Rust by Example

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 is 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 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 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:

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, 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");

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX