collections::btree_map::BTreeMap - Rust
Description: API documentation for the Rust `BTreeMap` struct in crate `collections`.
Keywords: rust, rustlang, rust-lang, BTreeMap

# Struct collections::btree_map::BTreeMap 1.0.0 [−] [src]

    pub struct BTreeMap<K, V> { /* fields omitted */ }

A 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 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 O(B logBn) 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.

# 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 alot.");

    // 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 book in &to_find {
        match movie_reviews.get(book) {
           Some(review) => println!("{}: {}", book, review),
           None => println!("{} is unreviewed.", book)
        }
    }

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

`BTreeMap` also implements an `Entry API`, which allows for more 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();Run

## Methods

### `impl<K: Ord, V> BTreeMap<K, V>`
[src]

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

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

# Examples

Basic usage:

    use std::collections::BTreeMap;

    let mut map = BTreeMap::new();

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

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

Clears the map, removing all values.

# Examples

Basic usage:

    use std::collections::BTreeMap;

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

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

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

Basic usage:

    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);Run

#### `fn contains_key<Q: ?Sized>(&self, key: &Q) -> bool where K: Borrow<Q>, Q: Ord`

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

Basic usage:

    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

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

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

Basic usage:

    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

#### `fn insert(&mut self, key: K, value: 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

Basic usage:

    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

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

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

Basic usage:

    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

#### `fn append(&mut self, other: &mut Self)`
1.11.0

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

# Examples

    use std::collections::BTreeMap;

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

    let mut b = BTreeMap::new();
    b.insert(3, "d");
    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");
    assert_eq!(a[&4], "e");
    assert_eq!(a[&5], "f");Run

#### `fn range<T: ?Sized, R>(&self, range: R) -> Range<K, V> where T: Ord, K: Borrow<T>, R: RangeArgument<T>`

🔬 This is a nightly-only experimental API. (`btree_range` #27787)

matches collection reform specification, waiting for dust to settle

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.

# Examples

Basic usage:

    #![feature(btree_range, collections_bound)]

    use std::collections::BTreeMap;
    use std::collections::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());Run

#### `fn range_mut<T: ?Sized, R>(&mut self, range: R) -> RangeMut<K, V> where T: Ord, K: Borrow<T>, R: RangeArgument<T>`

🔬 This is a nightly-only experimental API. (`btree_range` #27787)

matches collection reform specification, waiting for dust to settle

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.

# Examples

Basic usage:

    #![feature(btree_range)]

    use std::collections::BTreeMap;

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

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

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

# Examples

Basic usage:

    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 vec!["a","b","a","c","a","b"] {
        *count.entry(x).or_insert(0) += 1;
    }

    assert_eq!(count["a"], 3);Run

#### `fn split_off<Q: ?Sized + Ord>(&mut self, key: &Q) -> Self where K: Borrow<Q>`
1.11.0

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

# Examples

Basic usage:

    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");Run

### `impl<K, V> BTreeMap<K, V>`
[src]

#### `fn iter(&self) -> Iter<K, V>`

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

# Examples

Basic usage:

    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"));Run

#### `fn iter_mut(&mut self) -> IterMut<K, V>`

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

# Examples

Basic usage:

    use std::collections::BTreeMap;

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

#### `fn keys<'a>(&'a self) -> Keys<'a, K, V>`

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

# Examples

Basic usage:

    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]);Run

#### `fn values<'a>(&'a self) -> Values<'a, K, V>`

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

# Examples

Basic usage:

    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"]);Run

#### `fn values_mut(&mut self) -> ValuesMut<K, V>`
1.10.0

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

# Examples

Basic usage:

    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

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

Returns the number of elements in the map.

# Examples

Basic usage:

    use std::collections::BTreeMap;

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

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

Returns true if the map contains no elements.

# Examples

Basic usage:

    use std::collections::BTreeMap;

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

## Trait Implementations

### `impl<K, V> Drop for BTreeMap<K, V>`
1.7.0
[src]

#### `fn drop(&mut self)`

A method called when the value goes out of scope. Read more

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

### `impl<'a, K: 'a, V: 'a> IntoIterator for &'a BTreeMap<K, V>`
[src]

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

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates an iterator from a value. Read more

### `impl<'a, K: 'a, V: 'a> IntoIterator for &'a mut BTreeMap<K, V>`
[src]

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

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates an iterator from a value. Read more

### `impl<K, V> IntoIterator for BTreeMap<K, V>`
[src]

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

The type of the elements being iterated over.

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

Which kind of iterator are we turning this into?

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

Creates an iterator from a value. Read more

### `impl<K: Ord, V> FromIterator<(K, V)> for BTreeMap<K, V>`
[src]

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

Creates a value from an iterator. Read more

### `impl<K: Ord, V> Extend<(K, V)> for BTreeMap<K, V>`
[src]

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

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

### `impl<'a, K: Ord + Copy, V: Copy> Extend<(&'a K, &'a V)> for BTreeMap<K, V>`
1.2.0
[src]

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

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

### `impl<K: Hash, V: Hash> Hash for BTreeMap<K, V>`
[src]

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

Feeds this value into the state given, updating the hasher as necessary.

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher`
1.3.0

Feeds a slice of this type into the state provided.

### `impl<K: Ord, V> Default for BTreeMap<K, V>`
[src]

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

Creates an empty `BTreeMap<K, V>`.

### `impl<K: PartialEq, V: PartialEq> PartialEq for BTreeMap<K, V>`
[src]

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

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

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

This method tests for `!=`.

### `impl<K: Eq, V: Eq> Eq for BTreeMap<K, V>`
[src]

### `impl<K: PartialOrd, V: PartialOrd> PartialOrd for BTreeMap<K, V>`
[src]

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

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

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

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

#### `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

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

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

#### `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

### `impl<K: Ord, V: Ord> Ord for BTreeMap<K, V>`
[src]

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

This method returns an `Ordering` between `self` and `other`. Read more

### `impl<K: Debug, V: Debug> Debug for BTreeMap<K, V>`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

### `impl<'a, K: Ord, Q: ?Sized, V> Index<&'a Q> for BTreeMap<K, V> where K: Borrow<Q>,
Q: Ord`
[src]

#### `type Output = V`

The returned type after indexing

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

The method for the indexing (`container[index]`) operation

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