Struct std::collections::HashMap
1.0.0 · source · pub struct HashMap<K, V, S = RandomState> { /* private fields */ }
Expand description
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.
§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!("{book} is unreviewed.")
}
}
// 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}\"");
}
RunA 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),
]);
RunHashMap
implements an Entry
API, which allows
for 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();
// modify an entry before an insert with in-place mutation
player_stats.entry("mana").and_modify(|mana| *mana += 200).or_insert(100);
RunThe 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 {
/// Creates 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 vikings = HashMap::from([
(Viking::new("Einar", "Norway"), 25),
(Viking::new("Olaf", "Denmark"), 24),
(Viking::new("Harald", "Iceland"), 12),
]);
// Use derived implementation to print the status of the vikings.
for (viking, health) in &vikings {
println!("{viking:?} has {health} hp");
}
RunImplementations§
source§impl<K, V> HashMap<K, V, RandomState>
impl<K, V> HashMap<K, V, RandomState>
1.0.0 · sourcepub fn new() -> HashMap<K, V, RandomState>
pub fn new() -> HashMap<K, V, RandomState>
1.0.0 · sourcepub fn with_capacity(capacity: usize) -> HashMap<K, V, RandomState>
pub fn with_capacity(capacity: usize) -> HashMap<K, V, RandomState>
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 0, the hash map will not allocate.
§Examples
use std::collections::HashMap;
let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);
Runsource§impl<K, V, S> HashMap<K, V, S>
impl<K, V, S> HashMap<K, V, S>
1.7.0 (const: unstable) · sourcepub fn with_hasher(hash_builder: S) -> HashMap<K, V, S>
pub 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.
The hash_builder
passed should implement the BuildHasher
trait for
the HashMap to be useful, see its documentation for details.
§Examples
use std::collections::HashMap;
use std::hash::RandomState;
let s = RandomState::new();
let mut map = HashMap::with_hasher(s);
map.insert(1, 2);
Run1.7.0 · sourcepub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashMap<K, V, S>
pub fn with_capacity_and_hasher(capacity: usize, hasher: S) -> HashMap<K, V, S>
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 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.
The hasher
passed should implement the BuildHasher
trait for
the HashMap to be useful, see its documentation for details.
§Examples
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);
Run1.0.0 · sourcepub fn capacity(&self) -> usize
pub 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<i32, i32> = HashMap::with_capacity(100);
assert!(map.capacity() >= 100);
Run1.0.0 · sourcepub fn keys(&self) -> Keys<'_, K, V> ⓘ
pub fn keys(&self) -> Keys<'_, K, V> ⓘ
An iterator visiting all keys in arbitrary order.
The iterator element type is &'a K
.
§Examples
use std::collections::HashMap;
let map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
for key in map.keys() {
println!("{key}");
}
Run§Performance
In the current implementation, iterating over keys takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.54.0 · sourcepub fn into_keys(self) -> IntoKeys<K, V> ⓘ
pub fn into_keys(self) -> IntoKeys<K, V> ⓘ
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
.
§Examples
use std::collections::HashMap;
let map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
let mut vec: Vec<&str> = map.into_keys().collect();
// The `IntoKeys` iterator produces keys in arbitrary order, so the
// keys must be sorted to test them against a sorted array.
vec.sort_unstable();
assert_eq!(vec, ["a", "b", "c"]);
Run§Performance
In the current implementation, iterating over keys takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.0.0 · sourcepub fn values(&self) -> Values<'_, K, V> ⓘ
pub fn values(&self) -> Values<'_, K, V> ⓘ
An iterator visiting all values in arbitrary order.
The iterator element type is &'a V
.
§Examples
use std::collections::HashMap;
let map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
for val in map.values() {
println!("{val}");
}
Run§Performance
In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.10.0 · sourcepub fn values_mut(&mut self) -> ValuesMut<'_, K, V> ⓘ
pub fn values_mut(&mut self) -> ValuesMut<'_, K, V> ⓘ
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::from([
("a", 1),
("b", 2),
("c", 3),
]);
for val in map.values_mut() {
*val = *val + 10;
}
for val in map.values() {
println!("{val}");
}
Run§Performance
In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.54.0 · sourcepub fn into_values(self) -> IntoValues<K, V> ⓘ
pub fn into_values(self) -> IntoValues<K, V> ⓘ
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
.
§Examples
use std::collections::HashMap;
let map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
let mut vec: Vec<i32> = map.into_values().collect();
// The `IntoValues` iterator produces values in arbitrary order, so
// the values must be sorted to test them against a sorted array.
vec.sort_unstable();
assert_eq!(vec, [1, 2, 3]);
Run§Performance
In the current implementation, iterating over values takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.0.0 · sourcepub fn iter(&self) -> Iter<'_, K, V> ⓘ
pub fn iter(&self) -> Iter<'_, K, V> ⓘ
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 map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
for (key, val) in map.iter() {
println!("key: {key} val: {val}");
}
Run§Performance
In the current implementation, iterating over map takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.0.0 · sourcepub fn iter_mut(&mut self) -> IterMut<'_, K, V> ⓘ
pub fn iter_mut(&mut self) -> IterMut<'_, K, V> ⓘ
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::from([
("a", 1),
("b", 2),
("c", 3),
]);
// Update all values
for (_, val) in map.iter_mut() {
*val *= 2;
}
for (key, val) in &map {
println!("key: {key} val: {val}");
}
Run§Performance
In the current implementation, iterating over map takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
1.6.0 · sourcepub fn drain(&mut self) -> Drain<'_, K, V> ⓘ
pub fn drain(&mut self) -> Drain<'_, K, V> ⓘ
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.
§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());
Runsourcepub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, K, V, F> ⓘ
🔬This is a nightly-only experimental API. (hash_extract_if
#59618)
pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, K, V, F> ⓘ
hash_extract_if
#59618)Creates an iterator which uses a closure to determine if an element 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.
Note that extract_if
lets you mutate every value in the filter 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.
§Examples
Splitting a map into even and odd keys, reusing the original map:
#![feature(hash_extract_if)]
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]);
Run1.18.0 · sourcepub fn retain<F>(&mut self, f: F)
pub fn retain<F>(&mut self, f: F)
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.
§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§Performance
In the current implementation, this operation takes O(capacity) time instead of O(len) because it internally visits empty buckets too.
source§impl<K, V, S> HashMap<K, V, S>
impl<K, V, S> HashMap<K, V, S>
1.0.0 · sourcepub fn reserve(&mut self, additional: usize)
pub 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 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
Panics if the new allocation size overflows usize
.
§Examples
use std::collections::HashMap;
let mut map: HashMap<&str, i32> = HashMap::new();
map.reserve(10);
Run1.57.0 · sourcepub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>
pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError>
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.
§Errors
If the capacity overflows, or the allocator reports a failure, then an error is returned.
§Examples
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?");
Run1.0.0 · sourcepub fn shrink_to_fit(&mut self)
pub 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<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);
Run1.56.0 · sourcepub fn shrink_to(&mut self, min_capacity: usize)
pub fn shrink_to(&mut self, min_capacity: usize)
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.
§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(10);
assert!(map.capacity() >= 10);
map.shrink_to(0);
assert!(map.capacity() >= 2);
Run1.0.0 · sourcepub fn entry(&mut self, key: K) -> Entry<'_, K, V>
pub 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() {
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);
Run1.0.0 · sourcepub fn get<Q>(&self, k: &Q) -> Option<&V>
pub fn get<Q>(&self, k: &Q) -> Option<&V>
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);
Run1.40.0 · sourcepub fn get_key_value<Q>(&self, k: &Q) -> Option<(&K, &V)>
pub fn get_key_value<Q>(&self, k: &Q) -> Option<(&K, &V)>
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
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);
Runsourcepub fn get_many_mut<Q, const N: usize>(
&mut self,
ks: [&Q; N],
) -> Option<[&mut V; N]>
🔬This is a nightly-only experimental API. (map_many_mut
#97601)
pub fn get_many_mut<Q, const N: usize>( &mut self, ks: [&Q; N], ) -> Option<[&mut V; N]>
map_many_mut
#97601)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 returned if any of the
keys are duplicates or missing.
§Examples
#![feature(map_many_mut)]
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 got = libraries.get_many_mut([
"Athenæum",
"Library of Congress",
]);
assert_eq!(
got,
Some([
&mut 1807,
&mut 1800,
]),
);
// Missing keys result in None
let got = libraries.get_many_mut([
"Athenæum",
"New York Public Library",
]);
assert_eq!(got, None);
// Duplicate keys result in None
let got = libraries.get_many_mut([
"Athenæum",
"Athenæum",
]);
assert_eq!(got, None);
Runsourcepub unsafe fn get_many_unchecked_mut<Q, const N: usize>(
&mut self,
ks: [&Q; N],
) -> Option<[&mut V; N]>
🔬This is a nightly-only experimental API. (map_many_mut
#97601)
pub unsafe fn get_many_unchecked_mut<Q, const N: usize>( &mut self, ks: [&Q; N], ) -> Option<[&mut V; N]>
map_many_mut
#97601)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 returned if
any of the keys are missing.
For a safe alternative see get_many_mut
.
§Safety
Calling this method with overlapping keys is undefined behavior even if the resulting references are not used.
§Examples
#![feature(map_many_mut)]
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 got = libraries.get_many_mut([
"Athenæum",
"Library of Congress",
]);
assert_eq!(
got,
Some([
&mut 1807,
&mut 1800,
]),
);
// Missing keys result in None
let got = libraries.get_many_mut([
"Athenæum",
"New York Public Library",
]);
assert_eq!(got, None);
Run1.0.0 · sourcepub fn contains_key<Q>(&self, k: &Q) -> bool
pub fn contains_key<Q>(&self, k: &Q) -> bool
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);
Run1.0.0 · sourcepub fn get_mut<Q>(&mut self, k: &Q) -> Option<&mut V>
pub fn get_mut<Q>(&mut self, k: &Q) -> Option<&mut V>
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");
Run1.0.0 · sourcepub fn insert(&mut self, k: K, v: V) -> Option<V>
pub 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");
Runsourcepub fn try_insert(
&mut self,
key: K,
value: V,
) -> Result<&mut V, OccupiedError<'_, K, V>>
🔬This is a nightly-only experimental API. (map_try_insert
#82766)
pub fn try_insert( &mut self, key: K, value: V, ) -> Result<&mut V, OccupiedError<'_, K, V>>
map_try_insert
#82766)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.
§Examples
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");
Run1.0.0 · sourcepub fn remove<Q>(&mut self, k: &Q) -> Option<V>
pub fn remove<Q>(&mut self, k: &Q) -> Option<V>
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);
Run1.27.0 · sourcepub fn remove_entry<Q>(&mut self, k: &Q) -> Option<(K, V)>
pub fn remove_entry<Q>(&mut self, k: &Q) -> Option<(K, V)>
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);
Runsource§impl<K, V, S> HashMap<K, V, S>where
S: BuildHasher,
impl<K, V, S> HashMap<K, V, S>where
S: BuildHasher,
sourcepub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<'_, K, V, S>
🔬This is a nightly-only experimental API. (hash_raw_entry
#56167)
pub fn raw_entry_mut(&mut self) -> RawEntryBuilderMut<'_, K, V, S>
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 initialize 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).
sourcepub fn raw_entry(&self) -> RawEntryBuilder<'_, K, V, S>
🔬This is a nightly-only experimental API. (hash_raw_entry
#56167)
pub fn raw_entry(&self) -> RawEntryBuilder<'_, K, V, S>
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§
1.4.0 · source§impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>
impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>
source§fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T)
fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T)
1.0.0 · source§impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.
1.0.0 · source§impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>
impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>
1.0.0 · source§impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S>
impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S>
1.0.0 · source§impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S>
impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S>
1.0.0 · source§impl<K, V, S> IntoIterator for HashMap<K, V, S>
impl<K, V, S> IntoIterator for HashMap<K, V, S>
source§fn into_iter(self) -> IntoIter<K, V> ⓘ
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 map = HashMap::from([
("a", 1),
("b", 2),
("c", 3),
]);
// Not possible with .iter()
let vec: Vec<(&str, i32)> = map.into_iter().collect();
Run