Struct bitflags::__core::collections::hash_map::HashMap

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

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

The hashes are all keyed by the thread-local random number generator on creation by default. This means that the ordering of the keys is randomized, but makes the tables more resistant to denial-of-service attacks (Hash DoS). This behavior can be overridden with one of the constructors.

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);
}

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();

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);
}

Methods

impl<K, V> HashMap<K, V, RandomState> where K: Eq + Hash

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

Creates an empty HashMap.

Examples

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

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

Creates an empty hash map with the given initial capacity.

Examples

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

impl<K, V, S> HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher

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

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);

fn with_hash_state(hash_state: S) -> HashMap<K, V, S>

Deprecated since 1.7.0

: renamed to with_hasher

Deprecated, renamed to with_hasher

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

Creates an empty HashMap with space for at least capacity elements, using hasher to hash the keys.

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);

fn with_capacity_and_hash_state(capacity: usize, hash_state: S) -> HashMap<K, V, S>

Deprecated since 1.7.0

: renamed to with_capacity_and_hasher

Deprecated, renamed to with_capacity_and_hasher

fn hasher(&self) -> &S

Unstable (hashmap_public_hasher)

: don't want to make insta-stable

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);

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);

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);

fn keys(&'a self) -> Keys<'a, 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);
}

fn values(&'a self) -> Values<'a, 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);
}

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);
}

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);
}

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);

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);

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());

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

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());

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());

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

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);

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

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);

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

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");

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");

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

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);

Trait Implementations

impl<K, V, S> PartialEq<HashMap<K, V, S>> for HashMap<K, V, S> where K: Eq + Hash, V: PartialEq<V>, S: BuildHasher

fn eq(&self, other: &HashMap<K, V, S>) -> bool

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

impl<K, V, S> Eq for HashMap<K, V, S> where V: Eq, S: BuildHasher, K: Eq + Hash

impl<K, V, S> Debug for HashMap<K, V, S> where K: Eq + Hash + Debug, S: BuildHasher, V: Debug

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

impl<K, V, S> Default for HashMap<K, V, S> where S: BuildHasher + Default, K: Eq + Hash

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

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

type Output = V

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

impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S> where S: BuildHasher, K: Eq + Hash

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

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

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

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

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

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

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

impl<K, V, S> IntoIterator for HashMap<K, V, S> where S: BuildHasher, K: Eq + Hash

type Item = (K, V)

type IntoIter = IntoIter<K, V>

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

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher + Default

fn from_iter<T>(iterable: T) -> HashMap<K, V, S> where T: IntoIterator<Item=(K, V)>

impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S> where S: BuildHasher, K: Eq + Hash

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

impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S> where S: BuildHasher, V: Copy, K: Eq + Hash + Copy

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

impl<K, S, Q> Recover<Q> for HashMap<K, (), S> where K: Eq + Hash + Borrow<Q>, Q: Eq + Hash + ?Sized, S: BuildHasher

type Key = K

fn get(&self, key: &Q) -> Option<&K>

fn take(&mut self, key: &Q) -> Option<K>

fn replace(&mut self, key: K) -> Option<K>

Derived Implementations

impl<K, V, S> Clone for HashMap<K, V, S> where K: Clone, S: Clone, V: Clone

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

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