💻AP Computer Science A Unit 6 – Array Basics in AP Computer Science A

What Are Arrays?

• Arrays are data structures that store a fixed-size sequential collection of elements of the same data type
• Provide a way to organize and efficiently access a group of related values using a single variable name
• Each element in an array is assigned a unique index, starting from 0 for the first element
• Array indices are used to access and modify individual elements within the array
• Arrays have a fixed length that is determined when the array is created and cannot be changed
• Once an array is created, its size remains constant throughout the program execution
• Arrays can store primitive data types (int, double, boolean) or object references (String, custom objects)

Creating and Initializing Arrays

• To create an array, specify the data type followed by square brackets

  []

  and the array variable name
  • Example:

    int[] numbers;

    creates an array of integers named

    numbers

• Arrays can be initialized using the

  new

  keyword followed by the data type and the desired length in square brackets
  • Example:

    String[] names = new String[5];

    creates an array of 5 String elements
• Arrays can also be initialized with a list of values enclosed in curly braces

  {}

  • Example:

    double[] prices = {9.99, 14.50, 7.75};

    creates an array with the specified values
• When an array is created without explicit initialization, its elements are automatically initialized to default values
  • Numeric types (int, double) are initialized to 0
  • Boolean arrays are initialized to false
  • Object arrays (String, custom objects) are initialized to null
• It is important to ensure that arrays are properly initialized before accessing their elements to avoid null pointer exceptions

Accessing and Modifying Array Elements

• Array elements are accessed using their index within square brackets

  []

  after the array variable name
  • Example:

    numbers[0]

    accesses the first element of the

    numbers

    array
• Array indices start from 0 for the first element and go up to

  length - 1

  for the last element
• To modify an array element, use the array variable name followed by the index and assign a new value
  • Example:

    names[2] = "John";

    assigns the value "John" to the third element of the

    names

    array
• Attempting to access an array element using an index that is out of bounds (negative or greater than or equal to the array length) will result in an

  ArrayIndexOutOfBoundsException

• When accessing array elements, it is crucial to ensure that the index is within the valid range to avoid runtime errors

Array Length and Bounds

• The length of an array determines the number of elements it can store
• The

  length

  property is used to obtain the length of an array
  • Example:

    int size = numbers.length;

    retrieves the length of the

    numbers

    array
• The length of an array is fixed and cannot be changed after the array is created
• Array indices range from 0 to

  length - 1

  , where

  length - 1

  represents the index of the last element
• Attempting to access an array element using an index that is out of bounds will throw an

  ArrayIndexOutOfBoundsException

  • Example: Accessing

    names[5]

    in an array of length 5 will result in an exception
• It is important to use the

  length

  property and valid indices when working with arrays to avoid accessing elements outside the array bounds

Common Array Operations

• Arrays support various common operations for manipulating and working with their elements
• Accessing elements: Individual elements can be accessed using their index within square brackets

  []

  • Example:

    int value = numbers[3];

    retrieves the fourth element of the

    numbers

    array
• Modifying elements: Array elements can be modified by assigning new values to specific indices
  • Example:

    prices[1] = 12.99;

    updates the second element of the

    prices

    array
• Searching for elements: Arrays can be searched to find the index of a specific element or to check if an element exists
  • Example:

    Arrays.binarySearch(numbers, 10)

    searches for the value 10 in the

    numbers

    array
• Sorting elements: Arrays can be sorted in ascending or descending order using various sorting algorithms
  • Example:

    Arrays.sort(names);

    sorts the elements of the

    names

    array in ascending order
• Copying arrays: Elements from one array can be copied to another array using the

  System.arraycopy()

  method or by manually iterating and copying elements
  • Example:

    System.arraycopy(source, 0, destination, 0, length);

    copies elements from the

    source

    array to the

    destination

    array
• Comparing arrays: Arrays can be compared for equality using the

  Arrays.equals()

  method, which checks if two arrays have the same elements in the same order

Iterating Through Arrays

• Iterating through an array allows you to access and process each element in a sequential manner
• The most common way to iterate through an array is using a

  for

  loop
  • The loop variable represents the index, starting from 0 and incrementing until it reaches

    length - 1

  • Example:

    for (int i = 0; i < numbers.length; i++) {
        System.out.println(numbers[i]);
    }
    

• Enhanced

  for

  loop (for-each loop) can also be used to iterate through an array without explicitly using an index
  • The loop variable represents each element of the array in sequential order
  • Example:

    for (String name : names) {
        System.out.println(name);
    }
    

• While iterating through an array, you can perform operations on each element, such as printing, modifying, or accumulating values
• It is important to ensure that the loop condition is properly defined to avoid out-of-bounds access and infinite loops

Multidimensional Arrays

• Multidimensional arrays are arrays that contain other arrays as elements, allowing for the representation of grid-like structures
• The most common multidimensional array is a 2D array, which can be thought of as a table with rows and columns
• To create a 2D array, specify the data type followed by two sets of square brackets

  [][]

  • Example:

    int[][] matrix = new int[3][4];

    creates a 2D array with 3 rows and 4 columns
• Elements in a 2D array are accessed using two indices: the first index represents the row, and the second index represents the column
  • Example:

    matrix[1][2]

    accesses the element at the second row and third column
• Multidimensional arrays can be initialized with nested curly braces

  {}

  • Example:

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

    creates a 3x3 2D array with the specified values
• Iterating through a multidimensional array requires nested loops, with the outer loop representing the rows and the inner loop representing the columns
  • Example:

    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[i].length; j++) {
            System.out.print(matrix[i][j] + " ");
        }
        System.out.println();
    }
    

• Multidimensional arrays can have more than two dimensions, representing higher-dimensional structures, but they are less commonly used

Arrays in AP CS A Exam Context

• The AP Computer Science A exam tests your understanding and application of arrays in various contexts
• Be prepared to analyze code snippets involving array creation, initialization, accessing elements, and iterating through arrays
• Understand the syntax for creating arrays of different data types and sizes
  • Example:

    double[] scores = new double[10];

    creates an array of 10 double elements
• Know how to access and modify array elements using the appropriate index
  • Example:

    scores[4] = 85.5;

    assigns the value 85.5 to the fifth element of the

    scores

    array
• Be familiar with common array operations such as searching, sorting, and copying elements
  • Example:

    Arrays.sort(scores);

    sorts the elements of the

    scores

    array in ascending order
• Understand how to iterate through arrays using both traditional

  for

  loops and enhanced

  for

  loops
  • Example:

    for (int i = 0; i < scores.length; i++) {
        System.out.println(scores[i]);
    }
    

• Be able to trace the execution of code involving arrays and determine the output or final state of the array
  • Example: Given an array initialization and a series of operations, determine the values of specific elements after the operations are performed
• Recognize common errors related to arrays, such as accessing elements out of bounds or using invalid indices
  • Example: Attempting to access

    scores[10]

    in an array of length 10 will throw an

    ArrayIndexOutOfBoundsException

• Practice solving problems that involve manipulating arrays, such as finding the maximum or minimum value, calculating averages, or removing duplicates
  • Example: Write a method that takes an array of integers and returns the sum of all even numbers in the array