Rust Foundations·Ownership as a design tool

Collections and iterators

Vec and HashMap cover common storage; iterators express transformations lazily.

Intermediate 14 min read Lesson 9 of 300 Challenge included

What you will be able to do

Use Vec<T> for ordered sequences.
Use HashMap<K, V> for key lookup.
Borrow iterators when you do not need ownership.

In plain English

A Vec is a growable array; a HashMap maps keys to values. Iterator adapters such as map, filter and collect are lazy until consumed. Prefer iterators when they make the intent clearer than index loops.

iterators.rsRust

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();
    scores.insert("HTML", 8);
    scores.insert("CSS", 12);

    let total: i32 = scores.values().sum();
    let short: Vec<&str> = scores
        .iter()
        .filter(|(_, minutes)| **minutes < 10)
        .map(|(title, _)| *title)
        .collect();

    println!("{total} {short:?}");
}

Your turn

Sum lesson minutes

Medium

Given a Vec of (title, minutes), sum the minutes and collect titles under 15 minutes.

RUST · starter

let lessons = vec![("HTML", 8), ("CSS", 12), ("Grid", 16)];

Check your understanding before moving on.

Knowledge checkQuestion 1

Which method turns an iterator into a collection?

Select an answer to continue

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