std::vector 结构学生示例

Question

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

struct StudentDataTypes
{
    std::string name{};
    int grade{};
};

int main()
{
    //Ask for Class_Size
    int Class_Size{};
    std::cout << "How big is the class?" << '\n';
    std::cin >> Class_Size;

    //Syntax format
    //std::vector<T> array(size);
    //Intialize a vector for the students called Vector_Student
    //T links to the struct StudentDataTypes
    //size is the Class_Size.
    std::vector<StudentDataTypes> Vector_Student(Class_Size);
    
    //Print Class Size
    std::cout << "There are " << Class_Size << " students." << '\n';

    //Get the Userinputs for the Class
    for (int i = 0; i < Class_Size; ++i)
    {
        std::cout << "Please input the name of Student #" << i + 1 << '\n';
        std::cin >> Vector_Student[i].name;
        std::cout << "Please input the grade of Student #" << i + 1 << '\n';
        std::cin >> Vector_Student[i].grade;
    }
    
    //Sort
    std::sort(Vector_Student.begin(), Vector_Student.end());

    //Print the required output
    for (int j = 0; j < Class_Size; ++j)
    {
        std::cout 
            << Vector_Student[j].name 
            << " got a grade of " 
            << Vector_Student[j].grade << '\n';
    }
    
    return 0;
}

I have an issue with a Vector Struct tutorial.

I'm using Visual Studio 2019 and there's a peculiar scenario where the compiler doesn't give me any warning at all. If I debug it, the first warning appears on line 1544, way out of bounds. The above code will actually sort of compile, run and crash.

I know the issue lies in the sorting but I can't figure it out.

Answer 1

std::sort requires you to implement operator< for your datatype. Here in this case adding following definition in your class will get your code to compile

bool operator<(const StudentDataTypes& that) {
    return this->grade < that.grade;
}

Update: Alternatively as sugested by Casey, we can use custom sort comparator. Here is the sample code for the same.

std::sort(Vector_Student.begin(), Vector_Student.end(), [](const StudentDataTypes& a, const StudentDataTypes&b) { return a.grade < b.grade; });

Answer 2

Here is the answer. I'm posting it in the answers for how to sort a vector-struct.

Step 1:

bool compareTwoStudents(StudentDataTypes a, StudentDataTypes b) {
    if (a.grade != b.grade)
        return a.grade > b.grade;
    return a.grade==b.grade;
} 

Step 2:

std::sort(Vector_Student.begin(), Vector_Student.end(),compareTwoStudents);