HashMap

向量通过整数索引存储值，而 HashMap 则通过键存储值。HashMap 的键可以是布尔值、整数、字符串或任何其他实现了 Eq 和 Hash 特性的类型。下一节会详细介绍。

与向量类似，HashMap 是可增长的，但当它们有多余空间时也可以自行收缩。你可以使用 HashMap::with_capacity(uint) 创建一个具有特定起始容量的 HashMap，或者使用 HashMap::new() 获取一个具有默认初始容量的 HashMap（推荐）。

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)); 
    }
}

有关哈希和哈希映射（有时称为哈希表）如何工作的更多信息，请查看哈希表维基百科