Writing first Rust program

Introduction

Writing your first Rust program helps you learn how to create and run simple code. It shows you how Rust works and how to see results on your screen.

When you want to learn how to write and run Rust code for the first time.

When you want to check if your Rust setup on your computer is working.

When you want to understand the basic structure of a Rust program.

When you want to print messages or simple results to the screen.

When you want to start practicing Rust programming step-by-step.

Syntax

Rust

fn main() {
    println!("Hello, world!");
}

fn main() defines the main function where the program starts.

println!("text") prints text to the screen with a new line.

Examples

This prints a simple greeting message.

Rust

fn main() {
    println!("Hello, Rust!");
}

This prints a message with a calculated value inside.

Rust

fn main() {
    println!("Sum of 2 + 3 is {}", 2 + 3);
}

This prints a welcome message to the user.

Rust

fn main() {
    println!("Welcome to Rust programming.");
}

Sample Program

This is the simplest Rust program. It prints "Hello, world!" to the screen.

Rust

fn main() {
    println!("Hello, world!");
}

Important Notes

Every Rust program starts with a main function.

Use println! to show messages on the screen.

Remember to save your file with .rs extension before running.

Summary

Rust programs start with fn main().

Use println! to print messages.

Running your first program helps you check your setup and learn basics.

Practice

1. What is the correct way to start a basic Rust program?

easy

A. By defining a fn main() function

B. By writing start() function

C. By using begin() function

D. By declaring a program() function

Solution

1. Step 1: Understand Rust program entry point

   Rust programs always start execution from the main function.

2. Step 2: Identify the correct function name

   The correct function to start a Rust program is fn main().

3. Final Answer:

   By defining a fn main() function -> Option A

4. Quick Check:

   Rust program entry point = fn main() [OK]

Hint: Rust programs always start with fn main() [OK]

Common Mistakes:

• Using incorrect function names like start() or begin()
• Omitting the main function entirely
• Confusing Rust with other languages' entry points

2. Which of the following is the correct syntax to print "Hello, world!" in Rust?

easy

A. print!("Hello, world!");

B. println!("Hello, world!");

C. echo("Hello, world!");

D. console.log("Hello, world!");

Solution

1. Step 1: Identify Rust's print macro

   Rust uses the macro println! to print text with a newline.

2. Step 2: Compare options

   Only println!("Hello, world!"); is valid Rust syntax for printing with a newline.

3. Final Answer:

   println!("Hello, world!"); -> Option B

4. Quick Check:

   Print with newline = println! macro [OK]

Hint: Use println! to print with a newline in Rust [OK]

Common Mistakes:

• Using print! without newline when newline is expected
• Using functions from other languages like echo or console.log
• Missing the exclamation mark after println

3. What will be the output of this Rust program?

fn main() {
    println!("{}", 5 + 3);
}

medium

A. 8

B. 5 + 3

C. {}

D. Compilation error

Solution

1. Step 1: Understand the println! macro with placeholders

   The {} inside the string is a placeholder for the value following the string.

2. Step 2: Calculate the expression inside println!

   The expression 5 + 3 evaluates to 8, which replaces the {}.

3. Final Answer:

   8 -> Option A

4. Quick Check:

   5 + 3 = 8 printed [OK]

Hint: {} prints evaluated expression inside println! [OK]

Common Mistakes:

• Expecting the literal string '5 + 3' to print
• Confusing {} as a string literal
• Thinking it causes a compile error

4. Identify the error in this Rust program:

fn main() {
    println!("Hello, world!")
}

medium

A. Missing quotation marks around string

B. Incorrect function name main

C. Missing semicolon after println! macro

D. println! macro does not exist

Solution

1. Step 1: Check syntax of println! macro call

   In Rust, statements must end with a semicolon unless they are the last expression in a block.

2. Step 2: Identify missing semicolon

   The println!("Hello, world!") line is missing a semicolon at the end.

3. Final Answer:

   Missing semicolon after println! macro -> Option C

4. Quick Check:

   Statements need semicolon unless last expression [OK]

Hint: Add semicolon after println! unless last expression [OK]

Common Mistakes:

• Forgetting semicolon after macro calls
• Thinking main function name is wrong
• Assuming println! is not a macro

5. You want to write a Rust program that prints "Hello, <name>!" where <name> is a variable. Which code correctly does this?

hard

A. fn main() {
let name = "Alice";
println!("Hello, {name}!");
}

B. fn main() {
let name = "Alice";
println!("Hello, name!");
}

C. fn main() {
let name = "Alice";
print!("Hello, {}!", name);
}

D. fn main() {
let name = "Alice";
println!("Hello, {}!", name);
}

Solution

1. Step 1: Understand variable interpolation in println!

   Rust uses {} as a placeholder and variables are passed after the string.

2. Step 2: Check each option for correct syntax and output

   fn main() { let name = "Alice"; println!("Hello, {}!", name); } correctly uses println!("Hello, {}!", name); to print the variable's value.

3. Final Answer:

   fn main() { let name = "Alice"; println!("Hello, {}!", name); } -> Option D

4. Quick Check:

   Use {} and pass variable to println! [OK]

Hint: Use {} and pass variable to println! for interpolation [OK]

Common Mistakes:

• Putting variable name inside quotes without {}
• Using print! instead of println! (missing newline)
• Trying to use {name} without Rust 1.58+ raw identifiers