Multi-dimensional arrays

Learn matrices with int[][].

Multi-dimensional arrays

A multi-dimensional array is an array of arrays. With int[][] you define a matrix:

int[][] m = new int[2][3];
// 2 rows, 3 columns
int[][] n = {{1, 2}, {3, 4}};
System.out.println(n[0][0]);  // 1

m[0][1] is the first row's second column. The first index is the row, the second is the column. In a literal each row is written in braces.

Matrix access

Each element of a matrix is accessed with two indexes: m[row][column].

int[][] m = {{1, 2}, {3, 4}};
System.out.println(m[0][0]);  // 1
System.out.println(m[0][1]);  // 2
System.out.println(m[1][0]);  // 3
System.out.println(m[1][1]);  // 4

The row count is m.length, a row's column count is m[0].length. To walk a matrix, use nested for loops.

Exercise Print matrix elements

Write a Main class with a main method. Assign the literal {{1, 2}, {3, 4}} to int[][] m. Print the top-left element with System.out.println(m[0][0]) and the bottom-right with System.out.println(m[1][1]).

Show example solution

public class Main {
    public static void main(String[] args) {
        int[][] m = {{1, 2}, {3, 4}};
        System.out.println(m[0][0]);
        System.out.println(m[1][1]);
    }
}

Test yourself

Match the matrix concepts

int[][]

Matrix
[0][0]

Top left
[1][1]

Bottom right
{{1,2},{3,4}}

Literal
How many elements does new int[2][3] create?
- 6
- 3
- 2
- 5
Show answer

6 — 2 rows × 3 columns = 6 elements.

Complete the matrix access

int v = matrix[____][0];

Show answer

int v = matrix[0][0];

Next lesson: Copying arrays

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