How to edit specific value in binary file? - c++

Question

I have a structure of flight, which is being writing into binary file and i want to edit flight destination and i have no idea how to do it.
This is my code for getting user input and writting to binary file.
struct Flight_Details {
char destination[99];
char departure[99];
char time_depart[80];
char time_arrive[80];
int flight_number;
};
switch (menu)
{
case MENU::NEW_FLIGHT: {
Flight_Details flight_d;
cout << "Enter Departure: ";
cin >> flight_d.destination;
cout << "Enter Destination: ";
cin >> flight_d.departure;
cout << "Enter Departure Time: ";
cin >> flight_d.time_depart;
cout << "Please enter arriving time: ";
cin >> flight_d.time_arrive;
cout << "Flight Number: ";
cin >> flight_d.flight_number;
ofstream file;
file.open("Flgiht_Details.data", ios::binary);
if (!file) cout << "Could create/open file";
else {
file.write((char*)&flight_d, sizeof(flight_d));
file.close();
}
break;
}
case MENU::OUTPUT_FILE: {
ifstream readFile;
readFile.open("Flgiht_Details.data");
if (!readFile) cout << "Couldn't open file";
else {
readFile.seekg(0, ios::end);
int fileSize = readFile.tellg();
int countOfFlights = fileSize / sizeof(Flight_Details);
readFile.seekg(0, ios::beg);
Flight_Details* flight = new Flight_Details[countOfFlights];
readFile.read((char*)flight, countOfFlights *sizeof(Flight_Details));
readFile.close();
for (int i = 0; i < countOfFlights; i++)
{
cout << flight[i].destination << "\n" << flight[i].departure << "\n" << flight[i].time_depart << "\n" << flight[i].time_arrive << "\n" << flight[i].flight_number << "\n\n";
}
break;
}
}
case MENU::EDIT: {
Flight_Details* flight_d;
ifstream readFile;
readFile.open("Flgiht_Details.data");
if (!readFile) cout << "Couldn't open file";
else {
readFile.seekg(0, ios::end);
int fileSize = readFile.tellg();
int countOfFlights = fileSize / sizeof(Flight_Details);
readFile.seekg(0, ios::beg);
Flight_Details* flight = new Flight_Details[countOfFlights];
readFile.read((char*)flight, countOfFlights * sizeof(Flight_Details));
readFile.close();
ofstream file;
char edit[50];
cout << "Edit: ";
cin.getline(edit, 50);
for (int i = 0; i < countOfFlights; i++)
{
if (strcmp(flight_d[i].destination, edit) == 0)
{
//edit file
}
}
}
}
}

Answer 1

Since your Flight_Details has a fixed size in bytes when written, it is easy to calculate where in the file each flight is. The general equation is flight_number * sizeof(Flight_Details)//where flight_number is 0..N. This will calculate a byte offset into the file to seek to. From there it is just a matter of reading exactly sizeof(Flight_Details) bytes from the file and converting that buffer into a Flight_Details which in this case could mean that you just fill each buffer in the Flight_Details with the bytes manually, or if you prefer you could just reinterpret_cast() the char* of data into the right type(this will only be a decent idea if your buffer is exactly sizeof(Flight_Details) long. It would be safer to just manually copy the bytes in via a constructor of some kind.

Answer 2

Here is what you will write under // edit file comment. This looks ugly but it works pretty fine
cout << "Yeah we are editing\n";
char newdestiny[99];
cout << "new dest: ";
cin.getline(newdestiny, 99);
strcpy(flight[i].destination, newdestiny);
file.write((char*)flight, sizeof(Flight_Details[countOfFlights]));
file.close();
return 0;
and you have got bugs in your program like when you ask for desparture you are getting destination and when you open file for writing that data you have to include ios::app flag so you can enter new flight information as well(?)
the file I am opening has only ios::out flag
and one more thing strcmp(flight_d[i].destination, edit) == 0 line should be strcmp(flight[i].destination, edit) == 0