Two-dimensional arrays in Java - Mini Project: Build & Apply

Working with Two-Dimensional Arrays in Java

📖 Scenario: You are organizing a small seating chart for a classroom. The classroom has 3 rows and 4 columns of seats. Each seat can hold the name of one student.

🎯 Goal: You will create a two-dimensional array to store student names, set up a helper variable for the number of rows, fill the array with student names, and finally print the seating chart.

📋 What You'll Learn

Create a two-dimensional array of strings called seatingChart with 3 rows and 4 columns.

Create an integer variable called rows and set it to 3.

Fill the seatingChart array with the exact student names provided.

Use nested for loops with variables i and j to print each student's name in the seating chart.

💡 Why This Matters

🌍 Real World

Two-dimensional arrays are useful for storing grid-like data such as seating charts, game boards, or tables.

💼 Career

Understanding two-dimensional arrays is important for software development roles that involve data organization, UI layouts, or matrix computations.

1

Create the two-dimensional array

Create a two-dimensional array of strings called seatingChart with 3 rows and 4 columns.

Java

// Your code here

Hint

Use new String[3][4] to create the array with 3 rows and 4 columns.

2

Add a variable for the number of rows

Add an integer variable called rows and set it to 3.

Java

public class Seating {
    public static void main(String[] args) {
        String[][] seatingChart = new String[3][4];
        // Your code here
    }
}

Hint

Declare int rows = 3; to store the number of rows.

3

Fill the seating chart with student names

Fill the seatingChart array with these exact student names in order by row and column:
"Alice", "Bob", "Charlie", "Diana" in row 0,
"Ethan", "Fiona", "George", "Hannah" in row 1,
and "Ian", "Jane", "Kyle", "Laura" in row 2.

Java

public class Seating {
    public static void main(String[] args) {
        String[][] seatingChart = new String[3][4];
        int rows = 3;
        // Your code here
    }
}

Hint

Assign each student name to the correct position using seatingChart[row][column] = "Name";.

4

Print the seating chart

Use nested for loops with variables i and j to print each student's name from seatingChart. Print each row on a new line with names separated by spaces.

Java

public class Seating {
    public static void main(String[] args) {
        String[][] seatingChart = new String[3][4];
        int rows = 3;
        seatingChart[0][0] = "Alice";
        seatingChart[0][1] = "Bob";
        seatingChart[0][2] = "Charlie";
        seatingChart[0][3] = "Diana";
        seatingChart[1][0] = "Ethan";
        seatingChart[1][1] = "Fiona";
        seatingChart[1][2] = "George";
        seatingChart[1][3] = "Hannah";
        seatingChart[2][0] = "Ian";
        seatingChart[2][1] = "Jane";
        seatingChart[2][2] = "Kyle";
        seatingChart[2][3] = "Laura";
        // Your code here
    }
}

Hint

Use for (int i = 0; i < rows; i++) and inside it for (int j = 0; j < seatingChart[i].length; j++) to print names with System.out.print and a space. Use System.out.println() after each row.

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