STL sort算法中的比较函数

排序，既陌生又熟悉的名词。

排序，成为面试官中喜欢问的算法问题。

c++ STL中为我们提供了std::sort， 所以今天我们不是来描述各种排序算法的实现，而是看看怎么使用stl为我们提供的sort。

先预热，代码：

#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;
}

从输入结果可以看出，sort默认是按照升序进行排序。

按照降序进行排序：

#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;
}

定义自己的比较函数–全局函数：

#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;
}

定义自己的比较函数–成员函数：

#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;
}

函数指针作为sort的比较函数：

#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表达式：

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