Struct std::collections::btree_map::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

fn main() { 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); } }
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:

fn main() { 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(); }
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>

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

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

Examples

Basic usage:

fn main() { 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")); }
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:

fn main() { 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; } } }
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:

fn main() { 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]); }
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:

fn main() { 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"]); }
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 #32551)

: recently added

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

Examples

Basic usage:

#![feature(map_values_mut)] fn main() { 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!")]); }
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:

fn main() { use std::collections::BTreeMap; let mut a = BTreeMap::new(); assert_eq!(a.len(), 0); a.insert(1, "a"); assert_eq!(a.len(), 1); }
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:

fn main() { use std::collections::BTreeMap; let mut a = BTreeMap::new(); assert!(a.is_empty()); a.insert(1, "a"); assert!(!a.is_empty()); }
use std::collections::BTreeMap;

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

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:

fn main() { use std::collections::BTreeMap; let mut map = BTreeMap::new(); // entries can now be inserted into the empty map map.insert(1, "a"); }
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:

fn main() { use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "a"); a.clear(); assert!(a.is_empty()); }
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:

fn main() { 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); }
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 Q: Ord + ?Sized, K: Borrow<Q>

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:

fn main() { 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); }
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:

fn main() { 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"); }
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:

fn main() { 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"); }
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 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:

fn main() { 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); }
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 #19986)

: recently added as part of collections reform 2

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

Examples

#![feature(btree_append)] fn main() { 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"); }
#![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 K: Borrow<Min> + Borrow<Max>, Max: Ord + ?Sized, Min: Ord + ?Sized

Unstable (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, 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)] fn main() { 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()); }
#![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, Max: Ord + ?Sized, K: Borrow<Min> + Borrow<Max>

Unstable (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, 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)] fn main() { 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); } }
#![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:

fn main() { 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); }
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);

Trait Implementations

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 #27783)

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

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

Unstable (collections #27783)

: 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 K: Debug, V: Debug

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

Unstable (collections #27783)

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

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

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

Unstable (collections #27783)

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

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

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

Unstable (collections #27783)

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

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

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

Unstable (collections #27783)

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

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

Unstable (collections #27783)

: 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 #27783)

: 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: Hasher1.3.0

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

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

Unstable (collections #27783)

: 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 #27783)

: 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 #27783)

: 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 #27783)

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

type IntoIter = IntoIter<K, V>

Unstable (collections #27783)

: 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 #27783)

: 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 V: Clone, K: Clone

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

Unstable (collections #27783)

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

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

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

fn drop(&mut self)

Unstable (collections #27783)

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