排序，既陌生又熟悉的名詞。

排序，成為面試官中喜歡問的演算法問題。

c++ STL中為我們提供了std::sort， 所以今天我們不是來描述各種排序演算法的實現，而是看看怎麼使用stl為我們提供的sort。

先預熱，程式碼：

#include <algorithm>
#include <iostream>
#include <string>

int main()
{
    int test_arr[5];
    test_arr[0] = 2;
    test_arr[1] = 3;
    test_arr[2] = 5;
    atest_arrr[3
 
] = 1;
    test_arr[4] = 4;

    std::sort(test_arr,test_arr+5);

    std::cout<<test_arr[0]<<"\n";
    std::cout<<test_arr[1]<<"\n";
    std::cout<<test_arr[2]<<"\n";
    std::cout<<test_arr[3]<<"\n";
    std::cout<<test_arr[4]<<std::endl;

    return
 
 0;
}

從輸入結果可以看出，sort預設是按照升序進行排序。

按照降序進行排序：

#include <algorithm>
#include <iostream>
#include <string>
#include <functional>

int main()
{
    int test_arr[5];
    test_arr[0] = 2;
    test_arr[1] = 3;
    test_arr[2] = 5;
    test_arr[3] = 1;
    test_arr[4] = 4;

    std::sort
 
(test_arr,test_arr+5,std::greater<int>());

    std::cout<<test_arr[0]<<"\n";
    std::cout<<test_arr[1]<<"\n";
    std::cout<<test_arr[2]<<"\n";
    std::cout<<test_arr[3]<<"\n";
    std::cout<<test_arr[4]<<std::endl;

    return 0;
}

其中用到了std::greater()。

sort中使用迭代器：

#include <vector>
#include <algorithm>
#include <iostream>
#include <string>

int main()
{
    std::vector<int> test_vec;
    test_vec.push_back(2);
    test_vec.push_back(3);
    test_vec.push_back(5);
    test_vec.push_back(1);
    test_vec.push_back(4);

    std::sort(test_vec.begin(),test_vec.end());

    for(size_t i=0; i<test_vec.size(); ++i)
        std::cout<<test_vec[i]<<std::endl;

    return 0;
}

定義自己的比較函式–全域性函式：

#include <vector>
#include <algorithm>
#include <iostream>
#include <string>

class Person
{
public:
    // default constructor
    Person() : age(0) {}
    Person(int age, std::string name) {
        this->age = age; this->name = name;
    }
    int age;
    std::string name;
};

inline bool operator<(const Person& a, const Person& b)
{
    return a.age < b.age;
}

int main()
{
    std::vector<Person> persons;
    persons.push_back(Person(24,"Calvin"));
    persons.push_back(Person(30,"Benny"));
    persons.push_back(Person(28,"Alison"));

    std::sort(persons.begin(),persons.end());

    for(size_t i=0; i<persons.size(); ++i)
        std::cout<<persons[i].age<<", "<<persons[i].name<<std::endl;

    return 0;
}

定義自己的比較函式–成員函式：

#include <vector>
#include <algorithm>
#include <iostream>
#include <string>

class Person
{
public:
    // default constructor
    Person() : age(0) {}
    Person(int age, std::string name) {
        this->age = age; this->name = name;
    }
    bool operator<(const Person& rhs)
    {
        return this->age < rhs.age;
    }
    int age;
    std::string name;
};

int main()
{
    std::vector<Person> persons;
    persons.push_back(Person(24,"Calvin"));
    persons.push_back(Person(30,"Benny"));
    persons.push_back(Person(28,"Alison"));

    std::sort(persons.begin(),persons.end());

    for(size_t i=0; i<persons.size(); ++i)
        std::cout<<persons[i].age<<", "<<persons[i].name<<std::endl;

    return 0;
}

函式指標作為sort的比較函式：

#include <vector>
#include <algorithm>
#include <iostream>
#include <string>

class Person
{
public:
    // default constructor
    Person() : age(0) {}
    Person(int age, std::string name) {
        this->age = age; this->name = name;
    }

    int age;
    std::string name;
};

bool Greater(const Person& a, const Person& b)
{
    if(a.age == b.age)
        return a.name < b.name;

    return a.age > b.age;
}

int main()
{
    std::vector<Person> persons;
    persons.push_back(Person(24,"Calvin"));
    persons.push_back(Person(30,"Benny"));
    persons.push_back(Person(30,"Alice"));
    persons.push_back(Person(28,"Alison"));

    std::sort(persons.begin(),persons.end(),Greater);

    for(size_t i=0; i<persons.size(); ++i)
        std::cout<<persons[i].age<<", "<<persons[i].name<<std::endl;

    return 0;
}

sort中使用lambda表示式：

std::sort(persons.begin(), persons.end(), [](const Person &a, const Person &b) { return ((*a).age < (*b).age); });