Two-dimensional arrays in Java - Practice Problems & Coding Challenges

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Challenge - 5 Problems

💻 code output

intermediate

Output of nested loop filling 2D array

What is the output of the following Java code that fills a 2D array and prints it?

Java

public class Main {
    public static void main(String[] args) {
        int[][] arr = new int[2][3];
        int count = 1;
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[i].length; j++) {
                arr[i][j] = count++;
            }
        }
        for (int i = 0; i < arr.length; i++) {
            for (int j = 0; j < arr[i].length; j++) {
                System.out.print(arr[i][j] + " ");
            }
            System.out.println();
        }
    }
}

A

0 0 0 
0 0 0 

B

1 3 5 
2 4 6 

C

1 2 3 
4 5 6 

D

1 2 3 4 5 6 

💻 code output

intermediate

Value of element after initialization

What is the value of arr[1][2] after this code runs?

Java

int[][] arr = {
    {5, 10, 15},
    {20, 25, 30},
    {35, 40, 45}
};
// What is arr[1][2]?

A 15

B 25

C 40

D 30

🔧 debug

advanced

Identify the error in 2D array declaration

What error does this code produce?

Java

int[][] arr = new int[2][];
arr[0] = new int[3];
arr[1] = new int[2];
arr[0][3] = 5;

A ArrayIndexOutOfBoundsException

B NullPointerException

C SyntaxError

D No error

🧠 conceptual

advanced

Number of elements in jagged 2D array

Given the jagged array below, how many total elements does it contain?

Java

int[][] arr = {
    {1, 2},
    {3, 4, 5},
    {6}
};

A 6

B 5

C 3

D 7

💻 code output

expert

Output of 2D array transpose code

What is the output of this Java code that transposes a 2D array?

Java

public class Main {
    public static void main(String[] args) {
        int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6}
        };
        int rows = matrix.length;
        int cols = matrix[0].length;
        int[][] transpose = new int[cols][rows];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                transpose[j][i] = matrix[i][j];
            }
        }
        for (int i = 0; i < transpose.length; i++) {
            for (int j = 0; j < transpose[i].length; j++) {
                System.out.print(transpose[i][j] + " ");
            }
            System.out.println();
        }
    }
}

A

1 2 3 
4 5 6 

B

1 4 
2 5 
3 6 

C

1 4 2 5 3 6 

D

4 1 
5 2 
6 3