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Comments

Part of the Fundamentals section of Coddy's Rust journey — lesson 3 of 75.

Comments are notes you write inside your code. The compiler completely ignores them - they exist only to help humans understand the code.

To write a single-line comment, use //. Everything after // until the end of the line is ignored:

// This is a comment
println!("Hello, world!");

A comment can also be written at the end of a line, after the code:

println!("Hello, world!"); // This prints Hello, world!

For comments that span several lines, use /* to start and */ to end:

/* This is a multi-line comment.
   The compiler ignores all of it. */
println!("Welcome!");

Comments can also temporarily disable a line of code without deleting it:

// println!("This line will NOT run");
println!("This line will run");

Challenge

Beginner

Fix the code so that only Hello, Rust! is printed.

Replace the ? with the symbols that turn a line into a comment
The line printing Goodbye! should become a comment so it does NOT run
Only change the line that starts with ?

Cheat sheet

Comments in Rust:

Notes for humans - the compiler ignores them
Can disable code temporarily without deleting it

Single-line comment:

// This is a comment
println!("Hello!"); // Comment after code

Multi-line comment:

/* This is a
   multi-line comment */

Disabling code:

// println!("This will NOT run");
println!("This will run");

Try it yourself

fn main() {
    // Type your code below
    ? println!("Goodbye!");
    println!("Hello, Rust!");
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Fundamentals

1Introduction

Rust Hello World!CommentsBasic Program Structure

4Operators Part 1

Arithmetic Operators Modulo Operator Arithmetic Shortcuts Comparison Operators String Comparison

7Basic IO

Printing to Console Reading User Input Recap - Till 120 Recap - True or False

10Variable Bindings

Scope Shadowing Part 1 Variable Binding Shadowing Part 2

13Arrays Continued

Mutable Reference Arrays Array Methods Recap - Product Array Recap - Reversed Array Better Print

16Final Projects

Number Pattern Pattern Finder Student Grade Calculator

2Variables Part 1

Numbers Char String Boolean Mutable Variables

5Operators Part 2

Logical Operators Part 1 Logical Operators Part 2 Recap - Simple Logic Logical Operators Part 3

8Calculator App

Welcome Message Getting Numbers

Project lesson

Basic Operations

Project lesson

Formatted Output

Project lesson

11Functions

Declaring Functions Parameters and Arguments Return Values Multiple Return Values Recap - Sigma Function Recap - Validation Function

14Iterating Over Arrays

Range Guide Enhanced For Loop Borrow In Loop Common Array Operations

3Variables Part 2

Type Declaration Naming Conventions Type Inference Recap - Initialize Variables Type Casting

6Decision Making

If Expressions If-Else Match Statement Recap - If Else Nested If - Else

9Loops

For Over Series While Loop Break Continue Nested Loop Loop Labels Infinite Loop Recap - Dynamic Input

12Arrays Basics

Declaring Arrays Array as Parameter Accessing Elements Modifying Arrays Recap - Pretty Print Array

15String Operations

String Methods format! Macro String Conversion Recap - Format