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.
Flash cards
Review the key moves
What is the main idea behind Rust HashMap?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ std::collections::HashMap;Put the learning moves in the order that makes the concept easiest to apply.
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.