#Lecture 005 — Comments

Comments are notes in your code that Dart ignores. We learn the three kinds and when each one is useful.

Contents

Prerequisites · Objectives · Lesson · Code Examples · Common Mistakes · Practice Questions · Answers · Takeaways

#Prerequisites

Lecture 004: you can write and run main with print.

#Learning Objectives

• Write single-line, multi-line and documentation comments
• Use comments to switch code off while testing
• Write comments that actually help

#1. Single-Line Comments

Everything after // to the end of the line is ignored.

// Prints a welcome message
print('Welcome!'); // a comment can also follow code

#2. Multi-Line Comments

Everything between /* and */ is ignored, however many lines.

/*
  This program prints the class schedule.
  Written for Day 2 of Dart Basic.
*/

#3. Documentation Comments

Three slashes, ///, document the thing right below them. Editors show the text when you hover over its name.

/// Prints a greeting for the given day.
void greet() {
  print('Good morning!');
}

#4. Good Comments

Write comments that explain why, not what — the code already says what.

// Bad: prints the total
print(total);

// Good: shown before the receipt so the cashier can confirm it
print(total);

#Code Examples

#Example 1 — Switching a Line Off

void main() {
  print('Welcome to Dart Basic');
  print('Lecture 005');
  // print('This line is switched off');

  /*
  print('So is this one');
  print('And this one');
  */

  print('Goodbye!');
}

Output

Welcome to Dart Basic
Lecture 005
Goodbye!

"Commenting out" is a quick way to test a program without some lines — without deleting them. In IntelliJ, Ctrl + / (⌘ + / on macOS) comments or uncomments the selected lines.

#Common Beginner Mistakes

1. Forgetting to close /* — without */, the rest of the file becomes a comment and you get confusing errors far below.

2. Commenting every line — // print hello above print('hello') adds nothing. Comment the reason, not the obvious.

3. Leaving old code commented out forever — once you are sure you don't need it, delete it.

#Practice Questions

1. Which symbol starts a single-line comment?
2. How do you write a comment over several lines?
3. What is special about ///?
4. What happens to a line that starts with // when the program runs?

#Practice Answers

1. //.
2. /* ... */.
3. It documents the code below it; editors show it on hover.
4. Nothing — Dart ignores it.

#Key Takeaways

• // comments one line; /* */ comments a block.
• /// documents code for editors and tools.
• Comment out lines to switch them off without deleting.
• Good comments explain why, not what.