Hash Maps: Matching Pairs

Imagine a phone book where you look up a friend’s name and instantly find their high score. 📖 You don’t read every page — you just flip right to the name. In Rust, that instant lookup book is called a hash map.

The Big Idea A hash map (written HashMap<K, V>) matches a key to a value — like matching a name to a score. The K is the key and the V is the value.

Opening the lookup book

Hash maps don’t come ready-to-go like vectors, so first you have to tell Rust where to find them with a use line. Then you can make an empty one.

Think of it like this… A hash map is like a coat-check at a party. You hand over your coat (the value) and get a numbered ticket (the key). Later, you show the ticket and get your exact coat back — almost instantly! 🎟️

Insert and look up

You add a pair with .insert(key, value). To find a value later, you use .get(&key). Rust wraps the answer in Some(...) to say “I found it!” — that’s Rust being careful in case the key isn’t there.

use std::collections::HashMap;

fn main() {
    let mut scores = HashMap::new();
    scores.insert("Ferris", 100);
    scores.insert("Ada", 95);

    let who = "Ferris";
    if let Some(points) = scores.get(who) {
        println!("{} scored {} points!", who, points);
    }
}

Ferris says: The if let Some(points) part means "if the name was really in the book, grab the score." If the name isn't there, Rust skips it safely instead of crashing. 🛡️

Watch out! Each key can only point to one value. If you insert the same name twice, the new score replaces the old one — like erasing and rewriting.

Try this! Add a third player with scores.insert("Gus", 80);, then change who to "Gus" and press ▶ Run.

Quick quiz

In a HashMap, how do you find a value once you know its key?

Exactly! .get(&key) flips right to the matching value. 📖

You learned… A hash map matches keys to values like a lookup book. You bring it in with use std::collections::HashMap;, add pairs with .insert, and find them with .get. You've packed your whole Collections backpack — next world, we head off on a brand-new adventure! 🎒