Rust by Example: Collections - Sets

Sets are a collection of values where each value must be unique, it allows for fast membership testing. Sets in Rust are implemented as a thin wrapper around HashMap<T, ()>. The () is a zero-sized type, which means that each value in the HashMap is just a key with no associated value.
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use std::collections::HashSet;

Creating a new set

 
fn main() {
    let mut a: HashSet<i32> = HashSet::new();

Inserting values into the set

 
    a.insert(1);
    a.insert(2);
    a.insert(3);

Trying to insert a value that is already in the set, returns false if this value is already in the set

 
    a.insert(2);

Removing a value from the set, returns true if the value was in the set

 
    a.remove(&2);

You can also create a set by using the collect() method with an iterator. You can use any iterator that yields elements, such as a range, a vector, or any other collection.

 
    let b: HashSet<i32> = [3, 4, 5].iter().cloned().collect();

Checking if a value is in the set by using contains()

 
    println!("Contains 1: {}", a.contains(&1));

HashSet supports whole-set operations like union,

 
    println!(
        "Union: {:?}", a.union(&b).collect::<HashSet<&i32>>()
    );

intersection,

 
    println!(
        "Intersection: {:?}",
        a.intersection(&b).collect::<HashSet<&i32>>()
    );

difference,

 
    println!(
        "Difference: {:?}",
        a.difference(&b).collect::<HashSet<&i32>>()
    );

and symmetric difference

 
    println!(
        "Symmetric Difference: {:?}",
        a.symmetric_difference(&b).collect::<HashSet<&i32>>()
    );

HashSet also supports three methods for testing relationships between sets. is_subset is true if all elements in this set are in the other set.

 
    println!("Is a subset of: {}", a.is_subset(&b));

is_superset is true if all elements in the other set are in this set

 
    println!("Is a superset of: {}", a.is_superset(&b));

is_disjoint is true if the sets have no elements in common

 
    println!("Is disjoint: {}", a.is_disjoint(&b));

In order to iterate over a set we can use .iter()

 
    for x in a.iter() {
        println!("{}", x);
    }
}

$ rustc sets.rs
$ ./sets
Contains 1: true
Union: {1, 3, 2, 4}
Intersection: {3}
Difference: {1}
Symmetric Difference: {2, 1, 4}
Is a subset of: false
Is a superset of: false
Is disjoint: false
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