详解Rust标准库：BTreeMap

std::collections::BTreeMap定义

B树也称B-树，注意不是减号，是一棵多路平衡查找树；理论上，二叉搜索树 （BST） 是最佳的选择排序映射，但是每次查找时层数越多I/O次数越多，B 树使每个节点都包含连续数组中的 B-1 到 2B-1 元素，可以减少树的高度，减少I/O次数

BTreeMap定义

pub struct BTreeMap<K, V, A: Allocator + Clone = Global> {// B 树的根节点root: Option<Root<K, V>>,// B 树映射中存储的键值对的数量length: usize,// 分配器pub(super) alloc: ManuallyDrop<A>,// PhantomData是一个零大小的类型，用于向编译器提供类型信息，但在运行时不占用任何空间_marker: PhantomData<crate::boxed::Box<(K, V), A>>,
}

方法

clear：用于清空BTreeMap，移除所有的键值对

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::new();map.insert("a", 1);map.insert("b", 2);map.clear();println!("After clear, is empty? {}", map.is_empty());// After clear, is empty? true
}

get：用于获取指定键对应的值的不可变引用，如果键不存在则返回None

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);if let Some(value) = map.get("a") {println!("Value for 'a': {}", value);// Value for 'a': 1} else {println!("'a' not found.");}
}

get_key_value：返回指定键对应的值和键的不可变引用，如果键不存在则返回None

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);if let Some((key, value)) = map.get_key_value("b") {println!("Key-value pair for 'b': {} -> {}", key, value);// Key-value pair for 'b': b -> 2} else {println!("'b' not found.");}
}

first_key_value：返回BTreeMap中的第一个键值对，如果地图为空则返回None

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);if let Some((first_key, first_value)) = map.first_key_value() {println!("First key-value pair: {} -> {}", first_key, first_value);// First key-value pair: a -> 1} else {println!("Map is empty.");}
}

first_entry：返回一个可变引用到BTreeMap中的第一个键值对，如果地图为空则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some(first_entry) = map.first_entry() {println!("First entry: {} -> {}", first_entry.key(), first_entry.get());// First entry: a -> 1} else {println!("Map is empty.");}
}

pop_first：移除并返回BTreeMap中的第一个键值对，如果地图为空则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some((popped_key, popped_value)) = map.pop_first() {println!("Popped first pair: {} -> {}", popped_key, popped_value);// Popped first pair: a -> 1} else {println!("Map is empty.");}
}

pop_last：移除并返回BTreeMap中的最后一个键值对，如果地图为空则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some((last_key, last_value)) = map.pop_last() {println!("Popped last pair: {} -> {}", last_key, last_value);// Popped last pair: b -> 2} else {println!("Map is empty.");}
}

contains_key：判断BTreeMap中是否存在指定的键

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);println!("Contains 'a'? {}", map.contains_key("a"));// Contains 'a'? true
}

get_mut：返回指定键对应的值的可变引用，如果键不存在则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some(value) = map.get_mut("a") {*value = 3;println!("Modified value for 'a': {}", value);// Modified value for 'a': 3} else {println!("'a' not found.");}
}

insert：插入一个键值对到BTreeMap中，如果键已经存在，则覆盖旧值

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::new();map.insert("a", 1);map.insert("a", 2);println!("After insert: {:?}", map);// After insert: {"a": 2}
}

remove：移除指定键对应的键值对，并返回被移除的值，如果键不存在则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some(removed_value) = map.remove("a") {println!("Removed value for 'a': {}", removed_value);// Removed value for 'a': 1} else {println!("'a' not found.");}
}

remove_entry：移除指定键对应的键值对，并返回被移除的键值对，如果键不存在则返回None

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);if let Some((removed_key, removed_value)) = map.remove_entry("b") {println!("Removed entry: {} -> {}", removed_key, removed_value);// Removed entry: b -> 2} else {println!("'b' not found.");}
}

retain：保留满足给定谓词的键值对

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),("c", 3),]);map.retain(|key, value| (*key > "b" || *value >= 2));println!("After retain: {:?}", map);// After retain: {"b": 2, "c": 3}
}

append：将另一个BTreeMap的键值对追加到当前BTreeMap中

use std::collections::BTreeMap;fn main() {let mut map1 = BTreeMap::from([("a", 1),("b", 2),]);let mut map2 = BTreeMap::from([("c", 3),("d", 4),]);map1.append(&mut map2);println!("After append: {:?}", map1);// After append: {"a": 1, "b": 2, "c": 3, "d": 4}
}

range：返回一个迭代器，遍历BTreeMap中满足给定范围条件的键值对

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),("c", 3),("d", 4),]);for (key, value) in map.range("b"..) {println!("In range: {} -> {}", key, value);}// In range: b -> 2// In range: c -> 3// In range: d -> 4
}

range_mut：返回一个可变迭代器，遍历BTreeMap中满足给定范围条件的键值对，并允许修改值

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),("c", 3),("d", 4),]);for (key, value) in map.range_mut("b"..) {*value = *value * 2;println!("Modified in range: {} -> {}", key, value);}// Modified in range: b -> 4// Modified in range: c -> 6// Modified in range: d -> 8
}

entry：返回一个Entry API，可以用于插入或更新键值对

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::new();let entry = map.entry("a").or_insert(1);println!("Entry value for 'a': {}", entry);// Entry value for 'a': 1
}

split_off：移除并返回给定键及其对应的值，如果键不存在则返回None和一个空BTreeMap

into_keys：将BTreeMap的键提取为一个可迭代的集合

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);let keys: Vec<_> = map.into_keys().collect();println!("Keys: {:?}", keys);// Keys: ["a", "b"]
}

into_values：将BTreeMap的值提取为一个可迭代的集合

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);let values: Vec<_> = map.into_values().collect();println!("Values: {:?}", values);// Values: [1, 2]
}

iter：返回一个迭代器，遍历BTreeMap的键值对

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);for (key, value) in map.iter() {println!("Iter: {} -> {}", key, value);}// Iter: a -> 1// Iter: b -> 2
}

iter_mut：返回一个可变迭代器，允许修改BTreeMap的键值对

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);for (key, value) in map.iter_mut() {*value = *value * 2;println!("Mut iter: {} -> {}", key, value);}// Mut iter: a -> 2// Mut iter: b -> 4
}

keys：返回一个迭代器，遍历BTreeMap的键

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);for key in map.keys() {println!("Key from keys: {}", key);}// Key from keys: a// Key from keys: b
}

values：返回一个迭代器，遍历BTreeMap的值

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);for value in map.values() {println!("Value from values: {}", value);}// Value from values: 1// Value from values: 2
}

values_mut：返回一个可变迭代器，允许修改BTreeMap的值

use std::collections::BTreeMap;fn main() {let mut map = BTreeMap::from([("a", 1),("b", 2),]);for value in map.values_mut() {*value = *value * 3;println!("Mut value from values_mut: {}", value);}// Mut value from values_mut: 3// Mut value from values_mut: 6
}

len：返回BTreeMap中键值对的数量

use std::collections::BTreeMap;fn main() {let map = BTreeMap::from([("a", 1),("b", 2),]);println!("Length: {}", map.len());// Length: 2
}

is_empty：判断BTreeMap是否为空

use std::collections::BTreeMap;fn main() {let map: BTreeMap<i32, &str> = BTreeMap::new();println!("Is empty: {}", map.is_empty());// Is empty: true
}