Two-dimensional arrays in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a two-dimensional array in Java?

A two-dimensional array in Java is like a table with rows and columns. It stores data in a grid format, where each element is accessed by two indexes: one for the row and one for the column.

beginner

How do you declare a two-dimensional array of integers with 3 rows and 4 columns in Java?

You declare it like this:
int[][] array = new int[3][4];
This creates a 3x4 grid of integers.

beginner

How do you access the element in the second row and third column of a 2D array named 'matrix'?

You use matrix[1][2]. Remember, counting starts at 0, so the second row is index 1 and the third column is index 2.

intermediate

How can you find the number of rows and columns in a two-dimensional array named 'data'?

The number of rows is data.length. The number of columns in the first row is data[0].length. This assumes all rows have the same number of columns.

beginner

What happens if you try to access an index outside the bounds of a two-dimensional array in Java?

Java throws an ArrayIndexOutOfBoundsException. This means you tried to use a row or column number that does not exist in the array.

How do you declare a 2D array of doubles with 5 rows and 2 columns in Java?

A double arr[][] = new double[2][5];

B double[][] arr = new double[5][2];

C double arr = new double[5,2];

D double arr[5][2];

Option B correctly declares a 2D array with 5 rows and 2 columns. Java uses new type[row][column] syntax.

What is the index of the first element in a two-dimensional array?

A 1,0

B 1,1

C 0,0

D 0,1

Java arrays start at index 0, so the first element is at row 0, column 0.

If you have int[][] grid = new int[4][3];, what is grid.length?

A 4

B 3

C 7

D 12

grid.length gives the number of rows, which is 4.

How do you access the element in the last row and last column of a 2D array named 'table'?

A table[table.length-1][table[0].length-1]

B table[table.length][table[0].length]

C table[0][0]

D table[table.length+1][table[0].length+1]

Since indexes start at 0, the last element is at length-1 for both rows and columns.

What exception is thrown if you try to access an invalid index in a 2D array?

A NullPointerException

B ClassCastException

C IllegalArgumentException

D ArrayIndexOutOfBoundsException

Accessing outside the array bounds throws ArrayIndexOutOfBoundsException.

Explain how to declare, initialize, and access elements in a two-dimensional array in Java.

Think of a table with rows and columns.

Describe how to find the size (number of rows and columns) of a two-dimensional array and why it matters.

Knowing size helps to loop safely through the array.

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