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Collections are data structures capable of holding multiple values. Unlike the primitive array and tuple types, the data these collections reference is stored on the heap, which means the amount of data does not need to be known at compile time and can grow or shrink dynamically during program execution.

The two most commonly used collections in Rust are vectors (Vec<T>) and key-value maps (HashMap<K, V>).

Vectors (Vec<T>)

Vectors allow you to store more than one value of the same type in a single data structure that puts all values next to each other in memory:

fn main() {
    // Creating an empty mutable vector
    let mut v: Vec<i32> = Vec::new();

    // Or using the vec! macro
    let mut v2 = vec![1, 2, 3];

    // Appending elements
    v.push(5);
    v.push(6);

    // Accessing elements by index
    let third = v2[2];
    println!("The third element is: {}", third);
}

HashMap (HashMap<K, V>)

The HashMap<K, V> collection stores a mapping of keys of type K to values of type V. It operates via a hashing function, which determines how these keys and values are placed in memory. It is extremely useful when you want to look up data not by a numeric index, but by a key (such as a name or an ID).

use std::collections::HashMap;

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

    // Inserting key-value pair
    scores.insert(String::from("Blue"), 10);
    scores.insert(String::from("Yellow"), 50);

    // Reading a value (returns an Option<&V>)
    let team_name = String::from("Blue");
    let score = scores.get(&team_name);

    match score {
        Some(val) => println!("Score: {}", val),
        None => println!("No score found."),
    }
}

Try it yourself

Exercise 1: Basic Operations on Vec

Exercise 2: User Scores with HashMap

Exercise 3: Doubling Values in a Vector