std::collections::HashSet - Rust
Description: API documentation for the Rust `HashSet` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, HashSet

# Struct std::collections::HashSet 1.0.0 [−] [src]

    pub struct HashSet<T, S = RandomState> { /* 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

    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);
    }Run

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`.

    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);
    }Run

HashSet with fixed list of elements can be initialized from an array:

    use std::collections::HashSet;

    fn main() {
        let viking_names: HashSet<&str> =
            [ "Einar", "Olaf", "Harald" ].iter().cloned().collect();
        // use the values stored in the set
    }Run

## Methods

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

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

Creates an empty HashSet.

# Examples

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

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

Creates an empty `HashSet` with the specified capacity.

The hash set will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash set will not allocate.

# Examples

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

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

#### `fn with_hasher(hasher: S) -> HashSet<T, S>`
1.7.0

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

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

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

#### `fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashSet<T, S>`
1.7.0

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

The hash set will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash set will not allocate.

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

    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);Run

#### `fn hasher(&self) -> &S`
1.9.0

Returns a reference to the set's hasher.

#### `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);Run

#### `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

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

#### `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);Run

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

An iterator visiting all elements in arbitrary order. 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);
    }Run

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

Visit the values representing the difference.

# Examples

    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());Run

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

Visit the values representing the symmetric difference.

# Examples

    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());Run

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

Visit the values representing the intersection.

# Examples

    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());Run

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

Visit the values representing the union.

# Examples

    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());Run

#### `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);Run

#### `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());Run

#### `fn drain(&mut self) -> Drain<T>`
1.6.0

Clears the set, returning all elements in an iterator.

#### `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());Run

#### `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

    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);Run

#### `fn get<Q: ?Sized>(&self, value: &Q) -> Option<&T> where T: Borrow<Q>, Q: Hash + Eq`
1.9.0

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

    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);Run

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

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

# Examples

    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);Run

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

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

# Examples

    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);Run

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

Adds a value to the set.

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

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

# Examples

    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);Run

#### `fn replace(&mut self, value: T) -> Option<T>`
1.9.0

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

    use std::collections::HashSet;

    let mut set = HashSet::new();

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

#### `fn take<Q: ?Sized>(&mut self, value: &Q) -> Option<T> where T: Borrow<Q>, Q: Hash + Eq`
1.9.0

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: Clone, S: Clone> Clone for HashSet<T, S>`
[src]

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

This method tests for `self` and `other` values to be equal, and is used by `==`. Read more

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

This method tests for `!=`.

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

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

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

Formats the value using the given formatter.

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

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

Creates a value from an iterator. Read more

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

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

Extends a collection with the contents of an iterator. Read more

### `impl<'a, T, S> Extend<&'a T> for HashSet<T, S> where T: 'a + Eq + Hash + Copy,
S: BuildHasher`
1.4.0
[src]

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

Extends a collection with the contents of an iterator. Read more

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

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

Creates an empty `HashSet<T, S>` with the `Default` value for the hasher.

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

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

The resulting type after applying the `|` operator

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

Returns the union of `self` and `rhs` as a new `HashSet<T, S>`.

# Examples

    use std::collections::HashSet;

    let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();

    let set = &a | &b;

    let mut i = 0;
    let expected = [1, 2, 3, 4, 5];
    for x in &set {
        assert!(expected.contains(x));
        i += 1;
    }
    assert_eq!(i, expected.len());Run

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

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

The resulting type after applying the `&` operator

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

Returns the intersection of `self` and `rhs` as a new `HashSet<T, S>`.

# Examples

    use std::collections::HashSet;

    let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    let b: HashSet<_> = vec![2, 3, 4].into_iter().collect();

    let set = &a & &b;

    let mut i = 0;
    let expected = [2, 3];
    for x in &set {
        assert!(expected.contains(x));
        i += 1;
    }
    assert_eq!(i, expected.len());Run

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

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

The resulting type after applying the `^` operator

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

Returns the symmetric difference of `self` and `rhs` as a new `HashSet<T, S>`.

# Examples

    use std::collections::HashSet;

    let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();

    let set = &a ^ &b;

    let mut i = 0;
    let expected = [1, 2, 4, 5];
    for x in &set {
        assert!(expected.contains(x));
        i += 1;
    }
    assert_eq!(i, expected.len());Run

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

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

The resulting type after applying the `-` operator

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

Returns the difference of `self` and `rhs` as a new `HashSet<T, S>`.

# Examples

    use std::collections::HashSet;

    let a: HashSet<_> = vec![1, 2, 3].into_iter().collect();
    let b: HashSet<_> = vec![3, 4, 5].into_iter().collect();

    let set = &a - &b;

    let mut i = 0;
    let expected = [1, 2];
    for x in &set {
        assert!(expected.contains(x));
        i += 1;
    }
    assert_eq!(i, expected.len());Run

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

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

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates an iterator from a value. Read more

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

#### `type Item = T`

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates a consuming iterator, that is, one that moves each value out of the set in arbitrary order. The set cannot be used after calling this.

# Examples

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

    // Not possible to collect to a Vec<String> with a regular `.iter()`.
    let v: Vec<String> = set.into_iter().collect();

    // Will print in an arbitrary order.
    for x in &v {
        println!("{}", x);
    }Run

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