#AP Computer Science A Study Guide

#2D Arrays

#Declaration and Initialization

• Declaration: To declare a 2D array, you specify the data type, followed by two sets of square brackets (one for rows, one for columns), and the array name.

java int[][] matrix; // Declares a 2D array named 'matrix' to store integers

- **Initialization:** You can initialize a 2D array during declaration or later in your code.

java // Initializing during declaration int[][] table = { {1, 2, 3}, {4, 5, 6} };

// Initializing later int[][] grid = new int[3][4]; // Creates a 3x4 grid (3 rows, 4 columns)

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Forgetting to initialize a 2D array with 'new' when you are not providing initial values during the declaration.

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### Accessing Elements

- **Indices:** Elements in a 2D array are accessed using two indices: row index and column index. 
- **Zero-based indexing:** Like 1D arrays, indexing starts from 0. - **Syntax:** `arrayName[rowIndex][columnIndex]`

java int value = table[1][0]; // Accesses the element at row 1, column 0 (value would be 4)

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Mixing up row and column indices is a frequent error. Remember: `array[row][column]`.

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### Iterating Through a 2D Array

- **Nested Loops:** The most common way to traverse a 2D array is using nested loops.

java public static void print2DArray(int[][] arr) { for (int row = 0; row < arr.length; row++) { for (int col = 0; col < arr[row].length; col++) { System.out.print(arr[row][col] + "