reading and writing to a file in C++

mgp6q · #1 01-Mar-2004, 12:14

I need to write a small program that reads a file and adds somethings to it. The file contains file paths. This is what it needs to look like

1
i:\home8\test_maintenance.drw\02_test.fld\02PC342M .dwg
i:\home8\test_maintenance.drw\02_test.fld\02PC342M
1

right now i just have
i:\home8\test_maintenance.drw\02_test.fld\02PC342M .dwg
in a file

The code below is what i have so far. Any help?? thanks

CPP / C++ / C Code:

//getline(array,array_size,delim);

#include <iostream>
#include <fstream>

int main()
{

	std::fstream iofile("input2.txt", 
			std::ios::in | std::ios::out);
	if (!iofile.fail()) {
		while (!iofile.eof()) {
			
			static char path[1000];	   	   
			iofile.getline(path);
			
			if (!iofile.eof())
				
				iofile << path;
			

		}
	}
	return 0;
}

WaltP · #2 01-Mar-2004, 13:03

Do you care how many lines are already in the file before writing a new line? If not, look into the append instead of /in addition to in and out for fstream.

mgp6q · #3 01-Mar-2004, 13:25

No it doesn't matter how many lines

Quote:

Originally Posted by WaltP

Do you care how many lines are already in the file before writing a new line? If not, look into the append instead of /in addition to in and out for fstream.

dsmith · #4 01-Mar-2004, 14:17

I think it may be easier to open a seperate file for reading and a seperate file for writing and then copy the output file to the input file.

Then you should be able to do something like this:

CPP / C++ / C Code:

outfile << "1\n";
outfile << path;
*(path + (strlen(path) - 4) ) = 0 //Take out .dwg text
outfile << path;
outfile << "1\n";

Please note that I haven't tested that. I would make sure that your output file is correct before putting in the code to copy it over your input file.

davekw7x · #5 01-Mar-2004, 15:35

If you want to append lines to a file, you can open it for reading
and writing, then write to the end of the file (no need to read
the lines that already in the file).

You could use iofile.seekg() to move around in the file, but that's
not necessary if you open the file with the pointer already at the
end.

Maybe this will help (note that you can eliminate having to write
"std::" everywhere by putting the statement "using namespace std;"
just before your main() statement).

Also, it's always a good idea to see whether the file was opened
properly.

The following compiles and executes OK on my Windows machine with
Borland Version 5.5 and g++ version 3.3.1

CPP / C++ / C Code:

#include <iostream>
#include <fstream>
using namespace std;
int main()
{

  fstream iofile("input2.txt", ios::in | ios::out |ios::ate);
  if (!iofile.fail()) { // Put whatever you need to generate new lines here

    for (int i = 1; i <= 3; i++ ) {
      iofile << "Put new line #" << i << " at end" << endl;
    }
  }
  else {
    cout << "Failed to open file <input2.txt> " << endl;
  }
  return 0;
}

Dave

aaroncohn · #6 01-Mar-2004, 15:59

Here's a tip to help you get rid of all that std wrapping.

For stream I/O, you can simply test for true or false to see if an error occured while opening the file, or if a read was successful. Here's an example of that:

CPP / C++ / C Code:

if (!iofile) // this test returns false if anything went wrong
  {
    cout << "Woops! I couldn't open the I/O file. Did you type the path correctly?\n";
    return 0;
  }
else
  . . .

iofile >> myVar;
while (iofile) // true if the read was successful, false if otherwise or end-of-file
  {
     cout << "Here's what I found in the file:" << myVar << endl;
     ioFile >> myVar;
  }

So I really see no point in wrapping your Stream I/O in a Standard I/O class. Stream I/O has plenty of native functions that work perfectly well. Here's how to declare various types of streams using Stream I/O.

CPP / C++ / C Code:

ifstream inFile("C:\\myfiles\\sometext.txt"); // opens sometext.txt for reading
ofstream outFile("C:\\myfiles\\sometext.txt",ios::app); // opens sometext.txt for writing and appending.

//You can also declare your streams independently of their paths as shown below.

ifstream inFile;
ofstream outFile;

inFile.open(filename);
outFile.open(filename,ios::app);

//Don't forget to close them!
inFile.close();
outFile.close();

davekw7x · #7 01-Mar-2004, 16:18

So, I tried the following:

CPP / C++ / C Code:

#include <fstream>

//using namespace std;

int main()
{
  ofstream outFile("sometext.txt",ios::app); // opens for appending

  outFile << "New line\n";

  outFile.close();

  return 0;
}

Neither Borland Version 5.5 or g++ version 3.3.1 compiled.
Error messages indicated that ofstream was undefined (undeclared).

When I un-commented the "using namespace std" statement, it
compiled and executed properly.

I have a lot less experience with C++ than with C, but so far, I have
treated "using namespace std;" as an idiom: just do it.

Is there any reason not to "just do it"?

Dave

aaroncohn · #8 01-Mar-2004, 18:24

You need to use the .h when including fstream.h, not just <fstream>, otherwise it won't compile. I don't see any problem with using namespace std, unless you don't need it. You only need it if you're going to use std functions on objects outside of the std class. If you write a program that doesn't use any std functions, then there's no need for the namespace statement.

davekw7x · #9 01-Mar-2004, 19:12

Gnu c (g++ version 3.3.1 will not compile, with or without
the .h.

Here are the g++ errors that prevent the compile with fstream.h
and with the "using namespace std" commented out:

Quote:

yy.cpp: In function `int main()':
yy.cpp:7: error: `ios' undeclared (first use this function)
yy.cpp:7: error: (Each undeclared identifier is reported only once for each
function it appears in.)
yy.cpp:7: error: syntax error before `::' token
yy.cpp:9: error: invalid operands of types `std::ofstream ()(...)' and `const
char[10]' to binary `operator<<'
yy.cpp:11: error: request for member `close' in `outFile(...)', which is of
non-aggregate type `std::ofstream ()(...)'

Dave

aaroncohn · #10 01-Mar-2004, 19:40

Why is it complaining about std:: if you don't have any in there? There must be something wrong. I've never tried G++. The exact code herein compiles just fine with no warnings under Borland 6 and VC 6.

CPP / C++ / C Code:

#include <fstream.h>

int main()
{
  ofstream outFile("sometext.txt",ios::app); // opens for appending

  outFile << "New line\n";

  outFile.close();

  return 0;
}

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