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

Challenge - 5 Problems

❓ Predict 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 

💡 Hint

Look at how the count variable increments inside the nested loops.

The code fills the 2D array row by row with increasing numbers starting from 1, then prints each row on a new line.

❓ Predict 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

💡 Hint

Remember that the first index is the row, the second is the column.

arr[1][2] means row 1 (second row), column 2 (third element), which is 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

💡 Hint

Check the valid indices for arr[0].

arr[0] has length 3, so valid indices are 0,1,2. Accessing index 3 causes ArrayIndexOutOfBoundsException.

🧠 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

💡 Hint

Count the elements in each inner array and add them up.

The first row has 2 elements, second has 3, third has 1. Total is 2+3+1=6.

❓ Predict 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 

💡 Hint

Transpose swaps rows and columns.

The transpose of a 2x3 matrix is a 3x2 matrix where rows and columns are swapped.

Practice

1. What does a two-dimensional array in Java represent?

easy

A. A collection of unrelated variables

B. A single list of elements

C. A table with rows and columns

D. A method to sort numbers

Solution

1. Step 1: Understand the structure of two-dimensional arrays

   Two-dimensional arrays store data in rows and columns, like a grid or table.

2. Step 2: Compare options with the definition

   Only A table with rows and columns correctly describes this as a table with rows and columns.

3. Final Answer:

   A table with rows and columns -> Option C

4. Quick Check:

   Two-dimensional array = table [OK]

Hint: Think of it as a spreadsheet with rows and columns [OK]

Common Mistakes:

• Confusing with one-dimensional arrays
• Thinking it's a list of unrelated variables
• Mixing arrays with methods

2. Which of the following is the correct way to declare a two-dimensional array of integers in Java?

easy

A. int matrix[3][4];

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

C. int matrix = new int[3,4];

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

Solution

1. Step 1: Recall Java syntax for 2D array declaration

   Java uses int[][] name = new int[rows][columns]; to declare 2D arrays.

2. Step 2: Check each option

   int[][] matrix = new int[3][4]; matches correct syntax. int matrix[3][4]; is C-style, invalid in Java. int matrix = new int[3,4]; uses comma, invalid. int[] matrix = new int[3][4]; declares a 1D array incorrectly.

3. Final Answer:

   int[][] matrix = new int[3][4]; -> Option B

4. Quick Check:

   Correct 2D array syntax = int[][] matrix = new int[3][4]; [OK]

Hint: Use double brackets [][] and new with sizes [OK]

Common Mistakes:

• Using commas instead of separate brackets
• Trying C-style declarations
• Mixing 1D and 2D array syntax

3. What is the output of the following Java code?

int[][] arr = {{1, 2}, {3, 4}, {5, 6}};
System.out.println(arr[1][0]);

medium

A. 3

B. 5

C. 4

D. 2

Solution

1. Step 1: Identify the element at arr[1][0]

   arr[1] is the second row: {3, 4}. The element at index 0 in this row is 3.

2. Step 2: Confirm the printed value

   System.out.println prints arr[1][0], which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   arr[1][0] = 3 [OK]

Hint: Remember indexing starts at 0 for rows and columns [OK]

Common Mistakes:

• Confusing row and column indices
• Starting count at 1 instead of 0
• Mixing up elements in nested arrays

4. Find the error in the following code snippet:

int[][] grid = new int[2][3];
grid[2][1] = 5;

medium

A. Array index out of bounds error

B. Syntax error in declaration

C. Missing initialization of array

D. No error, code runs fine

Solution

1. Step 1: Check array size and indices used

   Array grid has 2 rows (indices 0 and 1) and 3 columns (indices 0 to 2). Accessing grid[2][1] uses row index 2 which is out of bounds.

2. Step 2: Identify the error type

   Accessing an invalid index causes ArrayIndexOutOfBoundsException at runtime.

3. Final Answer:

   Array index out of bounds error -> Option A

4. Quick Check:

   Index 2 invalid for 2-row array [OK]

Hint: Check array size before accessing indices [OK]

Common Mistakes:

• Using indices equal to array length
• Confusing rows and columns sizes
• Assuming arrays auto-expand

5. Given a 2D array int[][] data = {{1,2},{3,4},{5,6}};, which code snippet correctly calculates the sum of all elements?

hard

A. int sum = 0;
for(int i=0; i

B. int sum = 0;
for(int i=0; i

C. int sum = 0;
for(int i=0; i

D. int sum = 0;
for(int i=0; i

Solution

1. Step 1: Understand array dimensions and iteration

   data.length is number of rows (3). data[i].length is number of columns (2). Nested loops must iterate rows then columns.

2. Step 2: Analyze each option's loop structure

   int sum = 0; for(int i=0; i

3. Final Answer:

   The nested loops with outer loop on data.length and inner on data[i].length -> Option D

4. Quick Check:

   Correct nested loops sum all elements [OK]

Hint: Outer loop rows, inner loop columns for 2D arrays [OK]

Common Mistakes:

• Swapping row and column loops
• Adding arrays directly to int
• Using wrong indices in loops