Rust Tutorial · Structs, Enums and Collections · Lesson 16 of 29

Rust Collections: Vec, HashMap and More

Store lists in Vec, key-value data in HashMap and BTreeMap, and unique values in HashSet.

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Vec

fn main() {
    let mut prices = vec![8.5, 24.0, 31.9];
    prices.push(5.0);
    prices.sort_by(|a, b| a.partial_cmp(b).unwrap());
    println!("{prices:?}");
    println!("cheapest: {}, len: {}", prices[0], prices.len());
    if let Some(last) = prices.last() {
        println!("most expensive: {last}");
    }
    prices.retain(|&p| p < 30.0);
    println!("under 30: {prices:?}");
}

Output

[5.0, 8.5, 24.0, 31.9]
cheapest: 5, len: 4
most expensive: 31.9
under 30: [5.0, 8.5, 24.0]

Run this on your computer — see “Get Started” for setup.

HashMap and BTreeMap

HashMap is the general-purpose map (iteration order is unspecified); BTreeMap keeps keys sorted. The entry API updates or inserts in one step:

Counting with the entry API

use std::collections::{BTreeMap, HashSet};

fn main() {
    let text = "the cat and the dog and the bird";
    let mut counts: BTreeMap<&str, u32> = BTreeMap::new();
    for word in text.split_whitespace() {
        *counts.entry(word).or_insert(0) += 1;
    }
    println!("{counts:?}");
    let unique: HashSet<&str> = text.split_whitespace().collect();
    println!("{} unique words", unique.len());
}

Output

{"and": 2, "bird": 1, "cat": 1, "dog": 1, "the": 3}
5 unique words

Run this on your computer — see “Get Started” for setup.

HashSet/BTreeSet store unique values, and VecDeque is a double-ended queue.

Test yourself

When would you choose BTreeMap over HashMap?

For faster lookups
When you need keys kept in sorted order
For string keys only
When values are numbers

Show answer

Answer: When you need keys kept in sorted order

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