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

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

    pub struct HashMap<K, V, S = RandomState> { /* fields omitted */ }

A hash map implementation which uses linear probing with Robin Hood bucket stealing.

By default, `HashMap` uses a hashing algorithm selected to provide resistance against HashDoS attacks. The algorithm is randomly seeded, and a reasonable best-effort is made to generate this seed from a high quality, secure source of randomness provided by the host without blocking the program. Because of this, the randomness of the seed is dependant on the quality of the system's random number generator at the time it is created. In particular, seeds generated when the system's entropy pool is abnormally low such as during system boot may be of a lower quality.

The default hashing algorithm is currently SipHash 1-3, though this is subject to change at any point in the future. While its performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings, though those algorithms will typically not protect against attacks such as HashDoS.

The hashing algorithm can be replaced on a per-`HashMap` basis using the `HashMap::default`, `HashMap::with_hasher`, and `HashMap::with_capacity_and_hasher` methods. Many alternative algorithms are available on crates.io, such as the `fnv` crate.

It is required that the keys implement the `Eq` and `Hash` traits, although 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 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.

Relevant papers/articles:

1. Pedro Celis. "Robin Hood Hashing"
2. Emmanuel Goossaert. "Robin Hood hashing"
3. Emmanuel Goossaert. "Robin Hood hashing: backward shift deletion"

# Examples

    use std::collections::HashMap;

    // type inference lets us omit an explicit type signature (which
    // would be `HashMap<&str, &str>` in this example).
    let mut book_reviews = HashMap::new();

    // review some books.
    book_reviews.insert("Adventures of Huckleberry Finn",    "My favorite book.");
    book_reviews.insert("Grimms' Fairy Tales",               "Masterpiece.");
    book_reviews.insert("Pride and Prejudice",               "Very enjoyable.");
    book_reviews.insert("The Adventures of Sherlock Holmes", "Eye lyked it alot.");

    // check for a specific one.
    if !book_reviews.contains_key("Les Misérables") {
        println!("We've got {} reviews, but Les Misérables ain't one.",
                 book_reviews.len());
    }

    // oops, this review has a lot of spelling mistakes, let's delete it.
    book_reviews.remove("The Adventures of Sherlock Holmes");

    // look up the values associated with some keys.
    let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
    for book in &to_find {
        match book_reviews.get(book) {
            Some(review) => println!("{}: {}", book, review),
            None => println!("{} is unreviewed.", book)
        }
    }

    // iterate over everything.
    for (book, review) in &book_reviews {
        println!("{}: \"{}\"", book, review);
    }Run

`HashMap` also implements an `Entry API`, which allows for more complex methods of getting, setting, updating and removing keys and their values:

    use std::collections::HashMap;

    // type inference lets us omit an explicit type signature (which
    // would be `HashMap<&str, u8>` in this example).
    let mut player_stats = HashMap::new();

    fn random_stat_buff() -> u8 {
        // could actually return some random value here - let's just return
        // some fixed value for now
        42
    }

    // insert a key only if it doesn't already exist
    player_stats.entry("health").or_insert(100);

    // insert a key using a function that provides a new value only if it
    // doesn't already exist
    player_stats.entry("defence").or_insert_with(random_stat_buff);

    // update a key, guarding against the key possibly not being set
    let stat = player_stats.entry("attack").or_insert(100);
    *stat += random_stat_buff();Run

The easiest way to use `HashMap` with a custom type as key is to derive `Eq` and `Hash`. We must also derive `PartialEq`.

    use std::collections::HashMap;

    #[derive(Hash, Eq, PartialEq, Debug)]
    struct Viking {
        name: String,
        country: String,
    }

    impl Viking {
        /// Create a new Viking.
        fn new(name: &str, country: &str) -> Viking {
            Viking { name: name.to_string(), country: country.to_string() }
        }
    }

    // Use a HashMap to store the vikings' health points.
    let mut vikings = HashMap::new();

    vikings.insert(Viking::new("Einar", "Norway"), 25);
    vikings.insert(Viking::new("Olaf", "Denmark"), 24);
    vikings.insert(Viking::new("Harald", "Iceland"), 12);

    // Use derived implementation to print the status of the vikings.
    for (viking, health) in &vikings {
        println!("{:?} has {} hp", viking, health);
    }Run

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

    use std::collections::HashMap;

    fn main() {
        let timber_resources: HashMap<&str, i32> =
        [("Norway", 100),
         ("Denmark", 50),
         ("Iceland", 10)]
         .iter().cloned().collect();
        // use the values stored in map
    }Run

## Methods

### `impl<K: Hash + Eq, V> HashMap<K, V, RandomState>`
[src]

#### `fn new() -> HashMap<K, V, RandomState>`

Creates an empty `HashMap`.

# Examples

    use std::collections::HashMap;
    let mut map: HashMap<&str, isize> = HashMap::new();Run

#### `fn with_capacity(capacity: usize) -> HashMap<K, V, RandomState>`

Creates an empty `HashMap` with the specified capacity.

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

# Examples

    use std::collections::HashMap;
    let mut map: HashMap<&str, isize> = HashMap::with_capacity(10);Run

### `impl<K, V, S> HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher`
[src]

#### `fn with_hasher(hash_builder: S) -> HashMap<K, V, S>`
1.7.0

Creates an empty `HashMap` which will use the given hash builder to hash keys.

The created map has the default initial capacity.

Warning: `hash_builder` is normally randomly generated, and is designed to allow HashMaps 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::HashMap;
    use std::collections::hash_map::RandomState;

    let s = RandomState::new();
    let mut map = HashMap::with_hasher(s);
    map.insert(1, 2);Run

#### `fn with_capacity_and_hasher(capacity: usize,
hash_builder: S)
-> HashMap<K, V, S>`
1.7.0

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

The hash map will be able to hold at least `capacity` elements without reallocating. If `capacity` is 0, the hash map will not allocate. Warning: `hasher` is normally randomly generated, and is designed to allow HashMaps 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::HashMap;
    use std::collections::hash_map::RandomState;

    let s = RandomState::new();
    let mut map = HashMap::with_capacity_and_hasher(10, s);
    map.insert(1, 2);Run

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

Returns a reference to the map's hasher.

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

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

This number is a lower bound; the `HashMap<K, V>` might be able to hold more, but is guaranteed to be able to hold at least this many.

# Examples

    use std::collections::HashMap;
    let map: HashMap<isize, isize> = HashMap::with_capacity(100);
    assert!(map.capacity() >= 100);Run

#### `fn reserve(&mut self, additional: usize)`

Reserves capacity for at least `additional` more elements to be inserted in the `HashMap`. The collection may reserve more space to avoid frequent reallocations.

# Panics

Panics if the new allocation size overflows `usize`.

# Examples

    use std::collections::HashMap;
    let mut map: HashMap<&str, isize> = HashMap::new();
    map.reserve(10);Run

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

Shrinks the capacity of the map 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::HashMap;

    let mut map: HashMap<isize, isize> = HashMap::with_capacity(100);
    map.insert(1, 2);
    map.insert(3, 4);
    assert!(map.capacity() >= 100);
    map.shrink_to_fit();
    assert!(map.capacity() >= 2);Run

#### `fn keys(&self) -> Keys<K, V>`

An iterator visiting all keys in arbitrary order. Iterator element type is `&'a K`.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    for key in map.keys() {
        println!("{}", key);
    }Run

#### `fn values(&self) -> Values<K, V>`

An iterator visiting all values in arbitrary order. Iterator element type is `&'a V`.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    for val in map.values() {
        println!("{}", val);
    }Run

#### `fn values_mut(&mut self) -> ValuesMut<K, V>`
1.10.0

An iterator visiting all values mutably in arbitrary order. Iterator element type is `&'a mut V`.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();

    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    for val in map.values_mut() {
        *val = *val + 10;
    }

    for val in map.values() {
        println!("{}", val);
    }Run

#### `fn iter(&self) -> Iter<K, V>`

An iterator visiting all key-value pairs in arbitrary order. Iterator element type is `(&'a K, &'a V)`.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    for (key, val) in map.iter() {
        println!("key: {} val: {}", key, val);
    }Run

#### `fn iter_mut(&mut self) -> IterMut<K, V>`

An iterator visiting all key-value pairs in arbitrary order, with mutable references to the values. Iterator element type is `(&'a K, &'a mut V)`.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    // Update all values
    for (_, val) in map.iter_mut() {
        *val *= 2;
    }

    for (key, val) in &map {
        println!("key: {} val: {}", key, val);
    }Run

#### `fn entry(&mut self, key: K) -> Entry<K, V>`

Gets the given key's corresponding entry in the map for in-place manipulation.

# Examples

    use std::collections::HashMap;

    let mut letters = HashMap::new();

    for ch in "a short treatise on fungi".chars() {
        let counter = letters.entry(ch).or_insert(0);
        *counter += 1;
    }

    assert_eq!(letters[&'s'], 2);
    assert_eq!(letters[&'t'], 3);
    assert_eq!(letters[&'u'], 1);
    assert_eq!(letters.get(&'y'), None);Run

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

Returns the number of elements in the map.

# Examples

    use std::collections::HashMap;

    let mut a = HashMap::new();
    assert_eq!(a.len(), 0);
    a.insert(1, "a");
    assert_eq!(a.len(), 1);Run

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

Returns true if the map contains no elements.

# Examples

    use std::collections::HashMap;

    let mut a = HashMap::new();
    assert!(a.is_empty());
    a.insert(1, "a");
    assert!(!a.is_empty());Run

#### `fn drain(&mut self) -> Drain<K, V>`
1.6.0

Clears the map, returning all key-value pairs as an iterator. Keeps the allocated memory for reuse.

# Examples

    use std::collections::HashMap;

    let mut a = HashMap::new();
    a.insert(1, "a");
    a.insert(2, "b");

    for (k, v) in a.drain().take(1) {
        assert!(k == 1 || k == 2);
        assert!(v == "a" || v == "b");
    }

    assert!(a.is_empty());Run

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

Clears the map, removing all key-value pairs. Keeps the allocated memory for reuse.

# Examples

    use std::collections::HashMap;

    let mut a = HashMap::new();
    a.insert(1, "a");
    a.clear();
    assert!(a.is_empty());Run

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

Returns a reference to the value corresponding to the key.

The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.get(&1), Some(&"a"));
    assert_eq!(map.get(&2), None);Run

#### `fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool where K: Borrow<Q>, Q: Hash + Eq`

Returns true if the map contains a value for the specified key.

The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.contains_key(&1), true);
    assert_eq!(map.contains_key(&2), false);Run

#### `fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V> where K: Borrow<Q>, Q: Hash + Eq`

Returns a mutable reference to the value corresponding to the key.

The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    if let Some(x) = map.get_mut(&1) {
        *x = "b";
    }
    assert_eq!(map[&1], "b");Run

#### `fn insert(&mut self, k: K, v: V) -> Option<V>`

Inserts a key-value pair into the map.

If the map did not have this key present, `None` is returned.

If the map did have this key present, the value is updated, and the old value is returned. The key is not updated, though; this matters for types that can be `==` without being identical. See the module-level documentation for more.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    assert_eq!(map.insert(37, "a"), None);
    assert_eq!(map.is_empty(), false);

    map.insert(37, "b");
    assert_eq!(map.insert(37, "c"), Some("b"));
    assert_eq!(map[&37], "c");Run

#### `fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V> where K: Borrow<Q>, Q: Hash + Eq`

Removes a key from the map, returning the value at the key if the key was previously in the map.

The key may be any borrowed form of the map's key type, but `Hash` and `Eq` on the borrowed form must match those for the key type.

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert(1, "a");
    assert_eq!(map.remove(&1), Some("a"));
    assert_eq!(map.remove(&1), None);Run

## Trait Implementations

### `impl<K: Clone, V: Clone, S: Clone> Clone for HashMap<K, V, S>`
[src]

#### `fn clone(&self) -> HashMap<K, V, S>`

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

### `impl<K, V, S> PartialEq for HashMap<K, V, S> where K: Eq + Hash, V: PartialEq, S: BuildHasher`
[src]

#### `fn eq(&self, other: &HashMap<K, V, 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<K, V, S> Eq for HashMap<K, V, S> where K: Eq + Hash, V: Eq, S: BuildHasher`
[src]

### `impl<K, V, S> Debug for HashMap<K, V, S> where K: Eq + Hash + Debug, V: Debug, S: BuildHasher`
[src]

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

Formats the value using the given formatter.

### `impl<K, V, S> Default for HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher + Default`
[src]

#### `fn default() -> HashMap<K, V, S>`

Creates an empty `HashMap<K, V, S>`, with the `Default` value for the hasher.

### `impl<'a, K, Q: ?Sized, V, S> Index<&'a Q> for HashMap<K, V, S> where K: Eq + Hash + Borrow<Q>, Q: Eq + Hash, S: BuildHasher`
[src]

#### `type Output = V`

The returned type after indexing

#### `fn index(&self, index: &Q) -> &V`

The method for the indexing (`container[index]`) operation

### `impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher`
[src]

#### `type Item = (&'a K, &'a V)`

The type of the elements being iterated over.

#### `type IntoIter = Iter<'a, K, V>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> Iter<'a, K, V>`

Creates an iterator from a value. Read more

### `impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher`
[src]

#### `type Item = (&'a K, &'a mut V)`

The type of the elements being iterated over.

#### `type IntoIter = IterMut<'a, K, V>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> IterMut<'a, K, V>`

Creates an iterator from a value. Read more

### `impl<K, V, S> IntoIterator for HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher`
[src]

#### `type Item = (K, V)`

The type of the elements being iterated over.

#### `type IntoIter = IntoIter<K, V>`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> IntoIter<K, V>`

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

# Examples

    use std::collections::HashMap;

    let mut map = HashMap::new();
    map.insert("a", 1);
    map.insert("b", 2);
    map.insert("c", 3);

    // Not possible with .iter()
    let vec: Vec<(&str, isize)> = map.into_iter().collect();Run

### `impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher + Default`
[src]

#### `fn from_iter<T: IntoIterator<Item=(K, V)>>(iter: T) -> HashMap<K, V, S>`

Creates a value from an iterator. Read more

### `impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher`
[src]

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

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

### `impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S> where K: Eq + Hash + Copy, V: Copy, S: BuildHasher`
1.4.0
[src]

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

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

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