重载小于号的两种方法

方法1：

struct Sum
{
    int s, c, d; // s表示c和d的平方
    bool operator<(const Sum& t) const //返回较小的那个
    {
        if(s != t.s) return s < t.s;
        if(c != t.c) return c < t.c;
        return d < t.d;
    }
} sum[N];

sort(sum, sum + n);

方法2：

struct Sum{
    int s, c, d;
    bool operator()(const Sum & s1, const Sum & s2){
        if(s1.s != s2.s) return s1.s < s2.s;
        if(s1.c != s2.c) return s1.c < s2.c;
        if(s1.d != s2.d) return s1.d < s2.d;
    }
}sum[N];

sort(sum, sum + n, Sum());

案例1

#include <iostream>
#include <algorithm>

using namespace std;

//从大到小排序
struct Rule1{
    bool operator()(const int & a1, const int & a2){
        return a1 > a2;
    }
};

//按个位数从小到大排序
struct Rule2{
    bool operator()(const int & a1, const int & a2){
        return a1 % 10 < a2 % 10;
    }
};

void Print(int *a, int size){
    for(int i = 0; i < size; i ++){
        cout << a[i] << ' ';
    }
    cout << endl;
}

int main(){
    int a[] = {12, 45, 3, 98, 21, 7};
    for(auto e : a) cout << e << ' ';
    cout << endl;

    //从小到大排序
    sort(a, a + sizeof a / sizeof (int));
    cout << "从小到大排序" << endl;
    cout << "1) "; Print(a, sizeof(a) / sizeof(int));

    //从大到小排序
    sort(a, a + sizeof a / sizeof (int),greater<int>());
    cout << "从大到小排序" << endl;
    cout << "2) "; Print(a, sizeof(a) / sizeof(int));

    //从大到小排序
    sort(a, a + sizeof a / sizeof (int), Rule1());
    cout << "从大到小排序" << endl;
    cout << "3) "; Print(a, sizeof(a) / sizeof(int));

    //按余数从小到大排序
    sort(a, a + sizeof a / sizeof (int), Rule2());
    cout << "按个位从小到大排序" << endl;
    cout << "4) "; Print(a, sizeof(a) / sizeof(int));
    return 0;
}

结果输出如下：
12 45 3 98 21 7 
从小到大排序
1) 3 7 12 21 45 98 
从大到小排序
2) 98 45 21 12 7 3 
从大到小排序
3) 98 45 21 12 7 3 
按个位从小到大排序
4) 21 12 3 45 7 98 

案例2

#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

//学生信息
struct Student{
    char name[20];
    int id;
    double gpa;
};

//学生信息初始化
Student students [] = {
    {"Jack", 112, 3.4}, {"Mary", 102, 3.8}, {"Mary", 117, 3.9},
    {"Ala", 333, 3.5}, {"Zero", 101, 4.0}
};

//按姓名从小到大排序
struct StudentRule1{
    bool operator()(const Student & s1, const Student & s2){
        if(strcmp(s1.name, s2.name) < 0){
            return true;
        }
        return false;
    }
};

//按id从小到大排序
struct StudentRule2{
    bool operator()(const Student & s1, const Student & s2){
        return s1.id < s2.id;
    }
};

//按gpa从高到低排序
struct StudentRule3{
    bool operator()(const Student & s1, const Student & s2){
        return s1.gpa > s2.gpa;
    }
};

void PrintStudents(Student *s, int size){
    for(int i = 0; i < size; i ++){
        cout << "(" << s[i].name << "," << s[i].id << " " << s[i].gpa << ")" << ' ';
    }
    cout << endl;
}

int main(){
    cout << "学生信息如下：" << endl;
    for(auto e : students){
        cout << e.name << ' ' << e.id << ' ' << e.gpa << ' ';
    }
    cout << endl;

    int n = sizeof(students) / sizeof(Student);

    cout << "按姓名从小到大排序：" << endl;
    sort(students, students + n, StudentRule1());
    PrintStudents(students, n);

    cout << "按id从小到大排序：" << endl;
    sort(students, students + n, StudentRule2());
    PrintStudents(students, n);

    cout << "按gpa从高到低排序：" << endl;
    sort(students, students + n, StudentRule3());
    PrintStudents(students, n);

    return 0;
}

输出如下：
学生信息如下：
Jack 112 3.4 Mary 102 3.8 Mary 117 3.9 Ala 333 3.5 Zero 101 4 
按姓名从小到大排序：
(Ala,333 3.5) (Jack,112 3.4) (Mary,102 3.8) (Mary,117 3.9) (Zero,101 4) 
按id从小到大排序：
(Zero,101 4) (Mary,102 3.8) (Jack,112 3.4) (Mary,117 3.9) (Ala,333 3.5) 
按gpa从高到低排序：
(Zero,101 4) (Mary,117 3.9) (Mary,102 3.8) (Ala,333 3.5) (Jack,112 3.4)