STL 集合类

set、multiset

概述

set 和 multiset 使得用户可以快速查找键（键是唯一存储在一维容器中的值）。二者区别：set只能存储唯一的值，multiset能存储重复值

set 和 multiset 的通过实现类似二叉树的内部结构进行排序，提高了查找速度。因此，这两种结构的特定位置元素不能被替换为值不同的新元素。

注意包含头文件：#include <set>

实例化

/*
desc：set/multiset的初始化
*/
#include <iostream>
#include <set>
using namespace std;

//指定排序标准
template <typename T>
struct SordDescending
{
    bool operator() (const &T lsh, const &T rsh) const
    {
        return lsh > rsh;
    }
};

int main()
{
    // 1. 简单初始化(此时的默认排序谓词为：std::less<T>。升序为std::greater<T>)
    set <int> setIntegers1;
    multiset <int> mulsetIntegers1;

    // 2. 带有用户自定义排序谓词的初始化
    set <int, SordDescending<int>> setIntegers2;
    multiset <int, SordDescending<int>> mulsetIntegers2;

    // 3. 用另一个容器（或者该容器的一部分）来初始化
    set <int> setIntegers3(setIntegers1);
    multiset <int> mulsetIntegers(setIntegers1.cbegin(), setIntegers1.end());
    system("pause");
    return 0;
}

插入

#include <iostream>
#include <set>
using namespace std;

template <typename T>
void DisplayElement (const T& input)
{
    for (auto iElementLocator = input.cbegin()
        ; iElementLocator != input.cend()
        ; ++iElementLocator)
    {
        cout << *iElementLocator << " "; 
    }
    cout << endl;
}

int main()
{
    set <int> setIntegers;
    multiset <int> msetIntegers;

    setIntegers.insert(60);
    setIntegers.insert(-1);
    setIntegers.insert(3000);
    DisplayElement(setIntegers);
    setIntegers.insert(3000);
    cout << "Inserting the number 3000 again:" << endl;
    DisplayElement(setIntegers);

    msetIntegers.insert(setIntegers.begin(), setIntegers.end());
    DisplayElement(msetIntegers);

    msetIntegers.insert(3000);
    cout << "The count of'3000' in multiset is: ";
    cout << msetIntegers.count(3000) << endl;
    system("pause");
    return 0;
}

务必使用 multiset.count(value) 来确定有多少个元素包含特定的value值

查找

#include <set>
#include <iostream>
using namespace std;

int main()
{
    //43 78 -1 124
    set <int> setIntegers;
    setIntegers.insert(43);
    setIntegers.insert(78);
    setIntegers.insert(-1);
    setIntegers.insert(124);
    for (auto iElement = setIntegers.begin(); iElement != setIntegers.end(); ++iElement)
    {
        cout << *iElement << " ";
    }
    cout << endl;

    //通过成员函数：find()查找 
    auto iElementFound = setIntegers.find(1223); 
    if (iElementFound != setIntegers.end()) //实参可以换成set中存在的
    {
        cout << "Element " << *iElementFound << " found!" << endl;
    }
    else
    {
        cout << "Element not found in set!" << endl;
    }
    system("pause");
    return 0;
}

find() 返回一个迭代器，指向找到的第一个元素。可将该迭代器递增以获取下一个元素。

删除

/*
通过成员函数:erase()
    1. setObject.erase (key); // 根据键删除值
    
    2. setObject.erase (element); // 迭代器作为参数，并删除该迭代器指向的对象
    MSETINT::iterator elementFound = msetInts.find(numberToErase); 
    if (elementFound != msetInts.end ()) 
        msetInts.erase (elementFound); 
    else 
        cout << "Element not found!" << endl; 

    3. setObject.erase (iLowerBound, iUpperBound); // 迭代器指定删除的边界
    MSETINT::iterator elementFound = msetInts.find(valueToErase); 
    if (elementFound != msetInts.end ()) 
        msetInts.erase (msetInts.begin (), elementFound);
*/
#include <set>
#include <iostream>
using namespace std;

template <typename T>
void DisplayElement (const T& input)
{
    for (auto iElement = input.cbegin()
        ; iElement != input.cend()
        ; ++iElement)
    {
        cout << *iElement << " ";
    }
    cout << endl;
}
typedef multiset <int> MSETINT;

int main()
{
    MSETINT msetIntegers;
    msetIntegers.insert(43);
    msetIntegers.insert(78);
    msetIntegers.insert(78);
    msetIntegers.insert(49);
    msetIntegers.insert(124);

    cout << "multiset contains " << msetIntegers.size() << " elements.";
    cout << " These are: " << endl;
    DisplayElement(msetIntegers);

    cout << "Please enter a number to be erased from the set" << endl;
    int nNumberToErase = 0;
    cin >> nNumberToErase;

    cout << "Erasing " << msetIntegers.count(nNumberToErase);
    cout << " instances of value " << nNumberToErase << endl;
    msetIntegers.erase(nNumberToErase);

    cout << "multiset contains " << msetIntegers.size() << " elements: ";
    DisplayElement(msetIntegers);
    system("pause");
    return 0;
}

通过 size() 指出容器包含多少个元素。

一个具体例子：存储对象的set（multiset）

#include <set>
#include <string>
#include <iostream>
using namespace std;

template <typename T>
void DisplayContents (const T& input)
{
    for (auto iElement = input.cbegin()
        ; iElement != input.cend()
        ; ++iElement)
    {
        cout << *iElement << endl;
    }
    cout << endl;
}

struct ContactItem
{
    string strContactsName;
    string strPhoneNumber;
    string strDisplayRepresentation;

    ContactItem (const string& strName, const string& strNumber)
    {
        strContactsName = strName;
        strPhoneNumber = strNumber;
        strDisplayRepresentation = (strContactsName + ": " + strPhoneNumber);
    }
    bool operator == (const ContactItem& itemToCompare) const
    {
        return (itemToCompare.strContactsName == this->strContactsName);
    }
    bool operator < (const ContactItem& itemToCompare) const
    {
        return (this->strContactsName < itemToCompare.strContactsName);
    }
    operator const char*() const
    {
        return strDisplayRepresentation.c_str();
    }
};
int main()
{
    set <ContactItem> setContacts;
    setContacts.insert(ContactItem("Jack Welsch", "+1 7889 879 879"));
    setContacts.insert(ContactItem("Bill Gates", "+1 97 7897 8799 8"));
    setContacts.insert(ContactItem("Angi Merkel", "+49 23456 5466")); 
    setContacts.insert(ContactItem("Vlad Putin", "+7 6645 4564 797")); 
    setContacts.insert(ContactItem("John Travolta", "91 234 4564 789")); 
    setContacts.insert(ContactItem("Ben Affleck", "+1 745 641 314"));
    DisplayContents(setContacts);

    cout << "Enter a person whose number you wish to delete: " ;
    string deletename;
    getline(cin, deletename);

    auto iContactFound = setContacts.find(ContactItem(deletename, ""));
    if (iContactFound != setContacts.end())
    {
        setContacts.erase(iContactFound);
        cout << "After deleting: " << endl;
        DisplayContents(setContacts);
    }
    else
    {
        cout << "Contact not found!" << endl;
    }
    system("pause");
    return 0;
}

对于存储对象的set、multiset，一定要实现运算符 < 和 ==，前者用于排序，后者用于 set::find()

---

unordered_set、unordered_multiset

通过hash函数计算排序索引，根据索引决定将元素放到哪个桶（bucket）内。

使用时注意添加头文件：#include <unordered_set>

对于初始化、插入、查找、删除等操作与set类似，注意一个重要特征：

负责确定排列顺序的散列函数：

unordered_set<int>::hasher HFn = usetInt.hash_function(); 

一个具体例子：

#include <iostream>
#include <unordered_set>
using namespace std;
template <typename T>
void DisplayContents (const T& input)
{
    cout << "Number of elements, size() = " << input.size() << endl;
    cout << "Max bucket count = " << input.max_bucket_count() << endl;
    cout << "Load factor: " << input.load_factor() << endl;
    cout << "Max load factor = " << input.max_load_factor() << endl;
    cout << "Unordered set contains: " << endl;

    for (auto iElement = input.cbegin()
        ; iElement != input.cend()
        ; ++iElement)
    {
        cout << *iElement << " ";
    }
    cout << endl;
}

int main()
{
    unordered_set <int> usetIntegers;
    cout << usetIntegers.max_bucket_count() << endl;
    cout << endl;
    usetIntegers.insert(1000);
    usetIntegers.insert(-3);
    usetIntegers.insert(2011);
    usetIntegers.insert(300);
    usetIntegers.insert(-1000);
    usetIntegers.insert(989);
    usetIntegers.insert(-300);
    usetIntegers.insert(111);
    DisplayContents(usetIntegers);
    usetIntegers.insert(999);
    cout << endl;
    cout << usetIntegers.max_bucket_count() << endl;
    cout << endl;
    DisplayContents(usetIntegers);

    cout << "Enter int you want to check for existence in set: ";
    int Key = 0;
    cin >> Key;
    auto iPairThousand = usetIntegers.find(Key);

    if (iPairThousand != usetIntegers.cend())
    {
        cout << *iPairThousand << " found in set" << endl;
    }
    else
    {
        cout << Key << " not available in set" << endl;
    }
    system("pause");
    return 0;
}