'read': identifier not found

chopficaro (24)
im just trying to get the hang of binary file io by making a simple program that can write a structure into a file, then read it, but the first error message i get is

'read': identifier not found

heres the code:
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#include <iostream>
#include <fstream>
using namespace std;

struct grades
{
	int grade1;
	int grade2;
	int grade3;
	int grade4;
} mygradesi,mygradeso;

int main ()
{
	int answer;
	do
	{
		cout<<"do u want to 1, write to the file or 2, read from the file, or 3, exit the program?";
		cin>>answer;
		if (answer==2)
		{
			struct grades * mygradesptri;
			mygradesptri=&mygradesi;
			ifstream myfilei;
			myfilei.open ("binary.bin", ios::in|ios::binary);
			if (myfilei.is_open())
			{
				read(mygradesptri, sizeof(grades));
				cout<< mygradesi.grade1<< mygradesi.grade2<< mygradesi.grade3<< mygradesi.grade4;
			}
			else cout << "Unable to open file for input";
		}
		if (answer==1)
		{
			struct grades * mygradesptro;
			mygradesptro=&mygradeso;
			ofstream myfileo;
			myfileo.open ("binary.bin", ios::out|ios::binary);
			cout>>"please enter 4 grades";
			cin<< mygradeso.grade1<< mygradeso.grade2<< mygradeso.grade3<< mygradeso.grade4;
			if (myfileo.is_open())
			{
				write (mygradesptro, sizeof(grades));
				myfileo.close();
			}
			else cout << "Unable to open file for output";
		}
	}while (answer!=3);
	return 0;
}
Duthomhas (13282)
read() and write() are methods belonging to a class. You must indicate what object you wish to operate on:
myfilei.read(mygradesptri, sizeof(grades));
and
myfileo.write (mygradesptro, sizeof(grades));
Hope this helps.
chopficaro (24)
omg! so simple! ty very much!
chopficaro (24)
k, the solution u gave the other day worked flawlessly, i know know how to write ONE structure to a file and then read it back, but now, im trying to learn more. i want to write many structures to a file, and read the whole file onto the screen. i wrote some code that should work, but it doesnt. no compilation errors, but strange behavior on screen when reading a file larger than one structure:
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#include <iostream>
#include <fstream>
using namespace std;

//the purpose of this program is to send structures into a file, and then read them from the same file
//structure to be written:
struct grades
{
	int grade1;
	int grade2;
	int grade3;
	int grade4;
}mygradesi,mygradeso,mygradesa;

int main ()
{
//declare pointers to structures for file io:
	grades * mygradesptro=&mygradeso;
	grades * mygradesptra=&mygradesa;
	grades * mygradesptri;

//give the user options:
	int answer;
	do
	{
		cout<<"do u want to \n1, read from the file, or \n2, write over the file or \n3, append the file or \n4, exit the program?\n";
		cin>>answer;
//option 1, read from the file:
		if (answer==1)
		{
//check the size of the file and allocate appropriate memory:
			long begin,end;
			ifstream myfilecheck ("binary.bin");
			if (myfilecheck.is_open())
			{
				begin = myfilecheck.tellg();
				myfilecheck.seekg (0, ios::end);
				end = myfilecheck.tellg();
				myfilecheck.close();
				int filesize=(end-begin)/(sizeof(grades));
				mygradesptri= new grades [filesize];
			}
			else cout << "Unable to open file for size check\n\n";
//read the file into memory:
			int mygradesnum=0;
			ifstream myfilei;
			myfilei.open ("binary.bin", ios::in|ios::binary);
			if (myfilei.is_open())
			{
				while ((myfilei.peek()!=EOF))
				{
					myfilei.read(reinterpret_cast<char*>(mygradesptri+mygradesnum*sizeof(grades)), sizeof(grades));
					mygradesnum++;
				}
//display memory on screen:
				for(int i=0;i<mygradesnum;i++)
				{
					cout<<"\ngrade1\n"<< (mygradesptri[i]).grade1<<"\ngrade2\n"<< (mygradesptri[i]).grade2<<"\ngrade3\n"<< (mygradesptri[i]).grade3<<"\ngrade4\n"<< (mygradesptri[i]).grade4<<"\n\n";
				}
			}
			else cout << "Unable to open file for input\n\n";
		}
//option 2, write over the file:
		if (answer==2)
		{
			ofstream myfileo;
			myfileo.open ("binary.bin", ios::out|ios::binary);
			cout << "please enter 4 grades separated by pressing enter\n";
			cin >> (mygradesptro[0]).grade1 >> (mygradesptro[0]).grade2 >> (mygradesptro[0]).grade3 >> (mygradesptro[0]).grade4;
			if (myfileo.is_open())
			{
				myfileo.write (reinterpret_cast<char*>(mygradesptro), sizeof(grades));
				myfileo.close();
			}
			else cout << "Unable to open file for output\n\n";
		}
//option 3, append the file:
		if (answer==3)
		{
			ofstream myfilea;
			myfilea.open ("binary.bin", ios::app|ios::binary);
			cout << "please enter 4 grades separated by pressing enter";
			cin >> (mygradesptra[0]).grade1 >> (mygradesptra[0]).grade2 >> (mygradesptra[0]).grade3 >> (mygradesptra[0]).grade4;
			if (myfilea.is_open())
			{
				myfilea.write (reinterpret_cast<char*>(mygradesptra), sizeof(grades));
				myfilea.close();
			}
			else cout << "Unable to open file for output\n\n";
		}
//option 4, exit:
	}while (answer!=4);
	return 0;
}
Duthomhas (13282)
You have a math error on line 52.
Hint: you have casted to char* after doing all your math. There are two ways to fix it.

Hope this helps.
chopficaro (24)
thanx, it works, i can read or write any number of structures to a file now, u have helped me a great deal, and i now have a piece of code that i will use many times!
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