Struct std::collections::BTreeMap

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 i^th 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 log_Bn) 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);
}

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,

pub 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");

pub 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());

pub fn get<Q>(&self, key: &Q) -> Option<&V> where     K: Borrow<Q>,     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

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);

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

🔬 This is a nightly-only experimental API. (map_get_key_value #49347)

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

#![feature(map_get_key_value)]
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);

pub 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);

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

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");

pub 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");

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

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);

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

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");

Important traits for Range<'a, K, V>

impl<'a, K, V> Iterator for Range<'a, K, V> type Item = (&'a K, &'a V);

pub fn range<T, R>(&self, range: R) -> Range<K, V> where     K: Borrow<T>,     R: RangeBounds<T>,     T: Ord + ?Sized,

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

Basic usage:

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());

Important traits for RangeMut<'a, K, V>

impl<'a, K, V> Iterator for RangeMut<'a, K, V> type Item = (&'a K, &'a mut V);

pub fn range_mut<T, R>(&mut self, range: R) -> RangeMut<K, V> where     K: Borrow<T>,     R: RangeBounds<T>,     T: Ord + ?Sized,

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

Basic usage:

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);
}

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

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);

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

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");

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

Important traits for Iter<'a, K, V>

impl<'a, K, V> Iterator for Iter<'a, K, V> where
    K: 'a,
    V: 'a,  type Item = (&'a K, &'a V);

pub 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"));

Important traits for IterMut<'a, K, V>

impl<'a, K, V> Iterator for IterMut<'a, K, V> where
    K: 'a,
    V: 'a,  type Item = (&'a K, &'a mut V);

pub 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;
    }
}

Important traits for Keys<'a, K, V>

impl<'a, K, V> Iterator for Keys<'a, K, V> type Item = &'a K;

pub 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]);

Important traits for Values<'a, K, V>

impl<'a, K, V> Iterator for Values<'a, K, V> type Item = &'a V;

pub 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"]);

Important traits for ValuesMut<'a, K, V>

impl<'a, K, V> Iterator for ValuesMut<'a, K, V> type Item = &'a mut V;

pub fn values_mut(&mut self) -> ValuesMut<K, V>

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!")]);

pub 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);

pub 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)

Executes the destructor for this type.

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

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

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

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

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

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

Feeds this value into the given [Hasher].

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

Feeds a slice of this type into the given [Hasher].

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)>,

Creates a value from an iterator.

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

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

Returns a copy of the value.

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

Performs copy-assignment from source.

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

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

Formats the value using the given formatter.

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

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?

Important traits for IntoIter<K, V>

impl<K, V> Iterator for IntoIter<K, V> type Item = (K, V);

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

Creates an iterator from a value.

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

Important traits for IterMut<'a, K, V>

impl<'a, K, V> Iterator for IterMut<'a, K, V> where
    K: 'a,
    V: 'a,  type Item = (&'a K, &'a mut V);

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

Creates an iterator from a value.

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

Important traits for Iter<'a, K, V>

impl<'a, K, V> Iterator for Iter<'a, K, V> where
    K: 'a,
    V: 'a,  type Item = (&'a K, &'a V);

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

Creates an iterator from a value.

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)>,

Extends a collection with the contents of an iterator.

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)>,

Extends a collection with the contents of an iterator.

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

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

Creates an empty BTreeMap<K, V>.

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

type Output = V

The returned type after indexing.

Important traits for &'a mut I

impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W

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.

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

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

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

This method tests for !=.

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>

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

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

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

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

This method tests less than or equal to (for self and other) and is used by the <= operator.

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

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

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

This method tests greater than or equal to (for self and other) and is used by the >= operator.