# Struct std::collections::hash_set::HashSet [−] [src]

    pub struct HashSet<T, S = RandomState> {
        // some fields omitted
    }

An implementation of a hash set using the underlying representation of 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.

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

# Examples

fn main() { use std::collections::HashSet; // Type inference lets us omit an explicit type signature (which // would be `HashSet<&str>` in this example). let mut books = HashSet::new(); // Add some books. books.insert("A Dance With Dragons"); books.insert("To Kill a Mockingbird"); books.insert("The Odyssey"); books.insert("The Great Gatsby"); // 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); } }

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

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

    // 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`, this will in the future be implied by `Eq`.

fn main() { use std::collections::HashSet; #[derive(Hash, Eq, PartialEq, Debug)] struct Viking<'a> { name: &'a str, power: usize, } let mut vikings = HashSet::new(); vikings.insert(Viking { name: "Einar", power: 9 }); vikings.insert(Viking { name: "Einar", power: 9 }); vikings.insert(Viking { name: "Olaf", power: 4 }); vikings.insert(Viking { name: "Harald", power: 8 }); // Use derived implementation to print the vikings. for x in &vikings { println!("{:?}", x); } }

    use std::collections::HashSet;
    #[derive(Hash, Eq, PartialEq, Debug)]
    struct Viking<'a> {
        name: &'a str,
        power: usize,
    }

    let mut vikings = HashSet::new();

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

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

## Methods

### `impl<T: Hash + Eq> HashSet<T, RandomState>`

#### `fn new() -> HashSet<T, RandomState>`

Creates an empty HashSet.

# Examples

fn main() { use std::collections::HashSet; let mut set: HashSet<i32> = HashSet::new(); }

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

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

Creates an empty HashSet with space for at least `n` elements in the hash table.

# Examples

fn main() { use std::collections::HashSet; let mut set: HashSet<i32> = HashSet::with_capacity(10); }

    use std::collections::HashSet;
    let mut set: HashSet<i32> = HashSet::with_capacity(10);

### `impl<T, S> HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

#### `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 `HashSet`s 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.

# Examples

fn main() { use std::collections::HashSet; use std::collections::hash_map::RandomState; let s = RandomState::new(); let mut set = HashSet::with_hasher(s); set.insert(2); }

    use std::collections::HashSet;
    use std::collections::hash_map::RandomState;

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

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

Creates an empty HashSet with space for at least `capacity` elements in the hash table, using `hasher` to hash the keys.

Warning: `hasher` is normally randomly generated, and is designed to allow `HashSet`s 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.

# Examples

fn main() { use std::collections::HashSet; use std::collections::hash_map::RandomState; let s = RandomState::new(); let mut set = HashSet::with_capacity_and_hasher(10, s); set.insert(1); }

    use std::collections::HashSet;
    use std::collections::hash_map::RandomState;

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

#### `fn with_hash_state(hash_state: S) -> HashSet<T, S>`

Deprecated since 1.7.0

: renamed to with_hasher

Deprecated, renamed to `with_hasher`

#### `fn with_capacity_and_hash_state(capacity: usize, hash_state: S) -> HashSet<T, S>`

Deprecated since 1.7.0

: renamed to with_capacity_and_hasher

Deprecated, renamed to `with_capacity_and_hasher`

#### `fn capacity(&self) -> usize`

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

# Examples

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

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

#### `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 avoid frequent reallocations.

# Panics

Panics if the new allocation size overflows `usize`.

# Examples

fn main() { use std::collections::HashSet; let mut set: HashSet<i32> = HashSet::new(); set.reserve(10); }

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

#### `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

fn main() { 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); }

    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);

#### `fn iter(&self) -> Iter<T>`

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

# Examples

fn main() { 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); } }

    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);
    }

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

Visit the values representing the difference.

# Examples

fn main() { use std::collections::HashSet; let a: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect(); // Can be seen as `a - b`. for x in a.difference(&b) { println!("{}", x); // Print 1 } let diff: HashSet<_> = a.difference(&b).cloned().collect(); assert_eq!(diff, [1].iter().cloned().collect()); // Note that difference is not symmetric, // and `b - a` means something else: let diff: HashSet<_> = b.difference(&a).cloned().collect(); assert_eq!(diff, [4].iter().cloned().collect()); }

    use std::collections::HashSet;
    let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();

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

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

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

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

Visit the values representing the symmetric difference.

# Examples

fn main() { use std::collections::HashSet; let a: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect(); // Print 1, 4 in arbitrary order. for x in a.symmetric_difference(&b) { println!("{}", x); } let diff1: HashSet<_> = a.symmetric_difference(&b).cloned().collect(); let diff2: HashSet<_> = b.symmetric_difference(&a).cloned().collect(); assert_eq!(diff1, diff2); assert_eq!(diff1, [1, 4].iter().cloned().collect()); }

    use std::collections::HashSet;
    let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();

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

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

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

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

Visit the values representing the intersection.

# Examples

fn main() { use std::collections::HashSet; let a: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect(); // Print 2, 3 in arbitrary order. for x in a.intersection(&b) { println!("{}", x); } let intersection: HashSet<_> = a.intersection(&b).cloned().collect(); assert_eq!(intersection, [2, 3].iter().cloned().collect()); }

    use std::collections::HashSet;
    let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();

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

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

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

Visit the values representing the union.

# Examples

fn main() { use std::collections::HashSet; let a: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect(); // Print 1, 2, 3, 4 in arbitrary order. for x in a.union(&b) { println!("{}", x); } let union: HashSet<_> = a.union(&b).cloned().collect(); assert_eq!(union, [1, 2, 3, 4].iter().cloned().collect()); }

    use std::collections::HashSet;
    let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let b: HashSet<_> = [4, 2, 3, 4].iter().cloned().collect();

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

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

#### `fn len(&self) -> usize`

Returns the number of elements in the set.

# Examples

fn main() { use std::collections::HashSet; let mut v = HashSet::new(); assert_eq!(v.len(), 0); v.insert(1); assert_eq!(v.len(), 1); }

    use std::collections::HashSet;

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

#### `fn is_empty(&self) -> bool`

Returns true if the set contains no elements.

# Examples

fn main() { use std::collections::HashSet; let mut v = HashSet::new(); assert!(v.is_empty()); v.insert(1); assert!(!v.is_empty()); }

    use std::collections::HashSet;

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

#### `fn drain(&mut self) -> Drain<T>`

Clears the set, returning all elements in an iterator.

#### `fn clear(&mut self)`

Clears the set, removing all values.

# Examples

fn main() { use std::collections::HashSet; let mut v = HashSet::new(); v.insert(1); v.clear(); assert!(v.is_empty()); }

    use std::collections::HashSet;

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

#### `fn contains<Q: ?Sized>(&self, value: &Q) -> bool where T: Borrow<Q>, Q: Hash + Eq`

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

fn main() { use std::collections::HashSet; let set: HashSet<_> = [1, 2, 3].iter().cloned().collect(); assert_eq!(set.contains(&1), true); assert_eq!(set.contains(&4), false); }

    use std::collections::HashSet;

    let set: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    assert_eq!(set.contains(&1), true);
    assert_eq!(set.contains(&4), false);

#### `fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T> where T: Borrow<Q>, Q: Hash + Eq`

Unstable (`set_recovery` #28050)

Returns a reference to the value in the set, if any, that is equal to the given 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.

#### `fn is_disjoint(&self, other: &HashSet<T, S>) -> bool`

Returns `true` if the set has no elements in common with `other`. This is equivalent to checking for an empty intersection.

# Examples

fn main() { use std::collections::HashSet; let a: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let mut b = HashSet::new(); assert_eq!(a.is_disjoint(&b), true); b.insert(4); assert_eq!(a.is_disjoint(&b), true); b.insert(1); assert_eq!(a.is_disjoint(&b), false); }

    use std::collections::HashSet;

    let a: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let mut b = HashSet::new();

    assert_eq!(a.is_disjoint(&b), true);
    b.insert(4);
    assert_eq!(a.is_disjoint(&b), true);
    b.insert(1);
    assert_eq!(a.is_disjoint(&b), false);

#### `fn is_subset(&self, other: &HashSet<T, S>) -> bool`

Returns `true` if the set is a subset of another.

# Examples

fn main() { use std::collections::HashSet; let sup: HashSet<_> = [1, 2, 3].iter().cloned().collect(); let mut set = HashSet::new(); assert_eq!(set.is_subset(&sup), true); set.insert(2); assert_eq!(set.is_subset(&sup), true); set.insert(4); assert_eq!(set.is_subset(&sup), false); }

    use std::collections::HashSet;

    let sup: HashSet<_> = [1, 2, 3].iter().cloned().collect();
    let mut set = HashSet::new();

    assert_eq!(set.is_subset(&sup), true);
    set.insert(2);
    assert_eq!(set.is_subset(&sup), true);
    set.insert(4);
    assert_eq!(set.is_subset(&sup), false);

#### `fn is_superset(&self, other: &HashSet<T, S>) -> bool`

Returns `true` if the set is a superset of another.

# Examples

fn main() { use std::collections::HashSet; let sub: HashSet<_> = [1, 2].iter().cloned().collect(); let mut set = HashSet::new(); assert_eq!(set.is_superset(&sub), false); set.insert(0); set.insert(1); assert_eq!(set.is_superset(&sub), false); set.insert(2); assert_eq!(set.is_superset(&sub), true); }

    use std::collections::HashSet;

    let sub: HashSet<_> = [1, 2].iter().cloned().collect();
    let mut set = HashSet::new();

    assert_eq!(set.is_superset(&sub), false);

    set.insert(0);
    set.insert(1);
    assert_eq!(set.is_superset(&sub), false);

    set.insert(2);
    assert_eq!(set.is_superset(&sub), true);

#### `fn insert(&mut self, value: T) -> bool`

Adds a value to the set.

If the set did not have a value present, `true` is returned.

If the set did have this key present, `false` is returned.

# Examples

fn main() { use std::collections::HashSet; let mut set = HashSet::new(); assert_eq!(set.insert(2), true); assert_eq!(set.insert(2), false); assert_eq!(set.len(), 1); }

    use std::collections::HashSet;

    let mut set = HashSet::new();

    assert_eq!(set.insert(2), true);
    assert_eq!(set.insert(2), false);
    assert_eq!(set.len(), 1);

#### `fn replace(&mut self, value: T) -> Option<T>`

Unstable (`set_recovery` #28050)

Adds a value to the set, replacing the existing value, if any, that is equal to the given one. Returns the replaced value.

#### `fn remove<Q: ?Sized>(&mut self, value: &Q) -> bool where T: Borrow<Q>, Q: Hash + Eq`

Removes a value from the set. Returns `true` if the value was present in the set.

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

fn main() { use std::collections::HashSet; let mut set = HashSet::new(); set.insert(2); assert_eq!(set.remove(&2), true); assert_eq!(set.remove(&2), false); }

    use std::collections::HashSet;

    let mut set = HashSet::new();

    set.insert(2);
    assert_eq!(set.remove(&2), true);
    assert_eq!(set.remove(&2), false);

#### `fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T> where T: Borrow<Q>, Q: Hash + Eq`

Unstable (`set_recovery` #28050)

Removes and returns the value in the set, if any, that is equal to the given one.

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.

## Trait Implementations

### `impl<T, S> PartialEq for HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

#### `fn eq(&self, other: &HashSet<T, S>) -> bool`

#### `fn ne(&self, other: &Rhs) -> bool`

### `impl<T, S> Eq for HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

### `impl<T, S> Debug for HashSet<T, S> where T: Eq + Hash + Debug, S: BuildHasher`

#### `fn fmt(&self, f: &mut Formatter) -> Result`

### `impl<T, S> FromIterator<T> for HashSet<T, S> where T: Eq + Hash, S: BuildHasher + Default`

#### `fn from_iter<I: IntoIterator<Item=T>>(iterable: I) -> HashSet<T, S>`

### `impl<T, S> Extend<T> for HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

#### `fn extend<I: IntoIterator<Item=T>>(&mut self, iter: I)`

### `impl<'a, T, S> Extend<&'a T> for HashSet<T, S> where T: 'a + Eq + Hash + Copy, S: BuildHasher`

#### `fn extend<I: IntoIterator<Item=&'a T>>(&mut self, iter: I)`

### `impl<T, S> Default for HashSet<T, S> where T: Eq + Hash, S: BuildHasher + Default`

#### `fn default() -> HashSet<T, S>`

### `impl<'a, 'b, T, S> BitOr<&'b HashSet<T, S>> for &'a HashSet<T, S> where T: Eq + Hash + Clone, S: BuildHasher + Default`

#### `type Output = HashSet<T, S>`

#### `fn bitor(self, rhs: &HashSet<T, S>) -> HashSet<T, S>`

### `impl<'a, 'b, T, S> BitAnd<&'b HashSet<T, S>> for &'a HashSet<T, S> where T: Eq + Hash + Clone, S: BuildHasher + Default`

#### `type Output = HashSet<T, S>`

#### `fn bitand(self, rhs: &HashSet<T, S>) -> HashSet<T, S>`

### `impl<'a, 'b, T, S> BitXor<&'b HashSet<T, S>> for &'a HashSet<T, S> where T: Eq + Hash + Clone, S: BuildHasher + Default`

#### `type Output = HashSet<T, S>`

#### `fn bitxor(self, rhs: &HashSet<T, S>) -> HashSet<T, S>`

### `impl<'a, 'b, T, S> Sub<&'b HashSet<T, S>> for &'a HashSet<T, S> where T: Eq + Hash + Clone, S: BuildHasher + Default`

#### `type Output = HashSet<T, S>`

#### `fn sub(self, rhs: &HashSet<T, S>) -> HashSet<T, S>`

### `impl<'a, T, S> IntoIterator for &'a HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

#### `type Item = &'a T`

#### `type IntoIter = Iter<'a, T>`

#### `fn into_iter(self) -> Iter<'a, T>`

### `impl<T, S> IntoIterator for HashSet<T, S> where T: Eq + Hash, S: BuildHasher`

#### `type Item = T`

#### `type IntoIter = IntoIter<T>`

#### `fn into_iter(self) -> IntoIter<T>`

### Derived Implementations

### `impl<T: Clone, S: Clone> Clone for HashSet<T, S>`

#### `fn clone(&self) -> HashSet<T, S>`

#### `fn clone_from(&mut self, source: &Self)`

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize`)