HashMap - Rust By Example
Description: A description

# HashMap

Where vectors store values by an integer index, `HashMap`s store values by key. `HashMap` keys can be booleans, integers, strings, or any other type that implements the `Eq` and `Hash` traits. More on this in the next section.

Like vectors, `HashMap`s are growable, but HashMaps can also shrink themselves when they have excess space. You can create a HashMap with a certain starting capacity using `HashMap::with_capacity(uint)`, or use `HashMap::new()` to get a HashMap with a default initial capacity (recommended).

    use std::collections::HashMap;

    fn call(number: &str) -> &str {
        match number {
            "798-1364" => "We're sorry, the call cannot be completed as dialed.
                Please hang up and try again.",
            "645-7689" => "Hello, this is Mr. Awesome's Pizza. My name is Fred.
                What can I get for you today?",
            _ => "Hi! Who is this again?"
        }
    }

    fn main() {
        let mut contacts = HashMap::new();

        contacts.insert("Daniel", "798-1364");
        contacts.insert("Ashley", "645-7689");
        contacts.insert("Katie", "435-8291");
        contacts.insert("Robert", "956-1745");

        // Takes a reference and returns Option<&V>
        match contacts.get(&"Daniel") {
            Some(&number) => println!("Calling Daniel: {}", call(number)),
            _ => println!("Don't have Daniel's number."),
        }

        // `HashMap::insert()` returns `None`
        // if the inserted value is new, `Some(value)` otherwise
        contacts.insert("Daniel", "164-6743");

        match contacts.get(&"Ashley") {
            Some(&number) => println!("Calling Ashley: {}", call(number)),
            _ => println!("Don't have Ashley's number."),
        }

        contacts.remove(&"Ashley");

        // `HashMap::iter()` returns an iterator that yields
        // (&'a key, &'a value) pairs in arbitrary order.
        for (contact, &number) in contacts.iter() {
            println!("Calling {}: {}", contact, call(number));
        }
    }

For more information on how hashing and hash maps (sometimes called hash tables) work, have a look at Hash Table Wikipedia