# Alternate/custom key types

Any type that implements the `Eq` and `Hash` traits can be a key in `HashMap`.
This includes:

* `bool` (though not very useful since there are only two possible keys)
* `int`, `uint`, and all variations thereof
* `String` and `&str` (protip: you can have a `HashMap` keyed by `String`
and call `.get()` with an `&str`)

Note that `f32` and `f64` do *not* implement `Hash`,
likely because [floating-point precision errors][floating]
would make using them as hashmap keys horribly error-prone.

All collection classes implement `Eq` and `Hash`
if their contained type also respectively implements `Eq` and `Hash`.
For example, `Vec<T>` will implement `Hash` if `T` implements `Hash`.

You can easily implement `Eq` and `Hash` for a custom type with just one line:
`#[derive(PartialEq, Eq, Hash)]`

The compiler will do the rest. If you want more control over the details,
you can implement `Eq` and/or `Hash` yourself.
This guide will not cover the specifics of implementing `Hash`.

To play around with using a `struct` in `HashMap`,
let's try making a very simple user logon system:

```rust,editable
use std::collections::HashMap;

// Eq requires that you derive PartialEq on the type.
#[derive(PartialEq, Eq, Hash)]
struct Account<'a>{
    username: &'a str,
    password: &'a str,
}

struct AccountInfo<'a>{
    name: &'a str,
    email: &'a str,
}

type Accounts<'a> = HashMap<Account<'a>, AccountInfo<'a>>;

fn try_logon<'a>(accounts: &Accounts<'a>,
        username: &'a str, password: &'a str){
    println!("Username: {}", username);
    println!("Password: {}", password);
    println!("Attempting logon...");

    let logon = Account {
        username,
        password,
    };

    match accounts.get(&logon) {
        Some(account_info) => {
            println!("Successful logon!");
            println!("Name: {}", account_info.name);
            println!("Email: {}", account_info.email);
        },
        _ => println!("Login failed!"),
    }
}

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

    let account = Account {
        username: "j.everyman",
        password: "password123",
    };

    let account_info = AccountInfo {
        name: "John Everyman",
        email: "j.everyman@email.com",
    };

    accounts.insert(account, account_info);

    try_logon(&accounts, "j.everyman", "psasword123");

    try_logon(&accounts, "j.everyman", "password123");
}
```

[hash]: https://en.wikipedia.org/wiki/Hash_function
[floating]: https://en.wikipedia.org/wiki/Floating_point#Accuracy_problems
