2D Array Creation and Access

Welcome! Let's dive into one of the most useful tools in your programming toolkit: the 2D array. If you've mastered 1D arrays, you're already halfway there.

What is a 2D Array?

Think of a 1D array as a single row of lockers. A 2D array is like a whole wall of lockers, organized into rows and columns.

In Java, a 2D array is technically an array of arrays. Imagine you have several 1D arrays (the rows of lockers). A 2D array is a single container that holds all of those 1D arrays.

This structure is perfect for representing any kind of grid data: a tic-tac-toe board, a spreadsheet of grades, or a map for a game. Just like a 1D array, a 2D array's size is fixed once you create it. You can't add or remove rows or columns later.

Creating a 2D Array

There are two main ways to create a 2D array.

1. Using the new keyword

This is useful when you know the dimensions of your grid but don't have the data ready yet. You specify the number of rows and columns.

// Syntax: dataType[][] arrayName = new dataType[numRows][numCols];

// Create a 2D array for a gradebook with 10 students and 5 quiz scores each.
int[][] studentScores = new int[10][5];

// Create a 2D array for a 3x3 tic-tac-toe board.
char[][] ticTacToeBoard = new char[3][3];

When you create a 2D array this way, Java fills it with default values:

• 0 for int
• 0.0 for double
• false for boolean
• null for any object type (like String)

So, our studentScores array is currently a 10x5 grid filled entirely with zeros.

2. Using an Initializer List

This is perfect when you already know the data you want to put in the array. You provide the values in a nested set of curly braces {}. The outer braces define the 2D array, and each inner set of braces defines a row.

// A 3x4 array of integers
int[][] numberGrid = {
    {10, 20, 30, 40},  // Row 0
    {50, 60, 70, 80},  // Row 1
    {90, 100, 110, 120} // Row 2
};

// A 2x2 array of player names
String[][] playerPairs = {
    {"Maya", "Carlos"},
    {"Priya", "Liam"}
};

Accessing and Modifying Elements

To get or change a value in a 2D array, you need to specify its address: its row and column index. Remember, just like with 1D arrays, indexing starts at 0.

The syntax is arrayName[row][col].

For the AP exam, always think [row] first, [column] second.

Let's use our numberGrid from before: int[][] numberGrid = { {10, 20, 30, 40}, {50, 60, 70, 80}, {90, 100, 110, 120} };

• To get the value 70, we go to row 1, column 2:

  int myVal = numberGrid[1][2]; // myVal is now 70

• To change the value 10 to 15, we go to row 0, column 0:

  numberGrid[0][0] = 15; // The grid now starts with 15

Finding the Dimensions of a 2D Array

How do you find out how many rows and columns a 2D array has? This is a very common task and a frequent source of mistakes.

Let's use a schedule array as an example: String[][] schedule = new String[5][7]; // 5 days, 7 periods

1. Getting the Number of Rows

The number of rows is the length of the outer array.

int numRows = schedule.length; // numRows will be 5

2. Getting the Number of Columns

This is the tricky part. Since a 2D array is an "array of arrays," you need to get the length of one of the inner arrays (one of the rows). The standard way is to check the length of the first row.

int numCols = schedule[0].length; // numCols will be 7

This is a critical point: schedule.length gives you the row count. schedule[0].length gives you the column count. Don't mix them up! For the AP exam, you can assume all rows have the same number of columns (the array is rectangular).

Accessing a Whole Row

Sometimes, you might want to work with an entire row at once. Since a 2D array is an array of arrays, you can grab a single row, which is itself a 1D array.

// Let's get the first row of our numberGrid
int[][] numberGrid = { {10, 20, 30, 40}, {50, 60, 70, 80}, {90, 100, 110, 120} };

// 'firstRow' is now a 1D array: {10, 20, 30, 40}
int[] firstRow = numberGrid[0];

// Now you can use it like any other 1D array
System.out.println(firstRow[1]); // Prints 20
System.out.println(firstRow.length); // Prints 4

This is a powerful concept that shows up when you start looping through 2D arrays, which we'll cover next.

Let's walk through a couple of examples to make these concepts concrete.

```

double[][] weeklyTemps = {
    {55.5, 62.0, 58.1, 65.0, 70.2, 68.0, 64.5}, // Week 1
    {58.0, 60.5, 63.2, 66.1, 71.0, 69.3, 67.8}  // Week 2
};

Ready to try a couple on your own? Think through the steps we just covered.

Problem 1: Tic-Tac-Toe

Create a 3x3 2D array of characters to represent a tic-tac-toe board. Initialize it using the new keyword. Remember what the default value for char is (it's a special whitespace character, but for this problem, just know it's not 'X' or 'O'). Then, write the line of code that places an 'X' in the center square.

Hint: What are the row and column indices for the center of a 3x3 grid?

Problem 2: Sales Data

You are given a 2D array representing weekly sales for 3 stores in Dallas. Row 0 is Store A, Row 1 is Store B, etc. Each column is a day, from Monday to Sunday.

double[][] salesData = {
    {120.50, 200.00, 150.75, 300.10, 450.00, 500.25, 250.00}, // Store A
    {90.00, 150.25, 180.00, 250.00, 350.50, 400.00, 200.00},  // Store B
    {210.10, 220.00, 190.50, 280.00, 410.25, 480.75, 300.00}   // Store C
};

Write the code to find and print the sales for Store C (row 2) on Friday (day 5).

Hint: Remember that indexing starts at 0 for both rows and columns.