Unit 8: 2D Arrays

Unit 8: 2D Arrays#

In the previous unit, we learned about ArrayLists, which allow us to manage collections of data that can dynamically grow or shrink. In this unit, we will explore 2D arrays, which are essentially arrays of arrays. 2D arrays are often used to represent grids, tables, or matrices, and are useful when working with multi-dimensional data structures.

What is a 2D Array?#

A 2D array in Java is an array that contains other arrays as its elements. It is commonly used to store data in a matrix format, where data is organized in rows and columns. Each element of the array can be accessed using two indices: one for the row and one for the column.

Example: Real-World Applications of 2D Arrays#

  • Storing a user database: A row might store the username, while the second column stores the corresponding password.

  • Game boards: Representing a chessboard or a grid-based game like Tic-Tac-Toe.

Row Major and Column Major Arrays#

There are two common ways to store data in a 2D array:

  • Row-major order: Data is stored row by row (Java uses row-major order).

  • Column-major order: Data is stored column by column (not used in Java but important in other languages like Fortran).

Declaring and Creating 2D Arrays#

Declaring a 2D Array#

To declare a 2D array, specify the type of elements that will be stored, followed by two sets of square brackets [][]:

int[][] matrix;

At this point, the array is declared, but it has not been created yet. Arrays in Java are objects, so the variable only holds a reference to the array object.

Creating a 2D Array#

To create a 2D array, use the new keyword, followed by the data type and the size of both dimensions (rows and columns):

matrix = new int[3][3];  // 3x3 matrix

This creates a 3x3 2D array of integers. The first number represents the number of rows, and the second number represents the number of columns.

Declaring and Creating a 2D Array Together#

You can combine the declaration and creation of a 2D array into one line:

int[][] matrix = new int[3][3];

Default Values in 2D Arrays#

When a 2D array is created:

  • Numeric types (like int or double) are initialized to 0.

  • Boolean types are initialized to false.

  • Object references (like String) are initialized to null.

Setting Values in a 2D Array#

You can assign values to specific elements in a 2D array by specifying the row and column indices:

matrix[0][0] = 1;  // Assigns 1 to the first row, first column
matrix[1][2] = 9;  // Assigns 9 to the second row, third column

Setting Multiple Values at Once#

You can also initialize a 2D array with predefined values when it is created:

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

In this example, the 2D array is initialized with specific values for each element.

Accessing Values in a 2D Array#

To retrieve a value from a 2D array, specify the row and column indices:

int value = matrix[2][1];  // Retrieves the element in the third row, second column

Traversing 2D Arrays#

Array Lengths in 2D Arrays#

A 2D array has both a row length and a column length. You can access the length of the outer array (the number of rows) and the length of the inner arrays (the number of columns) using the length property:

int rows = matrix.length;         // Number of rows
int cols = matrix[0].length;      // Number of columns in the first row

Using Nested Loops to Traverse a 2D Array#

To traverse a 2D array and access each element, you can use a nested for loop. The outer loop iterates over the rows, and the inner loop iterates over the columns:

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
    }
    System.out.println();  // New line after each row
}

This will print out each element of the 2D array in a grid format.

Homework Exercises#

  1. Create a 2D Integer Array

    • Create a 3x3 integer array and store the numbers 1 to 9 in it.

    • Print the array using nested loops.

  2. Boolean 2D Array

    • Create a 1x2 2D boolean array and store true and false in it.

    • Print the array using nested loops.

  3. String 2D Array for Grades

    • Create a 2D String array to store your grades for different subjects.

    • Find the average of all your grades and print it.

  4. Array Length and Traversal

    • Create a 5x5 2D array where each element is the sum of its row and column indices.

    • Print the array and its dimensions.

Runestone Exercises#

To practice further and explore the concepts from this unit, complete the exercises in Runestone’s Unit 8:

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