CodeBlanks Logo Icon CodeBlanks
Login Register

Multi-dimensional arrays

Learn matrices with int[][].

Multi-dimensional arrays

A multi-dimensional array is an array of arrays. With int[][] you define a matrix:

int[][] m = new int[2][3];
// 2 rows, 3 columns
int[][] n = {{1, 2}, {3, 4}};
System.out.println(n[0][0]);  // 1

m[0][1] is the first row's second column. The first index is the row, the second is the column. In a literal each row is written in braces.

Matrix access

Each element of a matrix is accessed with two indexes: m[row][column].

int[][] m = {{1, 2}, {3, 4}};
System.out.println(m[0][0]);  // 1
System.out.println(m[0][1]);  // 2
System.out.println(m[1][0]);  // 3
System.out.println(m[1][1]);  // 4

The row count is m.length, a row's column count is m[0].length. To walk a matrix, use nested for loops.

Practice Print matrix elements

Write a Main class with a main method. Assign the literal {{1, 2}, {3, 4}} to int[][] m. Print the top-left element with System.out.println(m[0][0]) and the bottom-right with System.out.println(m[1][1]).

Sample solution

Main.java
public class Main {
    public static void main(String[] args) {
        int[][] m = {{1, 2}, {3, 4}};
        System.out.println(m[0][0]);
        System.out.println(m[1][1]);
    }
}

Expected output

1
4

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Match the matrix concepts

Term Meaning
int[][] Matrix
[0][0] Top left
[1][1] Bottom right
{{1,2},{3,4}} Literal

2 How many elements does new int[2][3] create?

  • 6 Correct answer
  • 3
  • 2
  • 5

Explanation: 2 rows × 3 columns = 6 elements.

3 Complete the matrix access

Question 
int v = matrix[____][0];
Answer 
int v = matrix[0][0];

Explanation: matrix[0][0] gives the top-left element.

All lessons in Beginner

1 Java basics

  1. What is Java?
  2. The main method
  3. Comments
  4. Class skeleton
  5. javac and java
  6. Summary - Java basics

2 Variables

  1. int and double
  2. boolean
  3. String
  4. Naming variables
  5. final variables
  6. Summary - Variables

3 Data types

  1. Primitive types
  2. Type casting
  3. String concatenation
  4. Math class
  5. BigInteger and BigDecimal
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Assignment and increment
  6. Summary - Operators

6 String basics

  1. length and charAt
  2. equals and equalsIgnoreCase
  3. indexOf and substring
  4. toUpperCase and trim
  5. String.format
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. else and else if
  3. Nested if
  4. switch
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. do-while
  4. break and continue
  5. for-each
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Arrays

  1. Declaring arrays
  2. Array access
  3. Filling arrays
  4. Multi-dimensional arrays
  5. Copying arrays
  6. Summary - Arrays

11 Array methods

  1. Arrays.sort
  2. Arrays.toString
  3. Arrays.binarySearch
  4. Arrays.copyOf
  5. Arrays.fill
  6. Summary - Array methods

12 Collections

  1. ArrayList
  2. add and get
  3. HashMap
  4. put and get
  5. HashSet
  6. Summary - Collections

13 Methods

  1. Defining methods
  2. return
  3. void methods
  4. Method overloading
  5. Scope
  6. Summary - Methods

14 Method parameters

  1. Primitive parameters
  2. Object parameters
  3. varargs
  4. Multiple values via array
  5. final parameters
  6. Summary - Method parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 String builder

  1. StringBuilder
  2. append
  3. insert and delete
  4. reverse
  5. toString
  6. Summary - String builder

17 Input and output

  1. Scanner
  2. nextInt and nextLine
  3. System.out.printf
  4. BufferedReader
  5. try-with-resources
  6. Summary - Input and output

18 Enum

  1. Declaring enums
  2. values and valueOf
  3. Enum fields
  4. Enum methods
  5. switch with enum
  6. Summary - Enum

19 Exception basics

  1. try/catch
  2. finally
  3. throw
  4. Exception hierarchy
  5. Multiple catch
  6. Summary - Exception basics

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together