Using external crates in Rust - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Rust code using the rand crate?

Consider this Rust program that uses the rand crate to generate a random number between 1 and 3 inclusive. What will it print?

Rust

use rand::Rng;

fn main() {
    let mut rng = rand::thread_rng();
    let n = rng.gen_range(1..=3);
    println!("{}", n);
}

A Prints a random number: 0, 1, or 2

B Runtime panic due to invalid range

C Compilation error: gen_range requires exclusive upper bound

D Prints a random number: 1, 2, or 3

💡 Hint

Check the documentation for gen_range and how inclusive ranges are specified.

The gen_range method accepts inclusive ranges with ..= syntax. So gen_range(1..=3) generates 1, 2, or 3 randomly.

❓ Predict Output

intermediate

What does this code print using the chrono crate?

This Rust program uses the chrono crate to get the current year. What is the output?

Rust

use chrono::Utc;
use chrono::Datelike;

fn main() {
    let year = Utc::now().year();
    println!("{}", year);
}

A Prints the current year as a 4-digit number, e.g., 2024

B Compilation error: missing trait import for year()

C Prints the current month number

D Runtime error: Utc::now() returns a string, not a DateTime

💡 Hint

Look at the chrono crate's DateTime methods.

Utc::now() returns a DateTime object. The year() method returns the current year as an i32.

🔧 Debug

advanced

Why does this code fail to compile when using the serde_json crate?

This Rust code tries to parse a JSON string into a struct using serde_json but fails. What is the cause?

Rust

use serde::Deserialize;

#[derive(Deserialize)]
struct Person {
    name: String,
    age: u8,
}

fn main() {
    let data = "{\"name\": \"Alice\", \"age\": 30}";
    let p: Person = serde_json::from_str(data).unwrap();
    println!("{} is {} years old", p.name, p.age);
}

A Person struct missing Debug trait for unwrap()

B Missing serde_json crate in Cargo.toml

C Syntax error in JSON string due to unescaped quotes

D Age field type mismatch: u8 cannot hold 30

💡 Hint

Check the JSON string syntax carefully.

The JSON string uses double quotes inside double quotes without escaping, causing a syntax error.

📝 Syntax

advanced

Which option correctly adds the external crate in Cargo.toml and imports it in Rust?

You want to use the regex crate in your Rust project. Which option correctly shows the Cargo.toml dependency and the import statement?

A

[dependencies]
regex = 1

use regex::Regex;

B

[dependencies]
regex = "1"

use regex::Regex;

C

[dependencies]
regex = "^1.0"

use regex::Regex;

D

[dependencies]
regex = "=1.0"

use regex::regex;

💡 Hint

Check the correct syntax for specifying versions in Cargo.toml and the correct import path.

The version must be a string in quotes without extra symbols. The import path is regex::Regex, not regex::regex.

🚀 Application

expert

How many items does this code produce using itertools crate?

This Rust code uses the itertools crate to create a Cartesian product of two vectors. How many pairs are printed?

Rust

use itertools::iproduct;

fn main() {
    let a = vec![1, 2, 3];
    let b = vec![4, 5];
    let pairs: Vec<_> = iproduct!(a.iter(), b.iter()).collect();
    println!("{}", pairs.len());
}

A 6

B 5

C 3

D 2

💡 Hint

Cartesian product pairs every element of the first vector with every element of the second.

3 elements in a and 2 in b produce 3*2=6 pairs.