rust collections

HashMap<K, V> for key-value lookups

Marcus Chen Jan 2026

1 tab

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();
    scores.insert("Alice", 10);
    scores.insert("Bob", 20);

    if let Some(score) = scores.get("Alice") {
        println!("Alice's score: {}", score);
    }

    scores.entry("Charlie").or_insert(30);
    println!("{:?}", scores);
}

1 file · rust Explain with highlit

HashMap<K, V> provides O(1) average-case lookups, inserts, and deletes. Keys must implement Hash + Eq. I use it for caches, indexing, and associative data. Common methods: .insert(k, v) adds/updates, .get(&k) returns Option<&V>, .remove(&k) deletes. For iteration, .iter() yields (&K, &V) pairs. The .entry() API is powerful for conditional insert/update: .or_insert() inserts if missing. HashMap uses a randomized hasher by default to prevent DoS attacks. For ordered iteration, use BTreeMap. For small maps (< 10 entries), a Vec<(K, V)> can be faster due to cache locality. HashMap is versatile and efficient for most use cases.