Native lesson simulator
Hash map lookup
Hash the key, jump to a bucket, then compare entries there.
Ada hashes to bucket 2; lookup only scans entries in that bucket.
HashMap
A HashMap is a collection of key/value pairs.
HashMaps are great when you want to store values and find them by a key.
To use HashMap, you must import it from Rust's standard library:
use std::collections::HashMap;Create a HashMap
You can create a new, empty HashMap and add items to it:
Example
// Import HashMap
use std::collections::HashMap;
fn main() {
// Create a HashMap called capitalCities
let mut capitalCities = HashMap::new();
// Add keys and values (Country, City) capitalCities.insert("England", "London"); capitalCities.insert("Germany", "Berlin"); capitalCities.insert("Norway", "Oslo");
println!("{:?}", capitalCities);
}Access Values
You can use the .get() method to access a value in a HashMap by its key:
Example
let mut capitalCities = HashMap::new();
capitalCities.insert("England", "London");
capitalCities.insert("Germany", "Berlin");
capitalCities.insert("Norway", "Oslo");
if let Some(city) = capitalCities.get("England") {
println!("The capital of England is {}.", city);
} else {
println!("England is not in the map.");
}Update Values
If you insert a new value using a key that already exists, the old value is replaced with the new one:
Example
let mut
capitalCities = HashMap::new();
capitalCities.insert("England", "London");
capitalCities.insert("England", "Berlin");
println!("{:?}", capitalCities);Remove Values
To remove a key from a HashMap, use the .remove() method:
Example
let mut capitalCities = HashMap::new();
// Add keys and values (Country, City) capitalCities.insert("England", "London"); capitalCities.insert("Germany", "Berlin"); capitalCities.insert("Norway", "Oslo"); // Remove the key "England" capitalCities.remove("England");
println!("{:?}", capitalCities);Loop Through a HashMap
You can use a for loop to go through all key/value pairs:
Example
let mut capitalCities = HashMap::new();
// Add keys and values (Country, City) capitalCities.insert("England", "London"); capitalCities.insert("Germany", "Berlin"); capitalCities.insert("Norway", "Oslo"); // Loop through the HashMap
for (country, city) in
&capitalCities {
println!("The capital of {} is {}.", country, city);
}Why Use HashMaps?
- To store data by key
- To quickly look up values
- To group related data (like names and scores)
Note
HashMaps require keys to be unique. Inserting the same key again will overwrite the old value.