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Comments

Learn to write // and /* */ comments in Dart.

Comments

A comment is text the compiler ignores. It is used to leave notes explaining the code. Dart has two kinds of comments.

Use // for a single-line comment; it lasts until the end of the line. Use /* ... */ for a block comment spanning multiple lines.

There are also documentation comments starting with ///; external tools collect them. For now we use // and /* */.

When to write comments

Comments are most valuable to explain why the code is written that way. The code already shows what it does; so avoid useless comments like // multiply a by 2.

Temporarily disabling a line (turning it into a comment) is also common. This is called commenting out.

Adding a comment at the top of the file explaining its purpose helps you recall the context quickly when you return.

Practice Comment and output

Write a program that has a // comment line and uses print to output Comment did not run. The comment content does not matter; it just needs to exist.

Starter code

main.dart
// Add a comment and a print

Sample solution

main.dart
void main() {
  // This line is ignored
  print("Comment did not run");
}

Expected output

Comment did not run

Questions and answers

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

1 Single-line comment

Question 
____ This line is ignored by the compiler
Answer 
// This line is ignored by the compiler

Explanation: A line starting with // is a comment.

2 Which marks start a multi-line comment?

  • # ... #
  • -- ... --
  • /* ... */ Correct answer
  • // ... //

Explanation: The other marks are not comments in Dart.

3 Comment types

Term Meaning
// Single-line comment
/* */ Multi-line comment
/// Documentation comment
print Prints output

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

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

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

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

8 Loops

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

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

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

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together