How to sort a struct from binary file with sort c-CodePudding

The compiler gives a mistake :Expression: invalid operator<. I think there should be a problem with the parameters, which receive the comparator, but I am not sure. Here is the code.

#include<iostream>
#include<fstream>
#include<algorithm> 

using namespace std;

struct ticket{
    char destination[50];
    char  flightNumber[50];
    char Aircraft[50];
};

bool comparator(ticket a, ticket b)
{
    return a.destination < b.destination;
    
}

int main()
{
    const int SIZE = 6;
    char mydestination[40]; 
    ticket newA[SIZE];
    fstream f;
    f.open("records.dat", ios::in | ios::binary);
    
    if (f.is_open())
    {
        
        f.read(reinterpret_cast<char*>(newA), SIZE *sizeof(ticket));
        f.close();
    }
    else
        cout << "ERROR\n";
    
    sort(newA, newA   SIZE, comparator);
    
    for (ticket& s : newA)
    {
        cout << s.destination;
        cout << s.Aircraft;
        cout << s.flightNumber << endl;     
    }
    system("pause");
    return 0;
}

CodePudding user response：

As mentioned in the comments, using < will just compare addresses.

To actually compare the strings you can use std::lexicographical_compare. Change your comparator to

bool comparator(const ticket& a, const ticket& b) {
    return std::lexicographical_compare(std::begin(a.destination), std::end(a.destination), std::begin(b.destination), std::end(b.destination));
}

CodePudding user response：

Your "strings" are not c , they are arrays of characters in c style. Therefore the best option for you is, as is suggested in the comments, to use:

int strcmp (const char* str1, const char* str2);

bool comparator(ticket a, ticket b)

{
return strcmp(&a.destination,&b.destination);
}

Or just delete comparator function and use:

    sort(newA, newA   SIZE, strcmp(&a.destination,&b.destination));

You may need

#include <string.h>

But I would better rewrite my code in c style