Unit 8 Lesson 8.1 Lesson 8.2 ASCII image Homework

8.1 Lesson

By David, Torin, Josh, Nandan

• 4 min read

  • Declaring and Initializing 2D Arrays in Java
  • Typing Speed Test

    • What is a 2D array?

    Vocabulary

    Array

    • A data structure used to implement a list of primitive or object reference data.

      2D Array

    • Arrays of arrays, representing collections of related primitive or object reference data.
    • Created and indexed similar to 1D array

      Element

    • A single value in an array.

      Index

    • The position of an element in an array. The first element is at index 0 in Java.

      Length of an Array

    • The number of elements in the array. It’s a public final data member of an array.
      • Public: Accessible in any class.
      • Final: The length of the array cannot change after it is created.
      • The last element of an array is the length of the index - 1.

    Examples

    • 1D Array: Roster of students with strings.

    Image 1

    • 2D Array: Student grades (array of arrays).

    Image 1

    • This screenshot shows:
      • Collection of arrays representing student grades, where each entry is an array of a student’s grades.
      • A rectangular 2D array since each row array has the same number of entries.
      • 7 arrays (students), each with 4 grades (tests). Total: 28 elements.

    Declaring and Initializing 2D Arrays in Java

    Declaring Arrays

    • To declare a 2D array in Java, use the following format:
      • int[][] 2DArrayName = new datatype[# of rows][# of columns]
      • Example: int[][] grades = new int[3][4];
        • All the values in this array are zero
    • If you want to actually put values inside your array (initialize):
      • Use { }
      • Each row of a 2D array has its own initializer list
      • Separate elements of an array with a comma, same goes for separating a row array from another row array

    Example:

    public class Main {
    public static void main(String[] args) {
        int[][] grades = {
            {90, 85, 88, 92}, 
            {75, 80, 78, 85},
            {60, 65, 70, 75}
        };

        int rows = grades.length; // Attribute 1
        int columns = grades[0].length;

        System.out.println("Number of rows: " + rows);
        System.out.println("Number of columns: " + columns);

        int element = grades[1][2]; // Accessing an element 
        System.out.println("Element at row 2, column 3: " + element);
    }
}

Main.main(null)

    Number of rows: 3
Number of columns: 4
Element at row 2, column 3: 78
    Typing Test

    Typing Speed Test

    Type this sentence: int[][] Name = new datatype[rows][columns]

    Popcorn Hack!

    What’s wrong with this code:

    int[][] exampleArray = {
    {"Hello", "World"},
    {"Java", "Array"}
};

    The type is wrong. Instead of int, it should be string because we have words in our 2D array.

    Popcorn Hack!

    How many total elements can this 2D array hold, and how would you calculate it?

    int[][] matrix = new int[2][3];

    6 elements because there are two rows and three columns

    Popcorn Hack!

    Image 1

    How would you write the code to access the last score? MC

    • A. grades[6][3]
    • B. grades[7][4]
    • C. grades[grades.length - 1] [grades[0].length - 1]
    • D. A and C

    grades[6][3] and grades[grades.length - 1][grades[0].length - 1]
    Therefore: D. A and C

    Try on your own!

    Write a code to declare and initialize a 2D array that stores the following matrix: 1 2 3 4 5 6 7 8 9

    public class ArrayPractice {
    public static void main(String[] args) {
        // Write the code to declare and initialize the 2D array here
        int[][] array = {
            {1, 2, 3}, 
            {4, 5, 6},
            {7, 8, 9}
        };
        // Print the array
        System.out.println(java.util.Arrays.deepToString(array));
    }
}

ArrayPractice.main(null)

    [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

    Updating elements in arrays

    • Identify the position (row and column) of the element you want to update.
    • Use the array name followed by the index of the row and the index of the column inside square brackets.
    • Assign a new value to that specific position.
    • Example:
    int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6}
};

// To update the element in the first row, second column
matrix[0][1] = 10;

    10

    Try on your own!

    Write the code to change the element “Java” to “Programming”

    String[][] hack = {
    {"Hello", "World"},
    {"Java", "Array"}
};

//Insert code below:
hack[1][0] = "Programming";

    Programming

    Popcorn Hack!

    Image 1

    b) II only

    Usage of 2D arrays?

    • Collegeboard is stinky and are on the CSA test every year
      • Collegeboard tries to mess you up on 2D arrays!
        • They will me around with column order, nonunified arrays, and might throw garbage into the array like putting string into a numeric array. BE CAREFUL!
    • 2D array biggest application is managing images
      • They represent images as grids of pixels, where each element corresponds to a pixel’s color or intensity
      • Each pixel is a single value (0 to 255) indicating brightness.