C++ common syntax - unordered_set

1. Prerequisites

Import the header file:

#include <unordered_set>

Two, what is unordered_set

The unordered_set container can be literally translated as "unordered set container". That is, the unordered_set container is very similar to the set container, the only difference is that the set container will sort the stored data by itself, but the unordered_set container will not.

Several features of unordered_set:

1. Data is no longer stored in the form of key-value pairs, but the value of the data is stored directly;
2. The values ​​of each element stored in the container are not equal to each other and cannot be modified;
3. Does not sort the data stored internally

3. Initialization of unordered_set

Create an empty set

unordered_set<int> set1;

copy construction

unordered_set<int> set2(set1);

use iterator construction

unordered_set<int> set3(set1.begin(), set1.end());

Construct with an array as its initial value

unordered_set<int> set4(arr,arr+5);

mobile structure

unordered_set<int> set5(move(set2));

Construct with disposition list

unordered_set<int> set6 {
    
    1,2,10,10};

4. Commonly used built-in functions of unordered_set

empty() function - to determine whether it is empty
//If the container is empty, return true; otherwise false

set1.empty();

find() function - find
//find 2, return iterator if found, return end() on failure

set1.find(2);

count() function - number of occurrences
//returns the number of occurrences of 2, 0 or 1

set1.count(2);

insert() function - insert an element

//插入元素，返回pair<unordered_set<int>::iterator, bool>
set1.insert(3);
//使用initializer_list插入元素
set1.insert({
    
    1,2,3});
//指定插入位置，如果位置正确会减少插入时间，返回指向插入元素的迭代器
set1.insert(set1.end(), 4);
//使用范围迭代器插入
set1.insert(set2.begin(), set2.end());

Regarding the return value of the insert function:
insert() only passes in a single parameter (the element to be inserted)

1. will return a pair object
2. The pair object contains an iterator, and an additional boolean indicating whether the insertion was successful
3. If the element was inserted, the returned iterator will point to the new element
4. If not inserted, iterator to the element that prevented insertion

auto pr = words.insert("ninety"); // Returns a pair - an iterator & a bool value

insert() passes in two parameters (iterator + element to be inserted)

1. An iterator can be used as the first parameter of insert(), which specifies the position where the element is inserted
2. In this case, only an iterator will be returned

auto iter = words.insert (pr.first, "nine"); // 1st arg is a hint. Returns an iterator

insert() passes in the initialization list

1. insert elements in the initialization table
2. In this case nothing is returned

words.insert({
    
    "ten", "seven", "six"});  // Inserting an initializer list

emplace() function - insert elements (transfer construction)

//使用转移构造函数添加新元素3，比insert效率高
set1.emplace(3);

erase() function - delete an element

//删除操作，成功返回1，失败返回0
set1.erase(1);
//删除操作，成功返回下一个pair的迭代器
set1.erase(set1.find(1));
//删除set1的所有元素，返回指向end的迭代器
set1.erase(set1.begin(), set1.end());

bucket_count() function - the number of baskets

//返回容器中的篮子总数
set1.bucket_count();

bucket_size() function - the number of elements in the basket

//返回1号篮子中的元素数
set1.bucket_size(1);

bucket() function - in which basket

//元素1在哪一个篮子
set1.bucket(1);

clear() function - clear

set1.clear();

load_factor() function - load factor

//负载因子，返回每个篮子元素的平均数，即size/float(bucket_count);
set1.load_factor();

rehash() function - set the number of baskets and redistribute

//设置篮子的数量为20，并且重新rehash
set1.rehash(20);