What is a 2D Array?

A 2D array is an array whose elements are themselves arrays.

Think of a 2D array as a table, spreadsheet, seating chart, or game board. Instead of storing data in a single line like a 1D array, a 2D array organizes data into rows and columns.

Example: Student Grades

Student Quiz 1 Quiz 2 Quiz 3
Alice 90 85 92
Bob 78 88 81
Cara 95 91 97

This information can be stored in Java as:

int[][] grades = {
    {90, 85, 92},
    {78, 88, 81},
    {95, 91, 97}
};

Each row stores information about one student, while each column stores information about one quiz.

A 2D array requires two indices:

grades[row][column]

The first index selects a row.

The second index selects a column.

For example:

grades[1][2]
  • Row 1 → {78, 88, 81}
  • Column 2 → 81

Result:

81

Why Use 2D Arrays?

A 1D array stores information in a single list:

String[] students = {
    "Alice",
    "Bob",
    "Cara"
};

This works when there is only one category of information.

However, many real-world situations require data to be organized into rows and columns.

Examples include:

  • Classroom seating charts
  • Monthly calendars
  • Game boards
  • Student gradebooks
  • Tournament brackets
  • Pixel locations in an image

A 2D array allows related information to be grouped together in a structured way.

Visualizing Rows and Columns

Consider the following seating chart:

String[][] seating = {
    {"Ava", "Ben", "Chris"},
    {"Diana", "Ethan", "Faith"},
    {"Grace", "Henry", "Ivy"}
};

            Column
          0      1       2

Row 0   Ava    Ben    Chris
Row 1   Diana  Ethan  Faith
Row 2   Grace  Henry  Ivy

Examples:

seating[0][1]

returns:

"Ben"

seating[2][0]

returns:

"Grace"

seating[1][2]

returns:

"Faith"

Understanding rows and columns is one of the most important skills when working with 2D arrays.

Declaring a 2D Array

Declaring a 2D array is very similar to declaring a 1D array.

1D Array

int[] scores;

2D Array

int[][] scores;

The extra pair of brackets indicates that each element is itself an array.

Examples:

int[][] numbers;
String[][] names;
Student[][] roster;

Initializing a 2D Array

Method 1: Initialize with Values

int[][] grid = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Method 2: Create an Empty Grid

int[][] grid = new int[3][3];

This creates a 3-row by 3-column array.

All integer values start as 0.

0 0 0
0 0 0
0 0 0

Accessing Elements

The general format is:

array[row][column]

Example:

int[][] numbers = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

System.out.println(numbers[2][1]);

Step-by-step:

  1. Find row 2 → {7, 8, 9}
  2. Find column 1
  3. Return the value

Output:

8

Another example:

System.out.println(numbers[0][2]);

Output:

3

Updating Elements

Arrays are mutable, meaning their values can be changed after creation.

Example:

int[][] board = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

board[1][1] = 100;

Before:

1 2 3
4 5 6
7 8 9

After:

1 2 3
4 100 6
7 8 9

Only the specified location changes.

The rest of the array remains unchanged.

2D Arrays with Objects

2D arrays can store objects just like 1D arrays.

Example:

Student[][] classroom = new Student[3][3];

Each location can store a reference to a Student object.

classroom[0][0] = new Student("Alice", 16);
classroom[1][2] = new Student("Bob", 17);

Accessing methods works the same way:

System.out.println(classroom[0][0].getName());

Output:

Alice

This allows complex information to be organized into rows and columns.

HACK: AP CSA 2D Arrays

Part 1: AP CSA Multiple Choice

Question 1

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

What is the value of:

nums[1][2]

A. 3

B. 4

C. 5

D. 6

Question 2

String[][] names = {
    {"Ava", "Ben"},
    {"Cara", "David"}
};

What is printed?

System.out.println(names[0][1]);

A. Ava

B. Ben

C. Cara

D. David

Question 3

Which statement correctly updates the value 7 to 20?

int[][] arr = {
    {1,2,3},
    {4,5,6},
    {7,8,9}
};

A.

arr[2][0] = 20;

B.

arr[0][2] = 20;

C.

arr[2][2] = 20;

D.

arr[1][0] = 20;

Question 4

What does the first index represent in a 2D array?

A. Column

B. Row

C. Value

D. Length

Question 5

Which declaration creates a 2D array of Strings?

A.

String[] names;

B.

String[][] names;

C.

String names[][];

D. Both B and C

Part 2: Representing your Group

Create a 2D array containing the members of your group.

Requirements:

  • Each row represents your group of 2 or 3.
  • Each row represents a subgroup within your larger group.
  • Make sure every member of your group is in the array.
  • Add comments explaining what each subgroup represents.

Example:

String[][] groups = {
    {"Alice", "Bob", "Cara"},
    {"David", "Emma", "Frank"},
    {"Grace", "Henry", "Isabella"}
};

Reflection Questions:

  1. Why is a 2D array better than a 1D array for this situation?
  2. Which index represents the row?
  3. Which index represents the column?
  4. What changes would need to be made if each subgroup had 4 students instead of 3?
HOMEWORK DELIVERY
STEP 1
STEP 2
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