HashMap::keys() / values() / iter()

// Returns an iterator over references to each key.
for key in map.keys() { }

// Returns an iterator over references to each value.
for value in map.values() { }

// Returns an iterator over key-value pairs as tuples.
for (key, value) in map.iter() { }

// Returns an iterator that consumes the map, yielding owned key-value pairs.
for (key, value) in map.into_iter() { }

use std::collections::HashMap;

fn main() {
    let mut scores: HashMap<String, i32> = HashMap::new();
    scores.insert(String::from("Alice"), 100);
    scores.insert(String::from("Bob"), 85);
    scores.insert(String::from("Carol"), 92);

    // Loop over keys only.
    println!("--- keys ---");
    for key in scores.keys() {
        println!("{}", key);
    }

    // Loop over values only.
    println!("--- values ---");
    for val in scores.values() {
        println!("{}", val);
    }

    // Loop over key-value pairs.
    println!("--- iter ---");
    for (name, score) in scores.iter() {
        println!("{}: {}", name, score);
    }

    // Get mutable references to values and modify them.
    for val in scores.values_mut() {
        *val += 5; // Add 5 points to every score.
    }
    println!("After adding points: {:?}", scores);
}

Choose among three iteration methods depending on your needs: use keys() when you only need the keys, values() when you only need the values, and iter() when you need both. Note that HashMap does not preserve insertion order — the iteration order depends on hash values and may differ between runs.

To modify values during iteration, use values_mut() or iter_mut(). If you no longer need the map after the loop, use into_iter() to take ownership of the entries directly, which can be more efficient. Keep in mind that the map becomes unavailable after calling into_iter().