Using external crates in Rust - Mini Project: Build & Apply

Using external crates

📖 Scenario: You want to use a popular external crate called rand to generate random numbers in your Rust program. This is common when you need random values for games, simulations, or testing.

🎯 Goal: Learn how to add an external crate to your Rust project and use it to generate a random number between 1 and 10.

📋 What You'll Learn

Create a new Rust project with Cargo

Add the rand crate as a dependency in Cargo.toml

Import the rand crate in your main.rs

Generate and print a random number between 1 and 10

💡 Why This Matters

🌍 Real World

Random numbers are used in games, simulations, and testing to add unpredictability and variety.

💼 Career

Knowing how to use external crates is essential for Rust developers to reuse code and add functionality quickly.

1

Create a new Rust project

Use the command cargo new random_number in your terminal to create a new Rust project folder called random_number. Then open the main.rs file inside the src folder. Write a main function that prints "Hello, world!".

Rust

fn main() {
    // Your code here
}

Hint

Use println!("Hello, world!"); inside the main function.

2

Add the rand crate to Cargo.toml

Open the Cargo.toml file in your project root. Add the line rand = "0.8" under the [dependencies] section to include the rand crate version 0.8.

Rust

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

// Add 'rand = "0.8"' under [dependencies] in Cargo.toml

Hint

Find the [dependencies] section in Cargo.toml and add rand = "0.8" below it.

3

Import rand and generate a random number

In main.rs, add use rand::Rng; at the top. Inside main, create a variable random_number that uses rand::thread_rng().gen_range(1..=10) to generate a random number between 1 and 10.

Rust

use rand::Rng;

fn main() {
    // Your code here
}

Hint

Import rand::Rng and use rand::thread_rng().gen_range(1..=10) to get a random number.

4

Print the random number

Add a println! statement to print the text "Random number: {random_number}" where {random_number} is the variable you created.

Rust

use rand::Rng;

fn main() {
    let random_number = rand::thread_rng().gen_range(1..=10);
    // Your code here
}

Hint

Use println!("Random number: {}", random_number); to show the number.

Practice

1. What is the main purpose of using external crates in a Rust project?

easy

A. To make the code run faster automatically

B. To avoid writing any code at all

C. To add extra functionality without writing it yourself

D. To replace the Rust compiler

Solution

1. Step 1: Understand what external crates are

   External crates are packages of code others wrote that you can use in your project.

2. Step 2: Identify their purpose

   They help you add features without coding everything from scratch.

3. Final Answer:

   To add extra functionality without writing it yourself -> Option C

4. Quick Check:

   External crates = add features easily [OK]

Hint: External crates add features you don't code yourself [OK]

Common Mistakes:

• Thinking crates speed up code automatically
• Believing crates replace Rust compiler
• Assuming crates remove all coding

2. Which of the following is the correct way to add an external crate named rand to your Rust project?

easy

A. Add import rand; in main.rs

B. Write use rand; in Cargo.toml

C. Add extern crate rand; in Cargo.toml

D. Add rand = "0.8" under [dependencies] in Cargo.toml

Solution

1. Step 1: Know where to add crate versions

   Crate versions go under [dependencies] in Cargo.toml file.

2. Step 2: Identify correct syntax

   The correct syntax is rand = "0.8" under [dependencies].

3. Final Answer:

   Add rand = "0.8" under [dependencies] in Cargo.toml -> Option D

4. Quick Check:

   Cargo.toml dependencies = rand = "0.8" [OK]

Hint: Add crate and version under [dependencies] in Cargo.toml [OK]

Common Mistakes:

• Trying to use 'use' or 'import' in Cargo.toml
• Writing extern crate in Cargo.toml instead of code
• Adding crate info in main.rs instead of Cargo.toml

3. Given the following code and Cargo.toml setup, what will be the output? Cargo.toml snippet:

[dependencies]
regex = "1"

main.rs snippet:

use regex::Regex;

fn main() {
    let re = Regex::new(r"\d+").unwrap();
    let text = "abc123def";
    println!("{}", re.is_match(text));
}

medium

A. true

B. false

C. Compilation error

D. Runtime panic

Solution

1. Step 1: Understand what the regex checks

   The regex looks for one or more digits (\d+) in the text "abc123def".

2. Step 2: Check if the text contains digits

   The text contains "123", so the regex matches and returns true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Regex match found = true [OK]

Hint: Regex \d+ matches digits in string, so output true [OK]

Common Mistakes:

• Thinking regex returns false if digits exist
• Expecting compilation error due to crate usage
• Assuming unwrap causes panic here

4. What is wrong with this Rust code using the chrono crate? Cargo.toml:

[dependencies]
chrono = "0.4"

main.rs:

use chrono::DateTime;

fn main() {
    let now = DateTime::now();
    println!("{}", now);
}

medium

A. Missing extern crate chrono declaration

B. DateTime::now() does not exist; use Utc::now() instead

C. Cargo.toml version is incorrect

D. println! macro cannot print DateTime

Solution

1. Step 1: Check DateTime usage

   DateTime::now() is not a valid method; chrono uses Utc::now() to get current time.

2. Step 2: Identify correct method to get current time

   Use chrono::Utc::now() to get current DateTime.

3. Final Answer:

   DateTime::now() does not exist; use Utc::now() instead -> Option B

4. Quick Check:

   Use Utc::now() for current time [OK]

Hint: Use Utc::now(), not DateTime::now() for current time [OK]

Common Mistakes:

• Trying to call DateTime::now() directly
• Forgetting to import chrono::Utc
• Assuming extern crate needed in 2018 edition

5. You want to use the serde_json crate to convert a Rust struct into a JSON string. Which steps are needed to do this correctly?

hard

A. Add serde and serde_json to Cargo.toml, derive Serialize on struct, then call serde_json::to_string

B. Only add serde_json to Cargo.toml and call to_string on struct directly

C. Add serde_json to Cargo.toml and use println!("{}", struct) without Serialize

D. Add serde to Cargo.toml and call struct.to_json() method

Solution

1. Step 1: Add required crates in Cargo.toml

   You must add both serde and serde_json under [dependencies] to use serialization features.

2. Step 2: Derive Serialize trait on your struct

   To convert struct to JSON, it must implement Serialize, usually by adding #[derive(Serialize)].

3. Step 3: Use serde_json::to_string to convert struct

   Call serde_json::to_string(&your_struct) to get JSON string.

4. Final Answer:

   Add serde and serde_json to Cargo.toml, derive Serialize on struct, then call serde_json::to_string -> Option A

5. Quick Check:

   Serialize + serde_json::to_string = JSON string [OK]

Hint: Add serde + serde_json, derive Serialize, then use to_string [OK]

Common Mistakes:

• Forgetting to add serde crate
• Not deriving Serialize on struct
• Trying to call to_string directly on struct