Two-dimensional arrays in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a 2D array of integers with 3 rows and 4 columns.

Java

int[][] matrix = new int[[1]][4];

Drag options to blanks, or click blank then click option'

A2

B3

C5

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing rows and columns and swapping the numbers.

Using only one dimension for a 2D array.

The first dimension specifies the number of rows. Here, 3 rows and 4 columns create a 3x4 matrix.

2fill in blank

medium

Complete the code to access the element in the second row and third column of a 2D array named 'grid'.

Java

int value = grid[[1]][2];

Drag options to blanks, or click blank then click option'

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 instead of 1 for the second row index.

Confusing row and column indices.

Array indices start at 0, so the second row is index 1.

3fill in blank

hard

Fix the error in the code to correctly initialize a 2D array with 2 rows and 3 columns.

Java

int[][] arr = new int[[1]][3];

Drag options to blanks, or click blank then click option'

A2

B3, 2

C2, 3

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Specifying only one dimension.

Swapping the order of rows and columns.

To create a 2D array with 2 rows and 3 columns, you must specify both dimensions inside the brackets.

4fill in blank

hard

Fill both blanks to create a 2D array 'table' with 5 rows and 6 columns and assign the value 10 to the element in the first row and last column.

Java

int[][] table = new int[[1]][[2]];
table[0][[2] - 1] = 10;

Drag options to blanks, or click blank then click option'

A5

B6

C4

D7

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong number of rows or columns.

Incorrectly calculating the last column index.

The array has 5 rows and 6 columns. The last column index is 6 - 1 = 5.

5fill in blank

hard

Fill all three blanks to create a 2D array 'matrix', assign 7 to the element at row 2, column 3, and print that element.

Java

int[][] matrix = new int[[1]][[2]];
matrix[[3]][3] = 7;
System.out.println(matrix[2][3]);

Drag options to blanks, or click blank then click option'

A4

B5

C2

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong dimensions for the array.

Assigning the value to the wrong row index.

The array has 4 rows and 5 columns. The value 7 is assigned to row 2, column 3 (indexing from 0).

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