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

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

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

A hash map implemented with linear probing and 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 depends on the output quality of the system's random number generator when the seed 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 `default`, `with_hasher`, and `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<String, String>` in this example).
    let mut book_reviews = HashMap::new();

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

    // Check for a specific one.
    // When collections store owned values (String), they can still be
    // queried using references (&str).
    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)
        }
    }

    // Look up the value for a key (will panic if the key is not found).
    println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);

    // 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 key type 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]

#### `pub fn new() -> HashMap<K, V, RandomState>`[src]

Creates an empty `HashMap`.

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

# Examples

    use std::collections::HashMap;
    let mut map: HashMap<&str, i32> = HashMap::new();Run

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

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, i32> = HashMap::with_capacity(10);Run

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

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

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

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

Creates an empty `HashMap` with the specified capacity, using `hash_builder` 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: `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_capacity_and_hasher(10, s);
    map.insert(1, 2);Run

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`pub fn hasher(&self) -> &S`
1.9.0
[src]

Returns a reference to the map's `BuildHasher`.

# Examples

    use std::collections::HashMap;
    use std::collections::hash_map::RandomState;

    let hasher = RandomState::new();
    let map: HashMap<i32, i32> = HashMap::with_hasher(hasher);
    let hasher: &RandomState = map.hasher();Run

#### `pub fn capacity(&self) -> usize`[src]

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<i32, i32> = HashMap::with_capacity(100);
    assert!(map.capacity() >= 100);Run

#### `pub fn reserve(&mut self, additional: usize)`[src]

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, i32> = HashMap::new();
    map.reserve(10);Run

#### `pub fn try_reserve(
&mut self,
additional: usize
) -> Result<(), CollectionAllocErr>`[src]

🔬 This is a nightly-only experimental API. (`try_reserve` #48043)

new API

Tries to reserve capacity for at least `additional` more elements to be inserted in the given `HashMap<K,V>`. The collection may reserve more space to avoid frequent reallocations.

# Errors

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

# Examples

    #![feature(try_reserve)]
    use std::collections::HashMap;
    let mut map: HashMap<&str, isize> = HashMap::new();
    map.try_reserve(10).expect("why is the test harness OOMing on 10 bytes?");Run

#### `pub fn shrink_to_fit(&mut self)`[src]

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<i32, i32> = 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

#### `pub fn shrink_to(&mut self, min_capacity: usize)`[src]

🔬 This is a nightly-only experimental API. (`shrink_to` #56431)

new API

Shrinks the capacity of the map 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.

Panics if the current capacity is smaller than the supplied minimum capacity.

# Examples

    #![feature(shrink_to)]
    use std::collections::HashMap;

    let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    map.insert(1, 2);
    map.insert(3, 4);
    assert!(map.capacity() >= 100);
    map.shrink_to(10);
    assert!(map.capacity() >= 10);
    map.shrink_to(0);
    assert!(map.capacity() >= 2);Run

####
ⓘImportant traits for Keys<'a, K, V>

### Important traits for Keys<'a, K, V>

`impl<'a, K, V> Iterator for Keys<'a, K, V> type Item = &'a K;`
`pub fn keys(&self) -> Keys<K, V>`[src]

An iterator visiting all keys in arbitrary order. The 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

####
ⓘImportant traits for Values<'a, K, V>

### Important traits for Values<'a, K, V>

`impl<'a, K, V> Iterator for Values<'a, K, V> type Item = &'a V;`
`pub fn values(&self) -> Values<K, V>`[src]

An iterator visiting all values in arbitrary order. The 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

####
ⓘImportant traits for ValuesMut<'a, K, V>

### Important traits for ValuesMut<'a, K, V>

`impl<'a, K, V> Iterator for ValuesMut<'a, K, V> type Item = &'a mut V;`
`pub fn values_mut(&mut self) -> ValuesMut<K, V>`
1.10.0
[src]

An iterator visiting all values mutably in arbitrary order. The 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

####
ⓘImportant traits for Iter<'a, K, V>

### Important traits for Iter<'a, K, V>

`impl<'a, K, V> Iterator for Iter<'a, K, V> type Item = (&'a K, &'a V);`
`pub fn iter(&self) -> Iter<K, V>`[src]

An iterator visiting all key-value pairs in arbitrary order. The 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

####
ⓘImportant traits for IterMut<'a, K, V>

### Important traits for IterMut<'a, K, V>

`impl<'a, K, V> Iterator for IterMut<'a, K, V> type Item = (&'a K, &'a mut V);`
`pub fn iter_mut(&mut self) -> IterMut<K, V>`[src]

An iterator visiting all key-value pairs in arbitrary order, with mutable references to the values. The 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

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

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

#### `pub fn len(&self) -> usize`[src]

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

#### `pub fn is_empty(&self) -> bool`[src]

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

####
ⓘImportant traits for Drain<'a, K, V>

### Important traits for Drain<'a, K, V>

`impl<'a, K, V> Iterator for Drain<'a, K, V> type Item = (K, V);`
`pub fn drain(&mut self) -> Drain<K, V>`
1.6.0
[src]

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

#### `pub fn clear(&mut self)`[src]

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

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

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

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

🔬 This is a nightly-only experimental API. (`map_get_key_value` #49347)

Returns the key-value pair corresponding to the supplied key.

The supplied 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

    #![feature(map_get_key_value)]
    use std::collections::HashMap;

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

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

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

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

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

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

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

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

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

#### `pub fn remove_entry<Q: ?Sized>(&mut self, k: &Q) -> Option<(K, V)> where
K: Borrow<Q>,
Q: Hash + Eq,`
1.27.0
[src]

Removes a key from the map, returning the stored key and value 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_entry(&1), Some((1, "a")));
    assert_eq!(map.remove(&1), None);Run

#### `pub fn retain<F>(&mut self, f: F) where
F: FnMut(&K, &mut V) -> bool,`
1.18.0
[src]

Retains only the elements specified by the predicate.

In other words, remove all pairs `(k, v)` such that `f(&k,&mut v)` returns `false`.

# Examples

    use std::collections::HashMap;

    let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
    map.retain(|&k, _| k % 2 == 0);
    assert_eq!(map.len(), 4);Run

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

#### `pub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<K, V, S>`[src]

🔬 This is a nightly-only experimental API. (`hash_raw_entry` #56167)

Creates a raw entry builder for the HashMap.

Raw entries provide the lowest level of control for searching and manipulating a map. They must be manually initialized with a hash and then manually searched. After this, insertions into a vacant entry still require an owned key to be provided.

Raw entries are useful for such exotic situations as:

- Hash memoization
- Deferring the creation of an owned key until it is known to be required
- Using a search key that doesn't work with the Borrow trait
- Using custom comparison logic without newtype wrappers

Because raw entries provide much more low-level control, it's much easier to put the HashMap into an inconsistent state which, while memory-safe, will cause the map to produce seemingly random results. Higher-level and more foolproof APIs like `entry` should be preferred when possible.

In particular, the hash used to initialized the raw entry must still be consistent with the hash of the key that is ultimately stored in the entry. This is because implementations of HashMap may need to recompute hashes when resizing, at which point only the keys are available.

Raw entries give mutable access to the keys. This must not be used to modify how the key would compare or hash, as the map will not re-evaluate where the key should go, meaning the keys may become "lost" if their location does not reflect their state. For instance, if you change a key so that the map now contains keys which compare equal, search may start acting erratically, with two keys randomly masking each other. Implementations are free to assume this doesn't happen (within the limits of memory-safety).

#### `pub fn raw_entry(&self) -> RawEntryBuilder<K, V, S>`[src]

🔬 This is a nightly-only experimental API. (`hash_raw_entry` #56167)

Creates a raw immutable entry builder for the HashMap.

Raw entries provide the lowest level of control for searching and manipulating a map. They must be manually initialized with a hash and then manually searched.

This is useful for

- Hash memoization
- Using a search key that doesn't work with the Borrow trait
- Using custom comparison logic without newtype wrappers

Unless you are in such a situation, higher-level and more foolproof APIs like `get` should be preferred.

Immutable raw entries have very limited use; you might instead want `raw_entry_mut`.

## Trait Implementations

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

#### `fn eq(&self, other: &HashMap<K, V, S>) -> bool`[src]

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

#### `
#[must_use]
fn ne(&self, other: &Rhs) -> bool`[src]

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`[src]

Formats the value using the given formatter. Read more

### `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.

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn index(&self, key: &Q) -> &V`[src]

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

# Panics

Panics if the key is not present in the `HashMap`.

### `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?

####
ⓘImportant traits for Iter<'a, K, V>

### Important traits for Iter<'a, K, V>

`impl<'a, K, V> Iterator for Iter<'a, K, V> type Item = (&'a K, &'a V);`
`fn into_iter(self) -> Iter<'a, K, V>`[src]

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?

####
ⓘImportant traits for IterMut<'a, K, V>

### Important traits for IterMut<'a, K, V>

`impl<'a, K, V> Iterator for IterMut<'a, K, V> type Item = (&'a K, &'a mut V);`
`fn into_iter(self) -> IterMut<'a, K, V>`[src]

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?

####
ⓘImportant traits for IntoIter<K, V>

### Important traits for IntoIter<K, V>

`impl<K, V> Iterator for IntoIter<K, V> type Item = (K, V);`
`fn into_iter(self) -> IntoIter<K, V>`[src]

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, i32)> = map.into_iter().collect();Run

### `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)`[src]

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)`[src]

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

### `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>`[src]

Creates a value from an iterator. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

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

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

## Auto Trait Implementations

### `impl<K, V, S> Send for HashMap<K, V, S> where
K: Send,
S: Send,
V: Send,`

### `impl<K, V, S> Sync for HashMap<K, V, S> where
K: Sync,
S: Sync,
V: Sync,`

## Blanket Implementations

### `impl<T> From for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<I> IntoIterator for I where
I: Iterator,` [src]

#### `type Item = <I as Iterator>::Item`

The type of the elements being iterated over.

#### `type IntoIter = I`

Which kind of iterator are we turning this into?

#### `fn into_iter(self) -> I`[src]

Creates an iterator from a value. Read more

### `impl<T, U> TryFrom for T where
T: From<U>,` [src]

#### `type Error = !`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

### `impl<T, U> TryInto for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

### `impl<T, U> Into for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> Borrow for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn get_type_id(&self) -> TypeId`[src]

🔬 This is a nightly-only experimental API. (`get_type_id` #27745)

this method will likely be replaced by an associated static

Gets the `TypeId` of `self`. Read more

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

#### `fn to_owned(&self) -> T`[src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

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