Arrays

## Learning Objectives - Create and initialize arrays - Access and modify array elements - Iterate over arrays - Work with multi-dimensional arrays ## Declaring Arrays ### Syntax ```java // Declaration int[] numbers; // Preferred int numbers[]; // Also valid (C-style) // Creation numbers = new int[5]; // Declaration + Creation int[] numbers = new int[5]; ``` ### Initialization ```java // By size (default values) int[] numbers = new int[3]; // [0, 0, 0] boolean[] flags = new boolean[2]; // [false, false] String[] names = new String[2]; // [null, null] // With values int[] numbers = {1, 2, 3, 4, 5}; String[] fruits = {"Apple", "Banana", "Orange"}; ``` ## Array Length ```java int[] numbers = {1, 2, 3, 4, 5}; System.out.println(numbers.length); // 5 ``` ## Accessing Elements ### Index ```java int[] numbers = {10, 20, 30, 40, 50}; System.out.println(numbers[0]); // First element: 10 System.out.println(numbers[4]); // Last element: 50 System.out.println(numbers[numbers.length - 1]); // Last element ``` ### Modifying Elements ```java int[] numbers = {10, 20, 30}; numbers[0] = 15; // Change first element numbers[2] = 35; // Change last element ``` ### ArrayIndexOutOfBoundsException ```java int[] numbers = {1, 2, 3}; // numbers[5] = 10; // Runtime error! ``` ## Iterating Arrays ### for Loop ```java int[] numbers = {1, 2, 3, 4, 5}; for (int i = 0; i < numbers.length; i++) { System.out.println(numbers[i]); } ``` ### Enhanced for-each ```java int[] numbers = {1, 2, 3, 4, 5}; for (int num : numbers) { System.out.println(num); } ``` ### While Loop ```java int[] numbers = {1, 2, 3, 4, 5}; int i = 0; while (i < numbers.length) { System.out.println(numbers[i]); i++; } ``` ## Common Operations ### Sum and Average ```java int[] numbers = {1, 2, 3, 4, 5}; int sum = 0; for (int num : numbers) { sum += num; } double average = (double) sum / numbers.length; System.out.println("Sum: " + sum + ", Average: " + average); ``` ### Find Maximum ```java int[] numbers = {3, 7, 2, 9, 4}; int max = numbers[0]; for (int num : numbers) { if (num > max) { max = num; } } System.out.println("Max: " + max); ``` ### Count Matches ```java int[] numbers = {1, 2, 3, 2, 4, 2, 5}; int target = 2; int count = 0; for (int num : numbers) { if (num == target) { count++; } } System.out.println("Count: " + count); ``` ## Arrays Utility Class ### java.util.Arrays ```java import java.util.Arrays; // Sort int[] numbers = {3, 1, 4, 1, 5, 9}; Arrays.sort(numbers); // Fill int[] arr = new int[5]; Arrays.fill(arr, 10); // [10, 10, 10, 10, 10] // Binary Search (must be sorted) int index = Arrays.binarySearch(numbers, 4); // Copy int[] copy = Arrays.copyOf(numbers, numbers.length); int[] partial = Arrays.copyOf(numbers, 3); // Compare boolean equal = Arrays.equals(arr1, arr2); // toString System.out.println(Arrays.toString(numbers)); ``` ## Two-Dimensional Arrays ### Declaration ```java int[][] matrix = new int[3][4]; // 3 rows, 4 columns ``` ### Matrix Initialization ```java int[][] matrix = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; ``` ### Accessing ```java System.out.println(matrix[0][0]); // First element: 1 System.out.println(matrix[2][2]); // Last element: 9 matrix[1][2] = 10; // Modify element ``` ### Iterating ```java for (int i = 0; i < matrix.length; i++) { for (int j = 0; j < matrix[i].length; j++) { System.out.printf("%d ", matrix[i][j]); } System.out.println(); } ``` ### Matrix for-each ```java for (int[] row : matrix) { for (int num : row) { System.out.printf("%d ", num); } System.out.println(); } ``` ## Jagged Arrays ### Different Row Lengths ```java int[][] jagged = new int[3][]; jagged[0] = new int[2]; // 2 elements jagged[1] = new int[4]; // 4 elements jagged[2] = new int[1]; // 1 element // Or with initialization int[][] jagged = { {1, 2}, {3, 4, 5, 6}, {7} }; ``` ## Arrays vs ArrayList | Feature | Array | ArrayList | |---------|-------|-----------| | Size | Fixed | Dynamic | | Performance | Faster | Slightly slower | | Type | Primitives & Objects | Objects only | | API | Limited | Rich methods | ## Summary - Arrays: `type[] name = new type[size]` or `type[] name = {values}` - Access: `array[index]` - Length: `array.length` (property, not method) - Iterate: regular for loop or enhanced for-each - Arrays utility: sort, fill, copy, binarySearch, equals - 2D arrays: `int[][] matrix = new int[rows][cols]` - Arrays have fixed size; use ArrayList for dynamic collections