rust collections

HashSet<T> for unique value collections

Marcus Chen Jan 2026

1 tab

use std::collections::HashSet;

fn main() {
    let mut words = HashSet::new();
    words.insert("hello");
    words.insert("world");
    words.insert("hello"); // Duplicate, not added

    println!("Unique words: {}", words.len());

    if words.contains("hello") {
        println!("Found 'hello'");
    }
}

1 file · rust Explain with highlit

HashSet<T> stores unique values with O(1) average-case membership tests. It's backed by a HashMap<T, ()>. I use it for deduplication, membership checks, and set operations (.union(), .intersection()). Common methods: .insert(v) adds, .contains(&v) checks, .remove(&v) deletes. For iteration, .iter() yields &T. Sets are unordered; use BTreeSet if you need ordering. HashSet is great for "have I seen this before?" checks in parsers, dedup in data pipelines, and implementing set-based algorithms. Like HashMap, it uses a randomized hasher for security. The API is simple and efficient for most set use cases.