# Struct alloc::collections::btree_map::BTreeMap

1.0.0 · source ·

    pub struct BTreeMap<K, V, A: Allocator + Clone = Global> { /* private fields */ }

Expand description

An ordered map based on a B-Tree.

B-Trees represent a fundamental compromise between cache-efficiency and actually minimizing the amount of work performed in a search. In theory, a binary search tree (BST) is the optimal choice for a sorted map, as a perfectly balanced BST performs the theoretical minimum amount of comparisons necessary to find an element (log2n). However, in practice the way this is done is very inefficient for modern computer architectures. In particular, every element is stored in its own individually heap-allocated node. This means that every single insertion triggers a heap-allocation, and every single comparison should be a cache-miss. Since these are both notably expensive things to do in practice, we are forced to, at the very least, reconsider the BST strategy.

A B-Tree instead makes each node contain B-1 to 2B-1 elements in a contiguous array. By doing this, we reduce the number of allocations by a factor of B, and improve cache efficiency in searches. However, this does mean that searches will have to do more comparisons on average. The precise number of comparisons depends on the node search strategy used. For optimal cache efficiency, one could search the nodes linearly. For optimal comparisons, one could search the node using binary search. As a compromise, one could also perform a linear search that initially only checks every ith element for some choice of i.

Currently, our implementation simply performs naive linear search. This provides excellent performance on small nodes of elements which are cheap to compare. However in the future we would like to further explore choosing the optimal search strategy based on the choice of B, and possibly other factors. Using linear search, searching for a random element is expected to take B * log(n) comparisons, which is generally worse than a BST. In practice, however, performance is excellent.

It is a logic error for a key to be modified in such a way that the key’s ordering relative to any other key, as determined by the `Ord` 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 such a logic error is not specified, but will be encapsulated to the `BTreeMap` that observed the logic error and not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

Iterators obtained from functions such as `BTreeMap::iter`, `BTreeMap::into_iter`, `BTreeMap::values`, or `BTreeMap::keys` produce their items in order by key, and take worst-case logarithmic and amortized constant time per item returned.

## §Examples

    use std::collections::BTreeMap;

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

    // review some movies.
    movie_reviews.insert("Office Space",       "Deals with real issues in the workplace.");
    movie_reviews.insert("Pulp Fiction",       "Masterpiece.");
    movie_reviews.insert("The Godfather",      "Very enjoyable.");
    movie_reviews.insert("The Blues Brothers", "Eye lyked it a lot.");

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

    // oops, this review has a lot of spelling mistakes, let's delete it.
    movie_reviews.remove("The Blues Brothers");

    // look up the values associated with some keys.
    let to_find = ["Up!", "Office Space"];
    for movie in &to_find {
        match movie_reviews.get(movie) {
           Some(review) => println!("{movie}: {review}"),
           None => println!("{movie} is unreviewed.")
        }
    }

    // Look up the value for a key (will panic if the key is not found).
    println!("Movie review: {}", movie_reviews["Office Space"]);

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

Run

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

    use std::collections::BTreeMap;

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

Run

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

    use std::collections::BTreeMap;

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

Run

## Implementations§

source§

### impl<K, V> BTreeMap<K, V>

1.0.0 (const: 1.66.0) · source

#### pub const fn new() -> BTreeMap<K, V>

Makes a new, empty `BTreeMap`.

Does not allocate anything on its own.

##### §Examples

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();

    // entries can now be inserted into the empty map
    map.insert(1, "a");

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source§

### impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A>

1.0.0 · source

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

Clears the map, removing all elements.

##### §Examples

    use std::collections::BTreeMap;

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

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source

#### pub const fn new_in(alloc: A) -> BTreeMap<K, V, A>

🔬This is a nightly-only experimental API. (`btreemap_alloc` #32838)

Makes a new empty BTreeMap with a reasonable choice for B.

##### §Examples

    use std::collections::BTreeMap;
    use std::alloc::Global;

    let mut map = BTreeMap::new_in(Global);

    // entries can now be inserted into the empty map
    map.insert(1, "a");

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source§

### impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A>

1.0.0 · source

#### pub fn get<Q>(&self, key: &Q) -> Option<&V>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

Returns a reference to the value corresponding to the key.

The key may be any borrowed form of the map’s key type, but the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

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1.40.0 · source

#### pub fn get_key_value<Q>(&self, k: &Q) -> Option<(&K, &V)>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

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 the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.66.0 · source

#### pub fn first_key_value(&self) -> Option<(&K, &V)>
where K: Ord,

Returns the first key-value pair in the map. The key in this pair is the minimum key in the map.

##### §Examples

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    assert_eq!(map.first_key_value(), None);
    map.insert(1, "b");
    map.insert(2, "a");
    assert_eq!(map.first_key_value(), Some((&1, &"b")));

Run
1.66.0 · source

#### pub fn first_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
where K: Ord,

Returns the first entry in the map for in-place manipulation. The key of this entry is the minimum key in the map.

##### §Examples

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    map.insert(1, "a");
    map.insert(2, "b");
    if let Some(mut entry) = map.first_entry() {
        if *entry.key() > 0 {
            entry.insert("first");
        }
    }
    assert_eq!(*map.get(&1).unwrap(), "first");
    assert_eq!(*map.get(&2).unwrap(), "b");

Run
1.66.0 · source

#### pub fn pop_first(&mut self) -> Option<(K, V)>
where K: Ord,

Removes and returns the first element in the map. The key of this element is the minimum key that was in the map.

##### §Examples

Draining elements in ascending order, while keeping a usable map each iteration.

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    map.insert(1, "a");
    map.insert(2, "b");
    while let Some((key, _val)) = map.pop_first() {
        assert!(map.iter().all(|(k, _v)| *k > key));
    }
    assert!(map.is_empty());

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1.66.0 · source

#### pub fn last_key_value(&self) -> Option<(&K, &V)>
where K: Ord,

Returns the last key-value pair in the map. The key in this pair is the maximum key in the map.

##### §Examples

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    map.insert(1, "b");
    map.insert(2, "a");
    assert_eq!(map.last_key_value(), Some((&2, &"a")));

Run
1.66.0 · source

#### pub fn last_entry(&mut self) -> Option<OccupiedEntry<'_, K, V, A>>
where K: Ord,

Returns the last entry in the map for in-place manipulation. The key of this entry is the maximum key in the map.

##### §Examples

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    map.insert(1, "a");
    map.insert(2, "b");
    if let Some(mut entry) = map.last_entry() {
        if *entry.key() > 0 {
            entry.insert("last");
        }
    }
    assert_eq!(*map.get(&1).unwrap(), "a");
    assert_eq!(*map.get(&2).unwrap(), "last");

Run
1.66.0 · source

#### pub fn pop_last(&mut self) -> Option<(K, V)>
where K: Ord,

Removes and returns the last element in the map. The key of this element is the maximum key that was in the map.

##### §Examples

Draining elements in descending order, while keeping a usable map each iteration.

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();
    map.insert(1, "a");
    map.insert(2, "b");
    while let Some((key, _val)) = map.pop_last() {
        assert!(map.iter().all(|(k, _v)| *k < key));
    }
    assert!(map.is_empty());

Run
1.0.0 · source

#### pub fn contains_key<Q>(&self, key: &Q) -> bool
where K: Borrow<Q> + Ord, Q: Ord + ?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 the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.0.0 · source

#### pub fn get_mut<Q>(&mut self, key: &Q) -> Option<&mut V>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

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 the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.0.0 · source

#### pub fn insert(&mut self, key: K, value: V) -> Option<V>
where K: Ord,

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::BTreeMap;

    let mut map = BTreeMap::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
source

#### pub fn try_insert( &mut self, key: K, value: V ) -> Result<&mut V, OccupiedError<'_, K, V, A>>
where K: Ord,

🔬This is a nightly-only experimental API. (`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

    #![feature(map_try_insert)]

    use std::collections::BTreeMap;

    let mut map = BTreeMap::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");

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1.0.0 · source

#### pub fn remove<Q>(&mut self, key: &Q) -> Option<V>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

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 the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.45.0 · source

#### pub fn remove_entry<Q>(&mut self, key: &Q) -> Option<(K, V)>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

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 the ordering on the borrowed form must match the ordering on the key type.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.53.0 · source

#### pub fn retain<F>(&mut self, f: F)
where K: Ord, F: FnMut(&K, &mut V) -> bool,

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 ascending key order.

##### §Examples

    use std::collections::BTreeMap;

    let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x*10)).collect();
    // Keep only the elements with even-numbered keys.
    map.retain(|&k, _| k % 2 == 0);
    assert!(map.into_iter().eq(vec![(0, 0), (2, 20), (4, 40), (6, 60)]));

Run
1.11.0 · source

#### pub fn append(&mut self, other: &mut Self)
where K: Ord, A: Clone,

Moves all elements from `other` into `self`, leaving `other` empty.

If a key from `other` is already present in `self`, the respective value from `self` will be overwritten with the respective value from `other`.

##### §Examples

    use std::collections::BTreeMap;

    let mut a = BTreeMap::new();
    a.insert(1, "a");
    a.insert(2, "b");
    a.insert(3, "c"); // Note: Key (3) also present in b.

    let mut b = BTreeMap::new();
    b.insert(3, "d"); // Note: Key (3) also present in a.
    b.insert(4, "e");
    b.insert(5, "f");

    a.append(&mut b);

    assert_eq!(a.len(), 5);
    assert_eq!(b.len(), 0);

    assert_eq!(a[&1], "a");
    assert_eq!(a[&2], "b");
    assert_eq!(a[&3], "d"); // Note: "c" has been overwritten.
    assert_eq!(a[&4], "e");
    assert_eq!(a[&5], "f");

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1.17.0 · source

#### pub fn range<T, R>(&self, range: R) -> Range<'_, K, V> ⓘ
where T: Ord + ?Sized, K: Borrow<T> + Ord, R: RangeBounds<T>,

Constructs a double-ended iterator over a sub-range of elements in the map. The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will yield elements from min (inclusive) to max (exclusive). The range may also be entered as `(Bound<T>, Bound<T>)`, so for example `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive range from 4 to 10.

##### §Panics

Panics if range `start > end`. Panics if range `start == end` and both bounds are `Excluded`.

##### §Examples

    use std::collections::BTreeMap;
    use std::ops::Bound::Included;

    let mut map = BTreeMap::new();
    map.insert(3, "a");
    map.insert(5, "b");
    map.insert(8, "c");
    for (&key, &value) in map.range((Included(&4), Included(&8))) {
        println!("{key}: {value}");
    }
    assert_eq!(Some((&5, &"b")), map.range(4..).next());

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1.17.0 · source

#### pub fn range_mut<T, R>(&mut self, range: R) -> RangeMut<'_, K, V> ⓘ
where T: Ord + ?Sized, K: Borrow<T> + Ord, R: RangeBounds<T>,

Constructs a mutable double-ended iterator over a sub-range of elements in the map. The simplest way is to use the range syntax `min..max`, thus `range(min..max)` will yield elements from min (inclusive) to max (exclusive). The range may also be entered as `(Bound<T>, Bound<T>)`, so for example `range((Excluded(4), Included(10)))` will yield a left-exclusive, right-inclusive range from 4 to 10.

##### §Panics

Panics if range `start > end`. Panics if range `start == end` and both bounds are `Excluded`.

##### §Examples

    use std::collections::BTreeMap;

    let mut map: BTreeMap<&str, i32> =
        [("Alice", 0), ("Bob", 0), ("Carol", 0), ("Cheryl", 0)].into();
    for (_, balance) in map.range_mut("B".."Cheryl") {
        *balance += 100;
    }
    for (name, balance) in &map {
        println!("{name} => {balance}");
    }

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1.0.0 · source

#### pub fn entry(&mut self, key: K) -> Entry<'_, K, V, A>
where K: Ord,

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

##### §Examples

    use std::collections::BTreeMap;

    let mut count: BTreeMap<&str, usize> = BTreeMap::new();

    // count the number of occurrences of letters in the vec
    for x in ["a", "b", "a", "c", "a", "b"] {
        count.entry(x).and_modify(|curr| *curr += 1).or_insert(1);
    }

    assert_eq!(count["a"], 3);
    assert_eq!(count["b"], 2);
    assert_eq!(count["c"], 1);

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1.11.0 · source

#### pub fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self
where K: Borrow<Q> + Ord, A: Clone,

Splits the collection into two at the given key. Returns everything after the given key, including the key.

##### §Examples

    use std::collections::BTreeMap;

    let mut a = BTreeMap::new();
    a.insert(1, "a");
    a.insert(2, "b");
    a.insert(3, "c");
    a.insert(17, "d");
    a.insert(41, "e");

    let b = a.split_off(&3);

    assert_eq!(a.len(), 2);
    assert_eq!(b.len(), 3);

    assert_eq!(a[&1], "a");
    assert_eq!(a[&2], "b");

    assert_eq!(b[&3], "c");
    assert_eq!(b[&17], "d");
    assert_eq!(b[&41], "e");

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source

#### pub fn extract_if<F>(&mut self, pred: F) -> ExtractIf<'_, K, V, F, A> ⓘ
where K: Ord, F: FnMut(&K, &mut V) -> bool,

🔬This is a nightly-only experimental API. (`btree_extract_if` #70530)

Creates an iterator that visits all elements (key-value pairs) in ascending key order and 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.

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.

##### §Examples

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

    #![feature(btree_extract_if)]
    use std::collections::BTreeMap;

    let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect();
    let evens: BTreeMap<_, _> = map.extract_if(|k, _v| k % 2 == 0).collect();
    let odds = map;
    assert_eq!(evens.keys().copied().collect::<Vec<_>>(), [0, 2, 4, 6]);
    assert_eq!(odds.keys().copied().collect::<Vec<_>>(), [1, 3, 5, 7]);

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1.54.0 · source

#### pub fn into_keys(self) -> IntoKeys<K, V, A> ⓘ

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

##### §Examples

    use std::collections::BTreeMap;

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

    let keys: Vec<i32> = a.into_keys().collect();
    assert_eq!(keys, [1, 2]);

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1.54.0 · source

#### pub fn into_values(self) -> IntoValues<K, V, A> ⓘ

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

##### §Examples

    use std::collections::BTreeMap;

    let mut a = BTreeMap::new();
    a.insert(1, "hello");
    a.insert(2, "goodbye");

    let values: Vec<&str> = a.into_values().collect();
    assert_eq!(values, ["hello", "goodbye"]);

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source§

### impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A>

1.0.0 · source

#### pub fn iter(&self) -> Iter<'_, K, V> ⓘ

Gets an iterator over the entries of the map, sorted by key.

##### §Examples

    use std::collections::BTreeMap;

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

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

    let (first_key, first_value) = map.iter().next().unwrap();
    assert_eq!((*first_key, *first_value), (1, "a"));

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1.0.0 · source

#### pub fn iter_mut(&mut self) -> IterMut<'_, K, V> ⓘ

Gets a mutable iterator over the entries of the map, sorted by key.

##### §Examples

    use std::collections::BTreeMap;

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

    // add 10 to the value if the key isn't "a"
    for (key, value) in map.iter_mut() {
        if key != &"a" {
            *value += 10;
        }
    }

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1.0.0 · source

#### pub fn keys(&self) -> Keys<'_, K, V> ⓘ

Gets an iterator over the keys of the map, in sorted order.

##### §Examples

    use std::collections::BTreeMap;

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

    let keys: Vec<_> = a.keys().cloned().collect();
    assert_eq!(keys, [1, 2]);

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1.0.0 · source

#### pub fn values(&self) -> Values<'_, K, V> ⓘ

Gets an iterator over the values of the map, in order by key.

##### §Examples

    use std::collections::BTreeMap;

    let mut a = BTreeMap::new();
    a.insert(1, "hello");
    a.insert(2, "goodbye");

    let values: Vec<&str> = a.values().cloned().collect();
    assert_eq!(values, ["hello", "goodbye"]);

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1.10.0 · source

#### pub fn values_mut(&mut self) -> ValuesMut<'_, K, V> ⓘ

Gets a mutable iterator over the values of the map, in order by key.

##### §Examples

    use std::collections::BTreeMap;

    let mut a = BTreeMap::new();
    a.insert(1, String::from("hello"));
    a.insert(2, String::from("goodbye"));

    for value in a.values_mut() {
        value.push_str("!");
    }

    let values: Vec<String> = a.values().cloned().collect();
    assert_eq!(values, [String::from("hello!"),
                        String::from("goodbye!")]);

Run
1.0.0 (const: unstable) · source

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

Returns the number of elements in the map.

##### §Examples

    use std::collections::BTreeMap;

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

Run
1.0.0 (const: unstable) · source

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

Returns `true` if the map contains no elements.

##### §Examples

    use std::collections::BTreeMap;

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

Run
source

#### pub fn lower_bound<Q>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

🔬This is a nightly-only experimental API. (`btree_cursors` #107540)

Returns a `Cursor` pointing at the gap before the smallest key greater than the given bound.

Passing `Bound::Included(x)` will return a cursor pointing to the gap before the smallest key greater than or equal to `x`.

Passing `Bound::Excluded(x)` will return a cursor pointing to the gap before the smallest key greater than `x`.

Passing `Bound::Unbounded` will return a cursor pointing to the gap before the smallest key in the map.

##### §Examples

    #![feature(btree_cursors)]

    use std::collections::BTreeMap;
    use std::ops::Bound;

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

    let cursor = map.lower_bound(Bound::Included(&2));
    assert_eq!(cursor.peek_prev(), Some((&1, &"a")));
    assert_eq!(cursor.peek_next(), Some((&2, &"b")));

    let cursor = map.lower_bound(Bound::Excluded(&2));
    assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
    assert_eq!(cursor.peek_next(), Some((&3, &"c")));

    let cursor = map.lower_bound(Bound::Unbounded);
    assert_eq!(cursor.peek_prev(), None);
    assert_eq!(cursor.peek_next(), Some((&1, &"a")));

Run
source

#### pub fn lower_bound_mut<Q>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

🔬This is a nightly-only experimental API. (`btree_cursors` #107540)

Returns a `CursorMut` pointing at the gap before the smallest key greater than the given bound.

Passing `Bound::Included(x)` will return a cursor pointing to the gap before the smallest key greater than or equal to `x`.

Passing `Bound::Excluded(x)` will return a cursor pointing to the gap before the smallest key greater than `x`.

Passing `Bound::Unbounded` will return a cursor pointing to the gap before the smallest key in the map.

##### §Examples

    #![feature(btree_cursors)]

    use std::collections::BTreeMap;
    use std::ops::Bound;

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

    let mut cursor = map.lower_bound_mut(Bound::Included(&2));
    assert_eq!(cursor.peek_prev(), Some((&1, &mut "a")));
    assert_eq!(cursor.peek_next(), Some((&2, &mut "b")));

    let mut cursor = map.lower_bound_mut(Bound::Excluded(&2));
    assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
    assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));

    let mut cursor = map.lower_bound_mut(Bound::Unbounded);
    assert_eq!(cursor.peek_prev(), None);
    assert_eq!(cursor.peek_next(), Some((&1, &mut "a")));

Run
source

#### pub fn upper_bound<Q>(&self, bound: Bound<&Q>) -> Cursor<'_, K, V>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

🔬This is a nightly-only experimental API. (`btree_cursors` #107540)

Returns a `Cursor` pointing at the gap after the greatest key smaller than the given bound.

Passing `Bound::Included(x)` will return a cursor pointing to the gap after the greatest key smaller than or equal to `x`.

Passing `Bound::Excluded(x)` will return a cursor pointing to the gap after the greatest key smaller than `x`.

Passing `Bound::Unbounded` will return a cursor pointing to the gap after the greatest key in the map.

##### §Examples

    #![feature(btree_cursors)]

    use std::collections::BTreeMap;
    use std::ops::Bound;

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

    let cursor = map.upper_bound(Bound::Included(&3));
    assert_eq!(cursor.peek_prev(), Some((&3, &"c")));
    assert_eq!(cursor.peek_next(), Some((&4, &"d")));

    let cursor = map.upper_bound(Bound::Excluded(&3));
    assert_eq!(cursor.peek_prev(), Some((&2, &"b")));
    assert_eq!(cursor.peek_next(), Some((&3, &"c")));

    let cursor = map.upper_bound(Bound::Unbounded);
    assert_eq!(cursor.peek_prev(), Some((&4, &"d")));
    assert_eq!(cursor.peek_next(), None);

Run
source

#### pub fn upper_bound_mut<Q>(&mut self, bound: Bound<&Q>) -> CursorMut<'_, K, V, A>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

🔬This is a nightly-only experimental API. (`btree_cursors` #107540)

Returns a `CursorMut` pointing at the gap after the greatest key smaller than the given bound.

Passing `Bound::Included(x)` will return a cursor pointing to the gap after the greatest key smaller than or equal to `x`.

Passing `Bound::Excluded(x)` will return a cursor pointing to the gap after the greatest key smaller than `x`.

Passing `Bound::Unbounded` will return a cursor pointing to the gap after the greatest key in the map.

##### §Examples

    #![feature(btree_cursors)]

    use std::collections::BTreeMap;
    use std::ops::Bound;

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

    let mut cursor = map.upper_bound_mut(Bound::Included(&3));
    assert_eq!(cursor.peek_prev(), Some((&3, &mut "c")));
    assert_eq!(cursor.peek_next(), Some((&4, &mut "d")));

    let mut cursor = map.upper_bound_mut(Bound::Excluded(&3));
    assert_eq!(cursor.peek_prev(), Some((&2, &mut "b")));
    assert_eq!(cursor.peek_next(), Some((&3, &mut "c")));

    let mut cursor = map.upper_bound_mut(Bound::Unbounded);
    assert_eq!(cursor.peek_prev(), Some((&4, &mut "d")));
    assert_eq!(cursor.peek_next(), None);

Run

## Trait Implementations§

1.0.0 · source§

### impl<K: Clone, V: Clone, A: Allocator + Clone> Clone for BTreeMap<K, V, A>

source§

#### fn clone(&self) -> BTreeMap<K, V, A>

Returns a copy of the value. Read more
1.0.0 · source§

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

Performs copy-assignment from `source`. Read more
1.0.0 · source§

### impl<K: Debug, V: Debug, A: Allocator + Clone> Debug for BTreeMap<K, V, A>

source§

#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
1.0.0 · source§

### impl<K, V> Default for BTreeMap<K, V>

source§

#### fn default() -> BTreeMap<K, V>

Creates an empty `BTreeMap`.

1.7.0 · source§

### impl<K, V, A: Allocator + Clone> Drop for BTreeMap<K, V, A>

source§

#### fn drop(&mut self)

Executes the destructor for this type. Read more
1.2.0 · source§

### impl<'a, K: Ord + Copy, V: Copy, A: Allocator + Clone> Extend<(&'a K, &'a V)> for BTreeMap<K, V, A>

source§

#### fn extend<I: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: I)

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

#### fn extend_one(&mut self, (k, v): (&'a K, &'a V))

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Extends a collection with exactly one element.
source§

#### fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Reserves capacity in a collection for the given number of additional elements. Read more
1.0.0 · source§

### impl<K: Ord, V, A: Allocator + Clone> Extend<(K, V)> for BTreeMap<K, V, A>

source§

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

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

#### fn extend_one(&mut self, (k, v): (K, V))

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Extends a collection with exactly one element.
source§

#### fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (`extend_one` #72631)
Reserves capacity in a collection for the given number of additional elements. Read more
1.56.0 · source§

### impl<K: Ord, V, const N: usize> From<[(K, V); N]> for BTreeMap<K, V>

source§

#### fn from(arr: [(K, V); N]) -> Self

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

    use std::collections::BTreeMap;

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

Run
1.0.0 · source§

### impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V>

source§

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

Creates a value from an iterator. Read more
1.0.0 · source§

### impl<K: Hash, V: Hash, A: Allocator + Clone> Hash for BTreeMap<K, V, A>

source§

#### fn hash<H: Hasher>(&self, state: &mut H)

Feeds this value into the given `Hasher`. Read more
1.3.0 · source§

#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
1.0.0 · source§

### impl<K, Q, V, A: Allocator + Clone> Index<&Q> for BTreeMap<K, V, A>
where K: Borrow<Q> + Ord, Q: Ord + ?Sized,

source§

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

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

##### §Panics

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

§

#### type Output = V

The returned type after indexing.
1.0.0 · source§

### impl<'a, K, V, A: Allocator + Clone> IntoIterator for &'a BTreeMap<K, V, A>

§

#### 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?
source§

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

Creates an iterator from a value. Read more
1.0.0 · source§

### impl<'a, K, V, A: Allocator + Clone> IntoIterator for &'a mut BTreeMap<K, V, A>

§

#### 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?
source§

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

Creates an iterator from a value. Read more
1.0.0 · source§

### impl<K, V, A: Allocator + Clone> IntoIterator for BTreeMap<K, V, A>

source§

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

Gets an owning iterator over the entries of the map, sorted by key.

§

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

The type of the elements being iterated over.
§

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

Which kind of iterator are we turning this into?
1.0.0 · source§

### impl<K: Ord, V: Ord, A: Allocator + Clone> Ord for BTreeMap<K, V, A>

source§

#### fn cmp(&self, other: &BTreeMap<K, V, A>) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
1.21.0 · source§

#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
1.21.0 · source§

#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
1.50.0 · source§

#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized + PartialOrd,

Restrict a value to a certain interval. Read more
1.0.0 · source§

### impl<K: PartialEq, V: PartialEq, A: Allocator + Clone> PartialEq for BTreeMap<K, V, A>

source§

#### fn eq(&self, other: &BTreeMap<K, V, A>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`.
1.0.0 · source§

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

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
1.0.0 · source§

### impl<K: PartialOrd, V: PartialOrd, A: Allocator + Clone> PartialOrd for BTreeMap<K, V, A>

source§

#### fn partial_cmp(&self, other: &BTreeMap<K, V, A>) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
1.0.0 · source§

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

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
1.0.0 · source§

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

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
1.0.0 · source§

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

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
1.0.0 · source§

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

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
1.0.0 · source§

### impl<K: Eq, V: Eq, A: Allocator + Clone> Eq for BTreeMap<K, V, A>

1.64.0 · source§

### impl<K, V, A> UnwindSafe for BTreeMap<K, V, A>
where A: UnwindSafe + Allocator + Clone, K: RefUnwindSafe, V: RefUnwindSafe,

## Auto Trait Implementations§

§

### impl<K, V, A> Freeze for BTreeMap<K, V, A>
where A: Freeze,

§

### impl<K, V, A> RefUnwindSafe for BTreeMap<K, V, A>
where A: RefUnwindSafe, K: RefUnwindSafe, V: RefUnwindSafe,

§

### impl<K, V, A> Send for BTreeMap<K, V, A>
where A: Send, K: Send, V: Send,

§

### impl<K, V, A> Sync for BTreeMap<K, V, A>
where A: Sync, K: Sync, V: Sync,

§

### impl<K, V, A> Unpin for BTreeMap<K, V, A>
where A: Unpin,

## Blanket Implementations§

source§

### impl<T> Any for T
where T: 'static + ?Sized,

source§

#### fn type_id(&self) -> TypeId

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

### impl<T> Borrow<T> for T
where T: ?Sized,

source§

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

### impl<T> BorrowMut<T> for T
where T: ?Sized,

source§

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source§

### impl<T> From<T> for T

source§

#### fn from(t: T) -> T

Returns the argument unchanged.

source§

### impl<T, U> Into<U> for T
where U: From<T>,

source§

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

source§

### impl<T> ToOwned for T
where T: Clone,

§

#### type Owned = T

The resulting type after obtaining ownership.
source§

#### fn to_owned(&self) -> T

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

#### fn clone_into(&self, target: &mut T)

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

### impl<T, U> TryFrom<U> for T
where U: Into<T>,

§

#### type Error = Infallible

The type returned in the event of a conversion error.
source§

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

Performs the conversion.
source§

### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

§

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

The type returned in the event of a conversion error.
source§

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

Performs the conversion.