# Struct bitflags::__core::collections::BTreeMap [−] [src]

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

1.0.0

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 books.
    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);
    }

`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();

## Methods

### `impl<K, V> BTreeMap<K, V> where K: Ord`

#### `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");

#### `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());

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

Returns a reference to the value corresponding to the key.

The key may be any borrowed form of the map's key type, but 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);

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

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);

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

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

The key may be any borrowed form of the map's key type, but 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");

#### `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");

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

Removes a key from the map, returning the value at the key if the key was previously in the map.

The key may be any borrowed form of the map's key type, but 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);

#### `fn append(&mut self, other: &mut BTreeMap<K, V>)`

Unstable (`btree_append`)

: recently added as part of collections reform 2

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

# Examples

    #![feature(btree_append)]
    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");

#### `fn range<Min, Max>(&self, min: Bound<&Min>, max: Bound<&Max>) -> Range<K, V> where Min: Ord + ?Sized, K: Borrow<Min> + Borrow<Max>, Max: Ord + ?Sized`

Unstable (`btree_range`)

: matches collection reform specification, waiting for dust to settle

Constructs a double-ended iterator over a sub-range of elements in the map, starting at min, and ending at max. If min is `Unbounded`, then it will be treated as "negative infinity", and if max is `Unbounded`, then it will be treated as "positive infinity". Thus range(Unbounded, Unbounded) will yield the whole collection.

# Examples

Basic usage:

    #![feature(btree_range, collections_bound)]

    use std::collections::BTreeMap;
    use std::collections::Bound::{Included, Unbounded};

    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(Included(&4), Unbounded).next());

#### `fn range_mut<Min, Max>(&mut self, min: Bound<&Min>, max: Bound<&Max>) -> RangeMut<K, V> where Min: Ord + ?Sized, K: Borrow<Min> + Borrow<Max>, Max: Ord + ?Sized`

Unstable (`btree_range`)

: matches collection reform specification, waiting for dust to settle

Constructs a mutable double-ended iterator over a sub-range of elements in the map, starting at min, and ending at max. If min is `Unbounded`, then it will be treated as "negative infinity", and if max is `Unbounded`, then it will be treated as "positive infinity". Thus range(Unbounded, Unbounded) will yield the whole collection.

# Examples

Basic usage:

    #![feature(btree_range, collections_bound)]

    use std::collections::BTreeMap;
    use std::collections::Bound::{Included, Excluded};

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

#### `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);

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

#### `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"));

#### `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;
        }
    }

#### `fn keys(&'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]);

#### `fn values(&'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"]);

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

Unstable (`map_values_mut`)

: recently added

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!")]);

#### `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);

#### `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());

## Trait Implementations

### `impl<K, V> Drop for BTreeMap<K, V>`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> Clone for BTreeMap<K, V> where K: Clone, V: Clone`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

### `impl<'a, K, V> IntoIterator for &'a BTreeMap<K, V> where K: 'a, V: 'a`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> IntoIterator for BTreeMap<K, V>`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> Extend<(K, V)> for BTreeMap<K, V> where K: Ord`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> Hash for BTreeMap<K, V> where K: Hash, V: Hash`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

### `impl<K, V> Default for BTreeMap<K, V> where K: Ord`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> PartialEq<BTreeMap<K, V>> for BTreeMap<K, V> where K: PartialEq<K>, V: PartialEq<V>`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

### `impl<K, V> Eq for BTreeMap<K, V> where K: Eq, V: Eq`

### `impl<K, V> PartialOrd<BTreeMap<K, V>> for BTreeMap<K, V> where K: PartialOrd<K>, V: PartialOrd<V>`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

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

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

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

### `impl<K, V> Ord for BTreeMap<K, V> where K: Ord, V: Ord`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

### `impl<K, V> Debug for BTreeMap<K, V> where V: Debug, K: Debug`

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

#### `type Output = V`

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

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

Unstable (`collections`)

: library is unlikely to be stabilized with the current layout and name, use std::collections instead

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)