Struct HashSet 

pub struct HashSet<T, S = RandomState, A: Allocator = Global> { /* private fields */ }

A hash set implemented as a HashMap where the value is ().

As with the HashMap type, a HashSet requires that the elements implement the Eq and Hash traits. This can frequently be achieved by using #[derive(PartialEq, Eq, Hash)]. If you implement these yourself, it is important that the following property holds:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must be equal. Violating this property is a logic error.

It is also a logic error for a key to be modified in such a way that the key’s hash, as determined by the Hash trait, or its equality, as determined by the Eq trait, changes while it is in the map. This is normally only possible through Cell, RefCell, global state, I/O, or unsafe code.

The behavior resulting from either logic error is not specified, but will be encapsulated to the HashSet that observed the logic error and not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

Examples

use std::collections::HashSet;
// Type inference lets us omit an explicit type signature (which
// would be `HashSet<String>` in this example).
let mut books = HashSet::new();

// Add some books.
books.insert("A Dance With Dragons".to_string());
books.insert("To Kill a Mockingbird".to_string());
books.insert("The Odyssey".to_string());
books.insert("The Great Gatsby".to_string());

// Check for a specific one.
if !books.contains("The Winds of Winter") {
    println!("We have {} books, but The Winds of Winter ain't one.",
             books.len());
}

// Remove a book.
books.remove("The Odyssey");

// Iterate over everything.
for book in &books {
    println!("{book}");
}

The easiest way to use HashSet with a custom type is to derive Eq and Hash. We must also derive PartialEq, which is required if Eq is derived.

use std::collections::HashSet;
#[derive(Hash, Eq, PartialEq, Debug)]
struct Viking {
    name: String,
    power: usize,
}

let mut vikings = HashSet::new();

vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
vikings.insert(Viking { name: "Einar".to_string(), power: 9 });
vikings.insert(Viking { name: "Olaf".to_string(), power: 4 });
vikings.insert(Viking { name: "Harald".to_string(), power: 8 });

// Use derived implementation to print the vikings.
for x in &vikings {
    println!("{x:?}");
}

A HashSet with a known list of items can be initialized from an array:

use std::collections::HashSet;

let viking_names = HashSet::from(["Einar", "Olaf", "Harald"]);

Usage in const and static

Like HashMap, HashSet is randomly seeded: each HashSet instance uses a different seed, which means that HashSet::new cannot be used in const context. To construct a HashSet in the initializer of a const or static item, you will have to use a different hasher that does not involve a random seed, as demonstrated in the following example. A HashSet constructed this way is not resistant against HashDoS!

use std::collections::HashSet;
use std::hash::{BuildHasherDefault, DefaultHasher};
use std::sync::Mutex;

const EMPTY_SET: HashSet<String, BuildHasherDefault<DefaultHasher>> =
    HashSet::with_hasher(BuildHasherDefault::new());
static SET: Mutex<HashSet<String, BuildHasherDefault<DefaultHasher>>> =
    Mutex::new(HashSet::with_hasher(BuildHasherDefault::new()));

Implementations

impl<T> HashSet<T, RandomState>

pub fn new() -> HashSet<T, RandomState>

Creates an empty HashSet.

The hash set is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

Examples

use std::collections::HashSet;
let set: HashSet<i32> = HashSet::new();

pub fn with_capacity(capacity: usize) -> HashSet<T, RandomState>

Creates an empty HashSet with at least the specified capacity.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, the hash set will not allocate.

Examples

use std::collections::HashSet;
let set: HashSet<i32> = HashSet::with_capacity(10);
assert!(set.capacity() >= 10);

impl<T, A: Allocator> HashSet<T, RandomState, A>

pub fn new_in(alloc: A) -> HashSet<T, RandomState, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty HashSet in the provided allocator.

The hash set is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

Examples

use std::alloc::Global;
use std::collections::HashSet;

let set: HashSet<i32> = HashSet::new_in(Global);

pub fn with_capacity_in(capacity: usize, alloc: A) -> HashSet<T, RandomState, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty HashSet with at least the specified capacity.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, the hash set will not allocate.

Examples

use std::collections::HashSet;
use std::alloc::Global;

let set: HashSet<i32> = HashSet::with_capacity_in(10, Global);

impl<T, S> HashSet<T, S>

pub const fn with_hasher(hasher: S) -> HashSet<T, S>

Creates a new empty hash set which will use the given hasher to hash keys.

The hash set is also created with the default initial capacity.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashSet to be useful, see its documentation for details.

Examples

use std::collections::HashSet;
use std::hash::RandomState;

let s = RandomState::new();
let mut set = HashSet::with_hasher(s);
set.insert(2);

pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S>

Creates an empty HashSet with at least the specified capacity, using hasher to hash the keys.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, the hash set will not allocate.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashSet to be useful, see its documentation for details.

Examples

use std::collections::HashSet;
use std::hash::RandomState;

let s = RandomState::new();
let mut set = HashSet::with_capacity_and_hasher(10, s);
set.insert(1);

impl<T, S, A: Allocator> HashSet<T, S, A>

pub fn with_hasher_in(hasher: S, alloc: A) -> HashSet<T, S, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates a new empty hash set which will use the given hasher to hash keys and will allocate memory using the provided allocator.

The hash set is also created with the default initial capacity.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashSet to be useful, see its documentation for details.

Examples

use std::alloc::Global;
use std::collections::HashSet;
use std::hash::RandomState;

let s = RandomState::new();
let set: HashSet<i32> = HashSet::with_hasher_in(s, Global);

pub fn with_capacity_and_hasher_in( capacity: usize, hasher: S, alloc: A, ) -> HashSet<T, S, A>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates an empty HashSet with at least the specified capacity, using hasher to hash the keys and alloc to allocate memory.

The hash set will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, the hash set will not allocate.

Warning: hasher is normally randomly generated, and is designed to allow HashSets to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

The hash_builder passed should implement the BuildHasher trait for the HashSet to be useful, see its documentation for details.

Examples

use std::alloc::Global;
use std::collections::HashSet;
use std::hash::RandomState;

let s = RandomState::new();
let set: HashSet<i32> = HashSet::with_capacity_and_hasher_in(10, s, Global);

pub fn capacity(&self) -> usize

Returns the number of elements the set can hold without reallocating.

Examples

use std::collections::HashSet;
let set: HashSet<i32> = HashSet::with_capacity(100);
assert!(set.capacity() >= 100);

pub fn iter(&self) -> Iter<'_, T>

An iterator visiting all elements in arbitrary order. The iterator element type is &'a T.

Examples

use std::collections::HashSet;
let mut set = HashSet::new();
set.insert("a");
set.insert("b");

// Will print in an arbitrary order.
for x in set.iter() {
    println!("{x}");
}

Performance

In the current implementation, iterating over set takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

pub fn len(&self) -> usize

Returns the number of elements in the set.

Examples

use std::collections::HashSet;

let mut v = HashSet::new();
assert_eq!(v.len(), 0);
v.insert(1);
assert_eq!(v.len(), 1);

pub fn is_empty(&self) -> bool

Returns true if the set contains no elements.

Examples

use std::collections::HashSet;

let mut v = HashSet::new();
assert!(v.is_empty());
v.insert(1);
assert!(!v.is_empty());

pub fn drain(&mut self) -> Drain<'_, T, A>

Clears the set, returning all elements as an iterator. Keeps the allocated memory for reuse.

If the returned iterator is dropped before being fully consumed, it drops the remaining elements. The returned iterator keeps a mutable borrow on the set to optimize its implementation.

Examples

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3]);
assert!(!set.is_empty());

// print 1, 2, 3 in an arbitrary order
for i in set.drain() {
    println!("{i}");
}

assert!(set.is_empty());

pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, T, F, A>

where F: FnMut(&T) -> bool,

Creates an iterator which uses a closure to determine if an element should be removed.

If the closure returns true, the element is removed from the set and yielded. If the closure returns false, or panics, the element remains in the set and will not be yielded.

If the returned ExtractIf is not exhausted, e.g. because it is dropped without iterating or the iteration short-circuits, then the remaining elements will be retained. Use retain with a negated predicate if you do not need the returned iterator.

Examples

Splitting a set into even and odd values, reusing the original set:

use std::collections::HashSet;

let mut set: HashSet<i32> = (0..8).collect();
let extracted: HashSet<i32> = set.extract_if(|v| v % 2 == 0).collect();

let mut evens = extracted.into_iter().collect::<Vec<_>>();
let mut odds = set.into_iter().collect::<Vec<_>>();
evens.sort();
odds.sort();

assert_eq!(evens, vec![0, 2, 4, 6]);
assert_eq!(odds, vec![1, 3, 5, 7]);

pub fn retain<F>(&mut self, f: F)

where F: FnMut(&T) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all elements e for which f(&e) returns false. The elements are visited in unsorted (and unspecified) order.

Examples

use std::collections::HashSet;

let mut set = HashSet::from([1, 2, 3, 4, 5, 6]);
set.retain(|&k| k % 2 == 0);
assert_eq!(set, HashSet::from([2, 4, 6]));

Performance

In the current implementation, this operation takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

pub fn clear(&mut self)

Clears the set, removing all values.

Examples

use std::collections::HashSet;

let mut v = HashSet::new();
v.insert(1);
v.clear();
assert!(v.is_empty());

pub fn hasher(&self) -> &S

Returns a reference to the set’s BuildHasher.

Examples

use std::collections::HashSet;
use std::hash::RandomState;

let hasher = RandomState::new();
let set: HashSet<i32> = HashSet::with_hasher(hasher);
let hasher: &RandomState = set.hasher();

impl<T, S, A> HashSet<T, S, A>

where T: Eq + Hash, S: BuildHasher, A: Allocator,

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional more elements to be inserted in the HashSet. The collection may reserve more space to speculatively avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to self.len() + additional. Does nothing if capacity is already sufficient.

Panics

Panics if the new allocation size overflows usize.

Examples

use std::collections::HashSet;
let mut set: HashSet<i32> = HashSet::new();
set.reserve(10);
assert!(set.capacity() >= 10);

pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>

Tries to reserve capacity for at least additional more elements to be inserted in the HashSet. The collection may reserve more space to speculatively avoid frequent reallocations. After calling try_reserve, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if capacity is already sufficient.

Errors

If the capacity overflows, or the allocator reports a failure, then an error is returned.

Examples

use std::collections::HashSet;
let mut set: HashSet<i32> = HashSet::new();
set.try_reserve(10).expect("why is the test harness OOMing on a handful of bytes?");

pub fn shrink_to_fit(&mut self)

Shrinks the capacity of the set as much as possible. It will drop down as much as possible while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

Examples

use std::collections::HashSet;

let mut set = HashSet::with_capacity(100);
set.insert(1);
set.insert(2);
assert!(set.capacity() >= 100);
set.shrink_to_fit();
assert!(set.capacity() >= 2);

pub fn shrink_to(&mut self, min_capacity: usize)

Shrinks the capacity of the set with a lower limit. It will drop down no lower than the supplied limit while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

If the current capacity is less than the lower limit, this is a no-op.

Examples

use std::collections::HashSet;

let mut set = HashSet::with_capacity(100);
set.insert(1);
set.insert(2);
assert!(set.capacity() >= 100);
set.shrink_to(10);
assert!(set.capacity() >= 10);
set.shrink_to(0);
assert!(set.capacity() >= 2);

pub fn difference<'a>( &'a self, other: &'a HashSet<T, S, A>, ) -> Difference<'a, T, S, A>

Visits the values representing the difference, i.e., the values that are in self but not in other.

Examples

use std::collections::HashSet;
let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// Can be seen as `a - b`.
for x in a.difference(&b) {
    println!("{x}"); // Print 1
}

let diff: HashSet<_> = a.difference(&b).collect();
assert_eq!(diff, [1].iter().collect());

// Note that difference is not symmetric,
// and `b - a` means something else:
let diff: HashSet<_> = b.difference(&a).collect();
assert_eq!(diff, [4].iter().collect());

pub fn symmetric_difference<'a>( &'a self, other: &'a HashSet<T, S, A>, ) -> SymmetricDifference<'a, T, S, A>

Visits the values representing the symmetric difference, i.e., the values that are in self or in other but not in both.

Examples

use std::collections::HashSet;
let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// Print 1, 4 in arbitrary order.
for x in a.symmetric_difference(&b) {
    println!("{x}");
}

let diff1: HashSet<_> = a.symmetric_difference(&b).collect();
let diff2: HashSet<_> = b.symmetric_difference(&a).collect();

assert_eq!(diff1, diff2);
assert_eq!(diff1, [1, 4].iter().collect());

pub fn intersection<'a>( &'a self, other: &'a HashSet<T, S, A>, ) -> Intersection<'a, T, S, A>

Visits the values representing the intersection, i.e., the values that are both in self and other.

When an equal element is present in self and other then the resulting Intersection may yield references to one or the other. This can be relevant if T contains fields which are not compared by its Eq implementation, and may hold different value between the two equal copies of T in the two sets.

Examples

use std::collections::HashSet;
let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// Print 2, 3 in arbitrary order.
for x in a.intersection(&b) {
    println!("{x}");
}

let intersection: HashSet<_> = a.intersection(&b).collect();
assert_eq!(intersection, [2, 3].iter().collect());

pub fn union<'a>(&'a self, other: &'a HashSet<T, S, A>) -> Union<'a, T, S, A>

Visits the values representing the union, i.e., all the values in self or other, without duplicates.

Examples

use std::collections::HashSet;
let a = HashSet::from([1, 2, 3]);
let b = HashSet::from([4, 2, 3, 4]);

// Print 1, 2, 3, 4 in arbitrary order.
for x in a.union(&b) {
    println!("{x}");
}

let union: HashSet<_> = a.union(&b).collect();
assert_eq!(union, [1, 2, 3, 4].iter().collect());

pub fn contains<Q>(&self, value: &Q) -> bool

where T: Borrow<Q>, Q: Hash + Eq + ?Sized,

Returns true if the set contains a value.

The value may be any borrowed form of the set’s value type, but Hash and Eq on the borrowed form must match those for the value type.

Examples

use std::collections::HashSet;

let set = HashSet::from([1, 2, 3]);
assert_eq!(set.contains(&1), true);
assert_eq!(set.contains(&4), false);