8.1: Declaring + Initializing 2D Arrays; Determining their

Review: Arrays are a collection (list) of elements (primitive or object reference type data)

So, a 2-Dimensional array is an array where the elements within that array are other arrays

2D arrays can be better at storing certain types of information

  • Especially if the data represents a space, with rows and columns

Seating chart: | . | Column 1 | Column 2 | Column 3 | | - | - | - | - | | Row 1 | Abby | Ben | Clara | | | Row 2 | Ethan | Frank | | | | Row 3 | Isabelle | John | Kim | Leo |

Note that this is a non-rectangular 2D array

  • Or if the data needs to be classified under categories
. Month 1 Month 2 Month 3
Winter December January February
Spring March April May
Summer June July August
Fall Summer October November

This is a rectangular 2D array. Non-rectangular 2D arrays are not a part of the CSA course

Declaring a 2D array

2D Arrays can be declared like this:

dataType[][] nameOfArray;

Initializing a 2D array

new dataType[r][c];

r: number of rows (number of arrays)
c: number of columns (length of each array)

public class Seasons {

    private String[][] Seasons = new String[2][3];

    // Or, if you already know what the elements should be:

    private String[][] Seasons2 = {
        {"December", "January", "February"},
        {"March", "April", "May"},
        {"June", "July", "August"},
        {"September", "October", "November"}
    };

}

Output: February

the value in the first bracket is the index of the rows, or which array we are accessing. In this case, the 0th index means we are accessing the first array

The value in the second bracket is the index within the array. So we are looking for the 2nd value within the first array.

To update the element of a 2D array, all you need to do is reference its location and change the value.

public class Seasons {

    private String[][] seasons = new String[2][3];

    private static String[][] seasons2 = {
        {"December", "January", "February"},
        {"March", "April", "May"},
        {"June", "July", "August"},
        {"September", "October", "November"}
    };

    public static void main(String[] args) {
        System.out.println(seasons2[0][2]);
        seasons2[0][2] = "Changed Value";  
        System.out.println(seasons2[0][2]);
    }
}

Seasons.main(null);

February
Changed Value

Popcorn Hack:

import java.util.Arrays;

public class TrimesterGrades {

    private static int[][] trimesterGrades = {
        {85, 90, 78, 92, 99}, // tri 1
        {92, 88, 91, 97, 80}, // tri 2
        {79, 85, 83, 95, 67}  // tri 3
    };

    public static void main(String[] args) {
        trimesterGrades[2][2] = 90;

        for (int[] trimester : trimesterGrades) {
            System.out.println(Arrays.toString(trimester));
        }
    }
}

TrimesterGrades.main(null);

[85, 90, 78, 92, 99]
[92, 88, 91, 97, 80]
[79, 85, 90, 95, 67]

The 2D array keeps track of a students grade, grouped by each trimester. The student, currently in Trimester 3, retook a test in their 3rd period, which raised that grade to 90.

Show how they would write code that changes the grade for the 3rd period class

8.2 - Traversing 2D Arrays

  • Learning Objective: Using nested + enhanced for loops to traverse arrays

  • How to use a nested loop to traverse a 2d array
    • Have an outer loop to iterate through each row
    • Have an inner loop that iterates through each column
    • It traverses from left to right and goes up to down (ABCDE → FGHIJ) image
  • How to use enhanced for loops
    • Get each row within the grid
    • Get each value within each row

image

  • How to traverse using row-major vs column-major order
  • Row-major order: get each row and then get the columns in it
    • Make the outer loop for each row and the inner loop for the columns when using nested loops
    • Get each row in the grid and then get each letter in the row when using enhanced loops
  • Column-major order: get each column and then get the rows in it
    • Can’t switch the row and col vars in the print statement
    • Have to switch the order that the row and column loops are in
    • Make the outer loop for each column and the inner loop for the row

image

public class RowMajorIndexing {
    public static void main(String[] args) {
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };
        int rows = matrix.length;
        int cols = matrix[0].length;
        System.out.println("Row-major indexing:");
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                int index = i * cols + j;
                System.out.println(matrix[i][j]);
            }
        }
    }
}
RowMajorIndexing.main(null)

Row-major indexing:
1
2
3
4
5
6
7
8
9

public class ColumnMajorIndexing {
    public static void main(String[] args) {
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
        };
        int rows = matrix.length;
        int cols = matrix[0].length;
        System.out.println("Column-major indexing:");
        for (int j = 0; j < cols; j++) {
            for (int i = 0; i < rows; i++) {
                int index = i + j * rows;
                System.out.println(matrix[i][j]);
            }
        }
    }
}
ColumnMajorIndexing.main(null)

Column-major indexing:
1
4
7
2
5
8
3
6
9
  • How to search for values in a 2d array
    • Use nested loops and in the inner loop make an if statement that returns True if a certain value is found
  • Finding the max/min
    • Save the index of the extrema and in the inner loop have a comparison statement that determines that value