Multi-Dimensional Arrays

Beginner ~25 min read

A multi-dimensional array is essentially an "array of arrays." The most common type is the 2D array, which represents a table or matrix with rows and columns. You can think of it like an Excel spreadsheet or a chessboard.

2D Arrays (Matrices)

To declare a 2D array, we use two sets of brackets [][]. The first bracket represents rows, and the second represents columns.

// Syntax: int[][] matrix = new int[rows][cols];
int[][] matrix = new int[3][3]; // A 3x3 table

Accessing Elements

You access elements by specifying the row index first, then the column index.

matrix[0][0] = 1;  // Top-left
matrix[1][2] = 5;  // Middle-right

public class Main {
    public static void main(String[] args) {
        System.out.println("--- 2D Array (Matrix) Demo ---");

// 1. Declaration & Initialization
        // A 3x3 matrix (3 rows, 3 columns)
        int[][] matrix = {
                { 1, 2, 3 }, // Row 0
                { 4, 5, 6 }, // Row 1
                { 7, 8, 9 } // Row 2
        };

// 2. Accessing elements: matrix[rowIndex][colIndex]
        System.out.println("Element at [0][0]: " + matrix[0][0]); // 1
        System.out.println("Element at [1][2]: " + matrix[1][2]); // 6
        System.out.println("Element at [2][1]: " + matrix[2][1]); // 8

// 3. Iterating through a 2D Array
        System.out.println("\n--- Matrix Print ---");

for (int i = 0; i < matrix.length; i++) { // Loop through rows
            for (int j = 0; j < matrix[i].length; j++) { // Loop through cols
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println(); // New line after each row
        }

// 4. Jagged Array (Rows with different lengths)
        System.out.println("\n--- Jagged Array ---");
        int[][] jagged = {
                { 1, 2 },
                { 3, 4, 5, 6 },
                { 7 }
        };

System.out.println("Row 0 length: " + jagged[0].length);
        System.out.println("Row 1 length: " + jagged[1].length);
    }
}

Output

Click Run to execute your code

Jagged Arrays

Since a 2D array is just an array of arrays, each "row" can actually be a different length! These are called ragged or jagged arrays.

int[][] jagged = new int[3][]; // Define 3 rows, but cols unknown
jagged[0] = new int[2]; // Row 0 has 2 cols
jagged[1] = new int[4]; // Row 1 has 4 cols
jagged[2] = new int[1]; // Row 2 has 1 col

Use Case: Jagged arrays are memory efficient if you have data rows of varying sizes (e.g., storing varying number of comments for different posts).

Summary

Use [][] to declare a 2D array.
Think of it as array[rowIndex][colIndex].
Use nested loops to iterate over all elements.
Rows in a 2D array can have different lengths (Jagged Arrays).

Module Complete!

Congratulations! You have completed Module 2: Variables & Data Types. You now have a solid understanding of primitives, strings, arrays, and how to store data in Java.

In the next module, we will bring your programs to life with Operators & Control Flow (If statements, loops, logic).

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