优先级队列从文件c ++中读取

Question

I have a priority queue and an "event" class and i want to read from a file called agenda.txt where i have: priority,date,year and name of the event. But when i read from the file i get only the first element and i want to see the element with the highest priority. Can you help?

agenda.txt
9
12.05
2016
meeting
16 13.05
2017
shopping
8 12.09
2056
swimming
60 45.76
2016
work

and this is the main:

int main(){

char filename[50];
ifstream bucky;
cin.getline(filename,50);
bucky.open(filename);
if(!bucky.is_open()){
    exit(EXIT_FAILURE);
}
string nume;
int prio;
double data;
int an;
bucky>>prio>>data>>an>>nume;

while(bucky.good()){
cout<<"prioritatea este "<<prio<<"    data este "<<data<<"   anul este "<<an<<"   numele este  "<<nume<<" "<<endl;
bucky>>prio>>data>>an>>nume;



priority_queue<Event> q;

q.push(Event(prio,data,an,nume));


    cout<< q.top().getEventPriority()<<q.top().getEventData()<<" "<<q.top().getEventAn()<<" "<<q.top().getEventName()<<endl;



system("pause");}}

Answer 1

priority_queue does not know how to sort your custom class. You Class needs to override the less than operator so that the priority_queue can sort the items by the Priority defined in your text file. A pseudo-implementation would look like this:

class Event
{
   int Priority;
};
bool operator<(const Event& lhs, const Event& rhs) {return (lhs.Priority < rhs.Priority);}

Answer 2

If I am understanding correctly what you want is to have the first record in the file be the record with the highest priority; if that is the case then you should be writing records in order of priority to the file with the first record having the highest priority. So for example

class Event {
public:
    int priority;
    int date;
    int year;
    string name;
};

bool operator<(const Event& first, const Event& second) { 
    return first.priority < second.priority;
}

priority_queue<event> pq;

for (const auto& event : events_array_from_before) {
    pq.push(event);
}

ofstream fout;
fout.open("agenda.txt");
if (!fout) {
    cerr << "Could not open file to write to" << endl;
    return 1;
}

while (!pq.empty()) {
    const auto& event = pq.top();
    pq.pop();
    fout << event.priority << '\n' << event.date << '\n' 
         << event.year << '\n' << event.name << '\n';
}

But if you have the file with the records in some random order you can do nothing (if using a priority queue) but to read all of the records and keep track of the one with the highest priority. So you would do the following

ifstream fin;
fin.open("agenda.txt");
if (!fin) { 
    cerr << "Could not open file agenda.txt" << endl;
    return 1;
}

Event temp;
Event top_record;
fin >> top_record.priority >> top_record.date >> top_record.year 
    >> top_record.name;
while (fin >> temp.priority >> temp.date >> temp.year >> temp.name) {
    if (temp.priority > top_record.priority) {
        top_record = std::move(temp); // for efficient moving of string name
    }
}

If you want all the records from the file in sorted order you will have to read all of them into a vector and call the std::sort method on the vector

If you have some indication of where the element is in the file you could open the file in a ifstream object and then call the seekg method on the object to read from that point on.