C++ STL unordered_set 与 unordered_map的基本用法完全指南

#include <unordered_set>
#include <unordered_map>

template < class Key,                        // unordered_set::key_type/value_type
           class Hash = hash<Key>,           // unordered_set::hasher
           class Pred = equal_to<Key>,       // unordered_set::key_equal
           class Alloc = allocator<Key>      // unordered_set::allocator_type
           > class unordered_set;

unordered_set<int> s1;                 // 空set
unordered_set<int> s2 = {1, 2, 3, 4};  // 初始化列表
unordered_set<int> s3(s2.begin(), s2.end());  // 范围构造

unordered_set<int> uset = {1, 2, 3, 4, 5};
cout << "size: " << uset.size() << endl;      // 元素个数: 5
cout << "empty: " << uset.empty() << endl;    // 是否为空: 0(false)
cout << "max_size: " << uset.max_size() << endl;  // 可存储的最大数量

unordered_set<int> uset = {10, 20, 30, 40, 50};
// 遍历（无序，但通常按哈希桶顺序）
for(auto it = uset.begin(); it != uset.end(); ++it) {
    cout << *it << " ";
}
cout<<endl;
// 使用范围for循环
for(const auto& val : uset) {
    cout << val << " ";
}

unordered_set<string> uset = {"apple", "banana", "orange"};
// find - 返回迭代器，未找到则返回end()
auto it = uset.find("banana");
if(it != uset.end()) {
    cout << "Found: " << *it << endl;
}
// count - 返回元素个数（0或1）
if(uset.count("apple") > 0) {
    cout << "apple exists" << endl;
}

unordered_set<int> uset;
// insert - 插入元素
auto result = uset.insert(10);  // 返回pair<iterator, bool>
if(result.second) {
    cout << "Insert successful" << endl;
}
uset.insert({20, 30, 40});  // 插入多个元素
// emplace - 原地构造
uset.emplace(50);
// erase - 删除元素
uset.erase(20);  // 通过值删除
auto it = uset.find(30);
if(it != uset.end()) {
    uset.erase(it);  // 通过迭代器删除
}
uset.erase(uset.begin(), uset.end());  // 范围删除
// clear - 清空所有元素
uset.clear();

unordered_set<int> uset = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
cout << "bucket_count: " << uset.bucket_count() << endl;  // 桶的数量
cout << "max_bucket_count: " << uset.max_bucket_count() << endl;
cout << "load_factor: " << uset.load_factor() << endl;  // 负载因子
cout << "max_load_factor: " << uset.max_load_factor() << endl;  // 最大负载因子
// 遍历桶
for(size_t i = 0; i < uset.bucket_count(); ++i) {
    cout << "Bucket " << i << " has " << uset.bucket_size(i) << " elements" << endl;
}
// 查找元素所在的桶
int val = 5;
cout << val << " is in bucket " << uset.bucket(val) << endl;

unordered_set<int> uset;
// 设置最大负载因子
uset.max_load_factor(0.7f);
// 预分配桶的数量
uset.reserve(100);  // 预留至少100个元素的空间
// 重新哈希
uset.rehash(50);  // 设置桶的数量至少为50

template < class Key,                                    // unordered_map::key_type
           class T,                                      // unordered_map::mapped_type
           class Hash = hash<Key>,                       // unordered_map::hasher
           class Pred = equal_to<Key>,                   // unordered_map::key_equal
           class Alloc = allocator< pair<const Key,T> >  // unordered_map::allocator_type
           > class unordered_map;

// 定义unordered_map
unordered_map<string, int> m1;  // 空map
unordered_map<string, int> m2 = {
    {"apple", 1},
    {"banana", 2},
    {"orange", 3}
};

unordered_map<string, int> umap = {{"apple", 5}, {"banana", 3}};
// operator[] - 访问或插入元素
umap["apple"] = 10;     // 修改现有元素
umap["orange"] = 7;    // 插入新元素
int val = umap["apple"];  // 访问元素
// at - 访问元素，越界时抛出异常
try {
    int value = umap.at("banana");
} catch(const out_of_range& e) {
    cout << "Key not found" << endl;
}

unordered_map<string, int> umap = {{"apple", 1}, {"banana", 2}};
// find
auto it = umap.find("apple");
if(it != umap.end()) {
    cout << it->first << ": " << it->second << endl;
}
// count
if(umap.count("banana") > 0) {
    cout << "banana exists" << endl;
}
// contains (C++20)
if(umap.contains("apple")) {
    cout << "apple exists" << endl;
}

unordered_map<string, int> umap;
// insert - 插入键值对
auto result = umap.insert({"apple", 5});
if(result.second) {
    cout << "Insert successful" << endl;
}
umap.insert({{"banana", 3}, {"orange", 2}});
// emplace - 原地构造
umap.emplace("grape", 4);
// emplace_hint - 带提示的插入
auto hint = umap.begin();
umap.emplace_hint(hint, "pear", 6);
// try_emplace (C++17) - 如果键不存在则插入
umap.try_emplace("apple", 10);  // 不会替换现有的"apple"
umap.try_emplace("mango", 8);   // 插入新的"mango"
// insert_or_assign (C++17) - 插入或赋值
umap.insert_or_assign("apple", 15);  // 替换现有值
umap.insert_or_assign("kiwi", 9);    // 插入新键值对
// erase
umap.erase("banana");  // 通过键删除
auto it = umap.find("orange");
if(it != umap.end()) {
    umap.erase(it);  // 通过迭代器删除
}

unordered_map<string, int> umap = {
    {"apple", 3},
    {"banana", 5},
    {"orange", 2}
};
// 使用迭代器
for(auto it = umap.begin(); it != umap.end(); ++it) {
    cout << it->first << ": " << it->second << endl;
}
// 结构化绑定 (C++17)
for(const auto& [key, value] : umap) {
    cout << key << ": " << value << endl;
}

unordered_map<string, int> umap = {
    {"apple", 1}, {"banana", 2}, {"orange", 3},
    {"grape", 4}, {"pear", 5}, {"kiwi", 6}
};
// 桶信息
cout << "Bucket count: " << umap.bucket_count() << endl;
cout << "Load factor: " << umap.load_factor() << endl;
cout << "Max load factor: " << umap.max_load_factor() << endl;

// 设置哈希策略
umap.max_load_factor(0.75f);
umap.reserve(50);  // 预留空间
umap.rehash(30);   // 重新哈希