consider objects instead (Level 2).

Practice

1. Easy: Print the total of each column (each subject) of the marks table.
2. Harder: Create a 4 × 4 grid and fill it so grid[i][j] = i * j, then print it as a table with printf("%4d", ...).

Key Takeaways