Rust for C-Programmers

12.4 Working with Closures

Closures shine when composing functional patterns, such as iterators, sorting, and lazy evaluation.

12.4.1 Using Closures with Iterators

fn main() {
    let numbers = vec![1, 2, 3, 4, 5, 6];
    let even_numbers: Vec<_> = numbers
        .into_iter()
        .filter(|x| x % 2 == 0)
        .collect();
    println!("{:?}", even_numbers); // [2, 4, 6]
}

12.4.2 Sorting with Closures

#[derive(Debug)]
struct Person {
    name: String,
    age: u32,
}

fn main() {
    let mut people = vec![
        Person { name: "Alice".to_string(), age: 30 },
        Person { name: "Bob".to_string(), age: 25 },
        Person { name: "Charlie".to_string(), age: 35 },
    ];
    people.sort_by_key(|person| person.age);
    println!("{:?}", people);
}

12.4.3 Lazy Defaults with unwrap_or_else

Closures provide lazy defaults in many standard library methods:

fn main() {
    let config: Option<String> = None;
    let config_value = config.unwrap_or_else(|| {
        println!("Using default configuration");
        "default_config".to_string()
    });
    println!("Config: {}", config_value);
}

Here, the closure is called only if config is None.