Struct std::collections::btree_map::BTreeMap 1.0.0[−][src]
pub struct BTreeMap<K, V> { /* fields omitted */ }
Expand description
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 * log(n)) 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.
The behavior resulting from such a logic error is not specified, but will not result in
undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and
non-termination.
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 a lot."); // 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 movie in &to_find { match movie_reviews.get(movie) { Some(review) => println!("{}: {}", movie, review), None => println!("{} is unreviewed.", movie) } } // Look up the value for a key (will panic if the key is not found). println!("Movie review: {}", movie_reviews["Office Space"]); // 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
Implementations
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
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
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);Run
Returns the first key-value pair in the map. The key in this pair is the minimum key in the map.
Examples
Basic usage:
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); assert_eq!(map.first_key_value(), None); map.insert(1, "b"); map.insert(2, "a"); assert_eq!(map.first_key_value(), Some((&1, &"b")));Run
Returns the first entry in the map for in-place manipulation. The key of this entry is the minimum key in the map.
Examples
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "a"); map.insert(2, "b"); if let Some(mut entry) = map.first_entry() { if *entry.key() > 0 { entry.insert("first"); } } assert_eq!(*map.get(&1).unwrap(), "first"); assert_eq!(*map.get(&2).unwrap(), "b");Run
Removes and returns the first element in the map. The key of this element is the minimum key that was in the map.
Examples
Draining elements in ascending order, while keeping a usable map each iteration.
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "a"); map.insert(2, "b"); while let Some((key, _val)) = map.pop_first() { assert!(map.iter().all(|(k, _v)| *k > key)); } assert!(map.is_empty());Run
Returns the last key-value pair in the map. The key in this pair is the maximum key in the map.
Examples
Basic usage:
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "b"); map.insert(2, "a"); assert_eq!(map.last_key_value(), Some((&2, &"a")));Run
Returns the last entry in the map for in-place manipulation. The key of this entry is the maximum key in the map.
Examples
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "a"); map.insert(2, "b"); if let Some(mut entry) = map.last_entry() { if *entry.key() > 0 { entry.insert("last"); } } assert_eq!(*map.get(&1).unwrap(), "a"); assert_eq!(*map.get(&2).unwrap(), "last");Run
Removes and returns the last element in the map. The key of this element is the maximum key that was in the map.
Examples
Draining elements in descending order, while keeping a usable map each iteration.
#![feature(map_first_last)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); map.insert(1, "a"); map.insert(2, "b"); while let Some((key, _val)) = map.pop_last() { assert!(map.iter().all(|(k, _v)| *k < key)); } assert!(map.is_empty());Run
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
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
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
pub fn try_insert(
&mut self,
key: K,
value: V
) -> Result<&mut V, OccupiedError<'_, K, V>> where
K: Ord,
[src]
pub fn try_insert(
&mut self,
key: K,
value: V
) -> Result<&mut V, OccupiedError<'_, K, V>> where
K: Ord,
[src]Tries to insert a key-value pair into the map, and returns a mutable reference to the value in the entry.
If the map already had this key present, nothing is updated, and an error containing the occupied entry and the value is returned.
Examples
Basic usage:
#![feature(map_try_insert)] use std::collections::BTreeMap; let mut map = BTreeMap::new(); assert_eq!(map.try_insert(37, "a").unwrap(), &"a"); let err = map.try_insert(37, "b").unwrap_err(); assert_eq!(err.entry.key(), &37); assert_eq!(err.entry.get(), &"a"); assert_eq!(err.value, "b");Run
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
Removes a key from the map, returning the stored key and value 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_entry(&1), Some((1, "a"))); assert_eq!(map.remove_entry(&1), None);Run
Retains only the elements specified by the predicate.
In other words, remove all pairs (k, v)
such that f(&k, &mut v)
returns false
.
Examples
use std::collections::BTreeMap; let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x*10)).collect(); // Keep only the elements with even-numbered keys. map.retain(|&k, _| k % 2 == 0); assert!(map.into_iter().eq(vec![(0, 0), (2, 20), (4, 40), (6, 60)]));Run
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
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());Run
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); }Run
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
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
pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, K, V, F>ⓘNotable traits for DrainFilter<'_, K, V, F>impl<'_, K, V, F> Iterator for DrainFilter<'_, K, V, F> where
F: FnMut(&K, &mut V) -> bool, type Item = (K, V);
where
F: FnMut(&K, &mut V) -> bool,
K: Ord,
[src]
pub fn drain_filter<F>(&mut self, pred: F) -> DrainFilter<'_, K, V, F>ⓘNotable traits for DrainFilter<'_, K, V, F>impl<'_, K, V, F> Iterator for DrainFilter<'_, K, V, F> where
F: FnMut(&K, &mut V) -> bool, type Item = (K, V);
where
F: FnMut(&K, &mut V) -> bool,
K: Ord,
[src]impl<'_, K, V, F> Iterator for DrainFilter<'_, K, V, F> where
F: FnMut(&K, &mut V) -> bool, type Item = (K, V);
Creates an iterator that visits all elements (key-value pairs) in
ascending key order and uses a closure to determine if an element should
be removed. If the closure returns true
, the element is removed from
the map and yielded. If the closure returns false
, or panics, the
element remains in the map and will not be yielded.
The iterator also lets you mutate the value of each element in the closure, regardless of whether you choose to keep or remove it.
If the iterator is only partially consumed or not consumed at all, each
of the remaining elements is still subjected to the closure, which may
change its value and, by returning true
, have the element removed and
dropped.
It is unspecified how many more elements will be subjected to the
closure if a panic occurs in the closure, or a panic occurs while
dropping an element, or if the DrainFilter
value is leaked.
Examples
Splitting a map into even and odd keys, reusing the original map:
#![feature(btree_drain_filter)] use std::collections::BTreeMap; let mut map: BTreeMap<i32, i32> = (0..8).map(|x| (x, x)).collect(); let evens: BTreeMap<_, _> = map.drain_filter(|k, _v| k % 2 == 0).collect(); let odds = map; assert_eq!(evens.keys().copied().collect::<Vec<_>>(), vec![0, 2, 4, 6]); assert_eq!(odds.keys().copied().collect::<Vec<_>>(), vec![1, 3, 5, 7]);Run
Creates a consuming iterator visiting all the keys, in sorted order.
The map cannot be used after calling this.
The iterator element type is K
.
Examples
use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(2, "b"); a.insert(1, "a"); let keys: Vec<i32> = a.into_keys().collect(); assert_eq!(keys, [1, 2]);Run
pub fn into_values(self) -> IntoValues<K, V>ⓘNotable traits for IntoValues<K, V>impl<K, V> Iterator for IntoValues<K, V> type Item = V;
1.54.0[src]
pub fn into_values(self) -> IntoValues<K, V>ⓘNotable traits for IntoValues<K, V>impl<K, V> Iterator for IntoValues<K, V> type Item = V;
1.54.0[src]impl<K, V> Iterator for IntoValues<K, V> type Item = V;
Creates a consuming iterator visiting all the values, in order by key.
The map cannot be used after calling this.
The iterator element type is V
.
Examples
use std::collections::BTreeMap; let mut a = BTreeMap::new(); a.insert(1, "hello"); a.insert(2, "goodbye"); let values: Vec<&str> = a.into_values().collect(); assert_eq!(values, ["hello", "goodbye"]);Run
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
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
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
Trait Implementations
impl<K, V> PartialOrd<BTreeMap<K, V>> for BTreeMap<K, V> where
K: PartialOrd<K>,
V: PartialOrd<V>,
[src]
impl<K, V> PartialOrd<BTreeMap<K, V>> for BTreeMap<K, V> where
K: PartialOrd<K>,
V: PartialOrd<V>,
[src]This method returns an ordering between self
and other
values if one exists. Read more
This method tests less than (for self
and other
) and is used by the <
operator. Read more
This method tests less than or equal to (for self
and other
) and is used by the <=
operator. Read more
This method tests greater than (for self
and other
) and is used by the >
operator. Read more
Auto Trait Implementations
impl<K, V> RefUnwindSafe for BTreeMap<K, V> where
K: RefUnwindSafe,
V: RefUnwindSafe,
impl<K, V> UnwindSafe for BTreeMap<K, V> where
K: RefUnwindSafe,
V: RefUnwindSafe,
Blanket Implementations
Mutably borrows from an owned value. Read more