Struct std::collections::BTreeMap
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pub struct BTreeMap<K, V> {
// some 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.
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.
fn with_b(b: usize) -> BTreeMap<K, V>
: niche API
Makes a new empty BTreeMap with the given B.
B cannot be less than 2.
fn clear(&mut self)
Clears the map, removing all values.
Examples
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
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 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
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
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 key is not updated, the value is updated and the old value is returned. See the module-level documentation for more.
Examples
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
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);
impl<K, V> BTreeMap<K, V>
fn iter(&self) -> Iter<K, V>
Gets an iterator over the entries of the map.
Examples
fn main() { use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "a"); map.insert(2, "b"); map.insert(3, "c"); 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(1, "a"); map.insert(2, "b"); map.insert(3, "c"); 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.
Examples
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.
Examples
fn main() { use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "a"); a.insert(2, "b"); let keys: Vec<_> = a.keys().cloned().collect(); assert_eq!(keys, [1, 2]); }use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "a"); a.insert(2, "b"); 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.
Examples
fn main() { use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "a"); a.insert(2, "b"); let values: Vec<&str> = a.values().cloned().collect(); assert_eq!(values, ["a", "b"]); }use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "a"); a.insert(2, "b"); let values: Vec<&str> = a.values().cloned().collect(); assert_eq!(values, ["a", "b"]);
fn len(&self) -> usize
Returns the number of elements in the map.
Examples
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
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 range<Min = K, Max = K>(&self, min: Bound<&Min>, max: Bound<&Max>) -> Range<K, V> where K: Borrow<Min> + Borrow<Max>, Max: Ord + ?Sized, Min: Ord + ?Sized
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
#![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 = K, Max = K>(&mut self, min: Bound<&Min>, max: Bound<&Max>) -> RangeMut<K, V> where K: Borrow<Min> + Borrow<Max>, Max: Ord + ?Sized, Min: Ord + ?Sized
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
#![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
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);