Fstream not creating file

[SourceCoded]

Hi,

I am writing code to read a specific line in a file. Here is my code:

  #include <iostream>
#include<fstream>
using namespace std;
int main()
{
	fstream myfile("file.txt");
	myfile << "1 \n 2 \n 3\n 4\n 5\n 6\n 7\n 8\n";
	myfile.open("file.txt");
	string line;
	for (int lineno = 0; getline (myfile,line) && lineno < 7; lineno++)
		if (lineno == 6)
			cout << line << endl;
	myfile.close();
	return 0;
}

The problem is that the program is not creating file.txt. Please help!

[andywestken]

Is the program actually building?

It looks like you're missing a header file to me: <string>

This is where getline() lives!

Andy

[LB]

std::string is from <string>
std::getline is the least of your worries

[SourceCoded]

YUp, just changed that, but it still does not make the file

[Chervil]

By default, the file is opened in input/output mode. If the file doesn't already exist, that will fail.

Instead, open it just in output mode the first time.
Remember to close the file before re-opening in input mode.

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    fstream myfile("file.txt", ios::out);
    myfile << "1 \n 2 \n 3\n 4\n 5\n 6\n 7\n 8\n";
    myfile.close();

    myfile.open("file.txt", ios::in);
    string line;

    for (int lineno = 0; getline (myfile,line) && lineno < 7; lineno++)
        if (lineno == 6)
            cout << line << endl;
    myfile.close();
    return 0;
}

[andywestken]

You can open a fstream in in/out mode it you you are happy to truncate the existing file...

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    fstream myfile("file.txt", ios::in | ios::out | ios::trunc);
    myfile << "1 \n 2 \n 3\n 4\n 5\n 6\n 7\n 8\n";
    myfile.close(); // remember to close

    myfile.open("file.txt"); // ios::in | ios::out by default
    string line;

    for (int lineno = 0; getline (myfile,line) && lineno < 7; lineno++)
        if (lineno == 6)
            cout << line << endl;
    myfile.close();
    return 0;
}

Note that if you don't close before you re-open, the file pointer is still at the end of the file. The second open actually does nothing (the file is already open) and the read starts from the end of the file (and sees nothing).

Rather than close and open you can just move the the file pointer back to the start of the file, with fstream::seek()

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    fstream myfile("file.txt", ios::in | ios::out | ios::trunc);
    myfile << "1 \n 2 \n 3\n 4\n 5\n 6\n 7\n 8\n";

    myfile.seekg(ios::beg); // move file pointer back to start

    string line;

    for (int lineno = 0; getline (myfile,line) && lineno < 7; lineno++)
        if (lineno == 6)
            cout << line << endl;

    myfile.close();

    return 0;
}

But in this case I'd prob take a similar approach to Chervil, but with ofstream and ifstream rather than the open flags.

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

// the anonymous scopes force destruction at the required points, to close file
int main()
{
    const char filename[] = "file.txt";

    {
        ofstream ofs(filename);
        ofs << "1 \n 2 \n 3\n 4\n 5\n 6\n 7\n 8\n";
    } 

    {
        ifstream ifs(filename);
        string line;
        for (int lineno = 0; getline(ifs, line) && lineno < 7; ++lineno)
            if (lineno == 6)
                cout << line << endl;
    }

    return 0;
}

Andy

[SourceCoded]

Thank you Chervil! Helped me again :D!