File I/O question... please help!

[sirSolarius]

Please help me with the following code!! I want to have a file that contains a list of recipes, with the first line of the file being the number of recipes. To test out the file IO code, I wrote up this little test program, but it keeps overwriting the file!

How can I make it append the data to the end, but change the first line at the end?

Thanks so much!
sirSolarius

#include <iostream>
#include <fstream.h>

void main()
{
	int *current, currenter;
	currenter = 80;
	current = &currenter;

	ofstream CoolFile("C:\\recipes.dat", ios::app);
	if (CoolFile)
	{
		CoolFile << "I'm" << endl;
		CoolFile << "One" << endl;
		CoolFile << "Crazy" << endl;
		CoolFile << "Guy" << endl;
		CoolFile.close();

		ofstream numberFile("C:\\recipes.dat");
		numberFile.seekp(0);
		numberFile << *current;
	}

	ifstream CoolFileIn("C:\\recipes.dat");
	char stringer[100];
	int x;
	for (x=0; x<6;x++)
	{
		CoolFileIn >> stringer;
		cout << "\n" << stringer;
	}
}

[KneeLess]

Originally posted by sirSolarius
How can I make it append the data to the end, but change the first line at the end?

I don't really understand your question. You want to append data to the end, that is not touching the rest of the data, but you want the first line at the end? Huh?

Well I think your problem is:

ofstream CoolFile("C:\\recipes.dat", ios::app);

Try this:

ofstream CoolFile("C:\\recipes.dat", ios::ate);

Notice the ios::ate, it doesn't call ios::out so it won't overwrite the file. Or maybe that just won't work, I don't know. Maybe you could throw a ios::nocreate in there for good measure.

Good Tutorial on File I/O.

:)

[sirSolarius]

Ok, here's what I'm saying:

Everything should be appended to the end of the file, so when I make a new entry, it goes onto the end of the file.

Example: (the desired file follows)

3
Entry 1:
Data
Data
Entry 2:
Data
Data
Entry 3:
Data
Data

Now I make a new entry:

4
Entry 1:
Data
Data
Entry 2:
Data
Data
Entry 3:
Data
Data
Entry 4:
Data
Data

So the new stuff is appended, but the first line is changed. See what I mean?

sirSolarius

[Sebastiani]

The problem is that unless you're careful, it's dangerous to write straight to the top of the file, it could creep into the successive data and cause corruption. You could pad the top of the file with an amount of bytes that is non-variable in size, write to the top, seek the end of file, then continue to write.

[sirSolarius]

Originally posted by Sebastiani
The problem is that unless you're careful, it's dangerous to write straight to the top of the file, it could creep into the successive data and cause corruption. You could pad the top of the file with an amount of bytes that is non-variable in size, write to the top, seek the end of file, then continue to write.

Ok... but how do I even do that??

[sirSolarius]

Can't anyone help me?

[alphaoide]

CoolFile.seekp(ios::beg);

will position the ostream pointer to the beginning of the stream

CoolFile.seekp(ios::end);

will position the ostream pointer to the end.

[sirSolarius]

The code still doesn't work.

#include <fstream.h>


void main()
{
	int *current, currenter;
	currenter = 80;
	current = &currenter;

	ofstream CoolFile("C:\\recipes.dat", ios::app);
	if (CoolFile)
	{
		CoolFile << "I'm" << endl;
		CoolFile << "One" << endl;
		CoolFile << "Crazy" << endl;
		CoolFile << "Guy" << endl;

		CoolFile.seekp(ios::beg);
		CoolFile << *current;
		CoolFile.close();
	}

	ifstream CoolFileIn("C:\\recipes.dat");
	char stringer[100];
	int x;
	for (x=0; x<6;x++)
	{
		CoolFileIn >> stringer;
		cout << "\n" << stringer;
	}
}

[elad]

Try this, might work.

#include <fstream.h>


void main()
{
                char * currenter = "80";
                char * filename = "recipes.txt";
	ofstream CoolFile(filename);
                CoolFile.close();
                CoolFile.open(filename, ios::app);
	if (CoolFile)
	{
		CoolFile << "I'm" << endl;
		CoolFile << "One" << endl;
		CoolFile << "Crazy" << endl;
		CoolFile << "Guy" << endl;

		CoolFile.seekp(ios::beg);
		CoolFile << currenter << endl;
		CoolFile.close();
	}

	ifstream CoolFileIn("C:\\recipes.dat");
	char stringer[100];
	int x;
	for (x=0; x<5;x++)
	{
		CoolFileIn >> stringer;
		cout << "\n" << stringer;
	}
}

output I would like to see:

80m
One
Crazy
Guy

At least that's what I would expect with if CoolFile were in binary mode and read() and write() were used to deal with file. Not sure what happens here however. ios::app may prevent the repositioning of the output file pointer using ios:: beg and behavior of << if file pointer is repositioned is unclear.
--------------------------------------------------------------------------------

[alphaoide]

I know ios::ate will let you to reposition put pointer (seekp()).
If you use write() you could specify how many bytes you want to overwrite, but note that you won't be able to view the output using notepad

int num1 =1;
int num2 =2;

outFile.write(reinterpret_cast<const char *>(&num1), sizeof(int));
outFile << bla... bla...
outFile << ....
outFile.seekp(ios::beg);
outFile.write(reinterpret_cast<const char *>(&num2), sizeof(int));

Using codes above, you'll always be able to overwrite the whole thing because you always write the same size (the size of an integer). If you view the output using VC++, you'll see it clearly.

[robquigley]

aplhaoide, how can u view the files if you use write()? Is the only way to read them back in or something? Or can u get a viewer of some sort?

[alphaoide]

If you view that output using VC++, you'll be able to read it. I don't know about Bloodshed. Otherwise, you need some hex dump utility.
To read back the data, (equivalent to "infile >>"), just use read().