Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://doc.rust-lang.org/std/collections/struct.HashMap.html below:

HashMap in std::collections - Rust

pub struct HashMap<K, V, S = RandomState> {  }

A hash map implemented with quadratic probing and SIMD lookup.

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 coroutine 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. There are many alternative hashing algorithms available on crates.io.

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. Violating this property is a logic error.

It is also 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.

The behavior resulting from either logic error is not specified, but will be encapsulated to the HashMap that observed the logic error and not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

The hash table implementation is a Rust port of Google’s SwissTable. The original C++ version of SwissTable can be found here, and this CppCon talk gives an overview of how the algorithm works.

use std::collections::HashMap;

let mut book_reviews = HashMap::new();

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(),
);

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

book_reviews.remove("The Adventures of Sherlock Holmes");

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!("{book} is unreviewed.")
    }
}

println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);

for (book, review) in &book_reviews {
    println!("{book}: \"{review}\"");
}

A HashMap with a known list of items can be initialized from an array:

use std::collections::HashMap;

let solar_distance = HashMap::from([
    ("Mercury", 0.4),
    ("Venus", 0.7),
    ("Earth", 1.0),
    ("Mars", 1.5),
]);

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

use std::collections::HashMap;

let mut player_stats = HashMap::new();

fn random_stat_buff() -> u8 {
    42
}

player_stats.entry("health").or_insert(100);

player_stats.entry("defence").or_insert_with(random_stat_buff);

let stat = player_stats.entry("attack").or_insert(100);
*stat += random_stat_buff();

player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);

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

let vikings = HashMap::from([
    (Viking::new("Einar", "Norway"), 25),
    (Viking::new("Olaf", "Denmark"), 24),
    (Viking::new("Harald", "Iceland"), 12),
]);

for (viking, health) in &vikings {
    println!("{viking:?} has {health} hp");
}

As explained above, HashMap is randomly seeded: each HashMap instance uses a different seed, which means that HashMap::new normally cannot be used in a const or static initializer.

However, if you need to use a HashMap in a const or static initializer while retaining random seed generation, you can wrap the HashMap in LazyLock.

Alternatively, you can construct a HashMap in a const or static initializer using a different hasher that does not rely on a random seed. Be aware that a HashMap created this way is not resistant to HashDoS attacks!

use std::collections::HashMap;
use std::hash::{BuildHasherDefault, DefaultHasher};
use std::sync::{LazyLock, Mutex};

const NONRANDOM_EMPTY_MAP: HashMap<String, Vec<i32>, BuildHasherDefault<DefaultHasher>> =
    HashMap::with_hasher(BuildHasherDefault::new());
static NONRANDOM_MAP: Mutex<HashMap<String, Vec<i32>, BuildHasherDefault<DefaultHasher>>> =
    Mutex::new(HashMap::with_hasher(BuildHasherDefault::new()));

const RANDOM_EMPTY_MAP: LazyLock<HashMap<String, Vec<i32>>> =
    LazyLock::new(HashMap::new);
static RANDOM_MAP: LazyLock<Mutex<HashMap<String, Vec<i32>>>> =
    LazyLock::new(|| Mutex::new(HashMap::new()));

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.

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

Creates an empty HashMap with at least the specified capacity.

The hash map will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, the hash map will not allocate.

use std::collections::HashMap;
let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);

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.

The hash_builder passed should implement the BuildHasher trait for the HashMap to be useful, see its documentation for details.

use std::collections::HashMap;
use std::hash::RandomState;

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

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

The hash map will be able to hold at least capacity elements without reallocating. This method is allowed to allocate for more elements than capacity. If capacity is zero, 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.

The hasher passed should implement the BuildHasher trait for the HashMap to be useful, see its documentation for details.

use std::collections::HashMap;
use std::hash::RandomState;

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

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.

use std::collections::HashMap;
let map: HashMap<i32, i32> = HashMap::with_capacity(100);
assert!(map.capacity() >= 100);

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

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

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

In the current implementation, iterating over keys takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

Creates a consuming iterator visiting all the keys in arbitrary order. The map cannot be used after calling this. The iterator element type is K.

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

let mut vec: Vec<&str> = map.into_keys().collect();
vec.sort_unstable();
assert_eq!(vec, ["a", "b", "c"]);

In the current implementation, iterating over keys takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

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

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

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

In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

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

use std::collections::HashMap;

let mut map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

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

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

In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

Creates a consuming iterator visiting all the values in arbitrary order. The map cannot be used after calling this. The iterator element type is V.

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

let mut vec: Vec<i32> = map.into_values().collect();
vec.sort_unstable();
assert_eq!(vec, [1, 2, 3]);

In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

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

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

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

In the current implementation, iterating over map takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

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).

use std::collections::HashMap;

let mut map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

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

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

In the current implementation, iterating over map takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

Returns the number of elements in the map.

use std::collections::HashMap;

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

Returns true if the map contains no elements.

use std::collections::HashMap;

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

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

If the returned iterator is dropped before being fully consumed, it drops the remaining key-value pairs. The returned iterator keeps a mutable borrow on the map to optimize its implementation.

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());

Creates an iterator which uses a closure to determine if an element (key-value pair) should be removed.

If the closure returns true, the element is removed from the map and yielded. If the closure returns false, or panics, the element remains in the map and will not be yielded.

The iterator also lets you mutate the value of each element in the closure, regardless of whether you choose to keep or remove it.

If the returned ExtractIf is not exhausted, e.g. because it is dropped without iterating or the iteration short-circuits, then the remaining elements will be retained. Use retain with a negated predicate if you do not need the returned iterator.

Splitting a map into even and odd keys, reusing the original map:

use std::collections::HashMap;

let mut map: HashMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
let extracted: HashMap<i32, i32> = map.extract_if(|k, _v| k % 2 == 0).collect();

let mut evens = extracted.keys().copied().collect::<Vec<_>>();
let mut odds = map.keys().copied().collect::<Vec<_>>();
evens.sort();
odds.sort();

assert_eq!(evens, vec![0, 2, 4, 6]);
assert_eq!(odds, vec![1, 3, 5, 7]);

Retains only the elements specified by the predicate.

In other words, remove all pairs (k, v) for which f(&k, &mut v) returns false. The elements are visited in unsorted (and unspecified) order.

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);

In the current implementation, this operation takes O(capacity) time instead of O(len) because it internally visits empty buckets too.

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

use std::collections::HashMap;

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

Returns a reference to the map’s BuildHasher.

use std::collections::HashMap;
use std::hash::RandomState;

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

Reserves capacity for at least additional more elements to be inserted in the HashMap. The collection may reserve more space to speculatively avoid frequent reallocations. After calling reserve, capacity will be greater than or equal to self.len() + additional. Does nothing if capacity is already sufficient.

Panics if the new allocation size overflows usize.

use std::collections::HashMap;
let mut map: HashMap<&str, i32> = HashMap::new();
map.reserve(10);

Tries to reserve capacity for at least additional more elements to be inserted in the HashMap. The collection may reserve more space to speculatively avoid frequent reallocations. After calling try_reserve, capacity will be greater than or equal to self.len() + additional if it returns Ok(()). Does nothing if capacity is already sufficient.

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

use std::collections::HashMap;

let mut map: HashMap<&str, isize> = HashMap::new();
map.try_reserve(10).expect("why is the test harness OOMing on a handful of bytes?");

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.

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);

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.

If the current capacity is less than the lower limit, this is a no-op.

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);

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

use std::collections::HashMap;

let mut letters = HashMap::new();

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

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

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.

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);

Returns the key-value pair corresponding to the supplied key. This is potentially useful:

• for key types where non-identical keys can be considered equal;
• for getting the &K stored key value from a borrowed &Q lookup key; or
• for getting a reference to a key with the same lifetime as the collection.

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.

use std::collections::HashMap;
use std::hash::{Hash, Hasher};

#[derive(Clone, Copy, Debug)]
struct S {
    id: u32,
    name: &'static str, }

impl PartialEq for S {
    fn eq(&self, other: &S) -> bool {
        self.id == other.id
    }
}

impl Eq for S {}

impl Hash for S {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.id.hash(state);
    }
}

let j_a = S { id: 1, name: "Jessica" };
let j_b = S { id: 1, name: "Jess" };
let p = S { id: 2, name: "Paul" };
assert_eq!(j_a, j_b);

let mut map = HashMap::new();
map.insert(j_a, "Paris");
assert_eq!(map.get_key_value(&j_a), Some((&j_a, &"Paris")));
assert_eq!(map.get_key_value(&j_b), Some((&j_a, &"Paris"))); assert_eq!(map.get_key_value(&p), None);

Attempts to get mutable references to N values in the map at once.

Returns an array of length N with the results of each query. For soundness, at most one mutable reference will be returned to any value. None will be used if the key is missing.

This method performs a check to ensure there are no duplicate keys, which currently has a time-complexity of O(n^2), so be careful when passing many keys.

Panics if any keys are overlapping.

use std::collections::HashMap;

let mut libraries = HashMap::new();
libraries.insert("Bodleian Library".to_string(), 1602);
libraries.insert("Athenæum".to_string(), 1807);
libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
libraries.insert("Library of Congress".to_string(), 1800);

let [Some(a), Some(b)] = libraries.get_disjoint_mut([
    "Athenæum",
    "Bodleian Library",
]) else { panic!() };

let got = libraries.get_disjoint_mut([
    "Athenæum",
    "Library of Congress",
]);
assert_eq!(
    got,
    [
        Some(&mut 1807),
        Some(&mut 1800),
    ],
);

let got = libraries.get_disjoint_mut([
    "Athenæum",
    "New York Public Library",
]);
assert_eq!(
    got,
    [
        Some(&mut 1807),
        None
    ]
);

use std::collections::HashMap;

let mut libraries = HashMap::new();
libraries.insert("Athenæum".to_string(), 1807);

let got = libraries.get_disjoint_mut([
    "Athenæum",
    "Athenæum",
]);

Attempts to get mutable references to N values in the map at once, without validating that the values are unique.

Returns an array of length N with the results of each query. None will be used if the key is missing.

For a safe alternative see get_disjoint_mut.

Calling this method with overlapping keys is undefined behavior even if the resulting references are not used.

use std::collections::HashMap;

let mut libraries = HashMap::new();
libraries.insert("Bodleian Library".to_string(), 1602);
libraries.insert("Athenæum".to_string(), 1807);
libraries.insert("Herzogin-Anna-Amalia-Bibliothek".to_string(), 1691);
libraries.insert("Library of Congress".to_string(), 1800);

let [Some(a), Some(b)] = (unsafe { libraries.get_disjoint_unchecked_mut([
    "Athenæum",
    "Bodleian Library",
]) }) else { panic!() };

let got = unsafe { libraries.get_disjoint_unchecked_mut([
    "Athenæum",
    "Library of Congress",
]) };
assert_eq!(
    got,
    [
        Some(&mut 1807),
        Some(&mut 1800),
    ],
);

let got = unsafe { libraries.get_disjoint_unchecked_mut([
    "Athenæum",
    "New York Public Library",
]) };
assert_eq!(got, [Some(&mut 1807), None]);

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.

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);

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.

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");

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.

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");

Tries to insert a key-value pair into the map, and returns a mutable reference to the value in the entry.

If the map already had this key present, nothing is updated, and an error containing the occupied entry and the value is returned.

Basic usage:

#![feature(map_try_insert)]

use std::collections::HashMap;

let mut map = HashMap::new();
assert_eq!(map.try_insert(37, "a").unwrap(), &"a");

let err = map.try_insert(37, "b").unwrap_err();
assert_eq!(err.entry.key(), &37);
assert_eq!(err.entry.get(), &"a");
assert_eq!(err.value, "b");

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.

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);

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.

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);

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

Extends a collection with the contents of an iterator.

Extends a collection with exactly one element.

Reserves capacity in a collection for the given number of additional elements.

Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.

Extends a collection with the contents of an iterator.

Extends a collection with exactly one element.

Reserves capacity in a collection for the given number of additional elements.

Converts a [(K, V); N] into a HashMap<K, V>.

If any entries in the array have equal keys, all but one of the corresponding values will be dropped.

use std::collections::HashMap;

let map1 = HashMap::from([(1, 2), (3, 4)]);
let map2: HashMap<_, _> = [(1, 2), (3, 4)].into();
assert_eq!(map1, map2);

Constructs a HashMap<K, V> from an iterator of key-value pairs.

If the iterator produces any pairs with equal keys, all but one of the corresponding values will be dropped.

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

Panics if the key is not present in the HashMap.

The returned type after indexing.

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

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.

use std::collections::HashMap;

let map = HashMap::from([
    ("a", 1),
    ("b", 2),
    ("c", 3),
]);

let vec: Vec<(&str, i32)> = map.into_iter().collect();

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

Tests for self and other values to be equal, and is used by ==.

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4