Collecting

Goal of This Episode

Learn to collect an iterator into various collection types with .collect().

Concept

We wouldn’t ordinarily spend this many episodes introducing methods, but iterators are simply too important — among the tools most used in everyday Rust — so the next few episodes take their time. Even then, plenty of methods will inevitably be missed. When you need more, consult the official documentation’s Iterator trait page.

.collect() — the Iterator’s Terminus

We’ve been building iterators, but an iterator is itself lazy (Episode 15 goes deep on this) — nothing actually runs until someone “pulls” on it. .collect() is the most common pull: gather all the iterator’s elements into a collection.

fn main() {
    let v: Vec<i32> = (1..=5).into_iter().collect();
}

Collecting into a String

.collect() isn’t limited to Vec. If the iterator produces chars or &strs, it can collect straight into a String:

fn main() {
    let chars = vec!['R', 'u', 's', 't'];
    let word: String = chars.into_iter().collect();
    println!("{}", word); // "Rust"
}

.last() — Taking the Final Element

.last() consumes the whole iterator and returns the final element (an Option<T>):

fn main() {
    let v = vec![10, 20, 30];
    let last = v.iter().last();
    println!("{:?}", last); // Some(&30)
}

Note it must walk the entire iterator to know which element is last.

Example Code

fn main() {
    // Basic collect — Range into Vec
    let numbers: Vec<i32> = (1..=10).into_iter().collect();
    println!("1 through 10: {:?}", numbers);

    // The turbofish syntax
    let numbers2 = (1..=5).into_iter().collect::<Vec<i32>>();
    println!("turbofish: {:?}", numbers2);

    // Collecting into a String
    let greeting: String = vec!['h', 'e', 'l', 'l', 'o'].into_iter().collect();
    println!("String: {}", greeting);

    // .last()
    let last_num = (1..=100).into_iter().last();
    println!("\nThe last of 1..=100: {:?}", last_num);

    let empty: Vec<i32> = vec![];
    let last_empty = empty.iter().last();
    println!("last of an empty Vec: {:?}", last_empty);
}

Recap

• .collect() gathers an iterator’s elements into a target collection type.
• Tell Rust the target type with an annotation let v: Vec<i32> or the turbofish .collect::<Vec<i32>>().
• Collection targets include Vec, String, and many other types.
• .last() consumes the whole iterator, returning the final element wrapped in Some.