I/O Problems - fstream

Ok. Be a little gentle. Im coming from microprocessor background so file i/o is a bit new to me. Anyway, here is some code below. Its pretty simple, its reading a file that has multiple lines of ints in it. It writes all the ints to the output file on one line. Then it is SUPPOSED to read the file again, this time computing the average and outputting it on the next line.

But, in the debugger I get a divide by 0 error...so something is wrong with the i/o operation. *sigh*

Can anyone lend a hand?

PHP Code:

---


#include <iostream>
#include <iomanip>
#include <fstream>

using namespace std;

void main()
{
    int count;
    int num;
    int total;
    int avg;

    ifstream inData;
    ofstream outData;

    // lets get them all on one line
    inData.open("datfile1.txt");
    outData.open("outdata.txt");

    while(inData >> num)
    {
        outData << num << " ";
    }

    outData << endl;

    inData.close();

    // lets get the avg of all ints in the file
    
    inData.open("datfile1.txt");

    count = 0;
    avg = 0;

    while(inData >> num)
    {
        total += num;
        count++;
    }
    
    avg = total/count;
    outData << avg << endl;

    inData.close();
} 


---

look at outdata.txt to make sure it data got written correctly. You can comment out everything after closing inData and before reopening it to try to get it to run to this point first, because what you have looks like it should work. If it writes to outdata.txt ok then display just total and count after reading them in and not do the avg calculation. Those steps should help you localize the problem.

Ok. I have been playing a bit with it and I have found that the problem must lie in how I am opening and closing the file. Reason: if I take the first routine out and only open the file one time then the second routine works. So, now my question is...

Am I doing this the right? Is there a better way to re-read a file that doesnt involve sticking all of the contents in ram? Am I performing the open and close correctly?

Look at this alternative method. You can do it all in one loop if
you like. You should check if you really opened the files
though. Also, main returns int. Always. And you might try
to close your outputfile after writing to it ;-)

PHP Code:

---

#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    int count = 0;
    int num = 0;
    int total = 0;
    int avg = 0;

    ifstream inData;
    ofstream outData;

    inData.open("datfile1.txt");
    outData.open("outdata.txt");

    // check if the open calls actually succeeded !!!

    while(inData >> num)
    {
        outData << num << " ";

        total += num;
        count++;
    }

    avg = total/count;

    outData << endl << avg << endl;

    inData.close();
    outData.close();

    return 0;
} 


---

Thats how I would do it as well, but for this one I have to make each read independent. I am still trying to figure it out. It is rather maddening.

Oh, and just so everyone knows. I have initialized all the variables, so that is not the problem I am having. :)

I ran your code and it works fine except that "total" needs to be initialized. Either:

int total = 0;

or before the loop.

I believe your problem may be it can't find datfile1.txt. One way to find out is see if the file got opened.

inData.open("datfile1.txt");
if (!inData)
{
cout << "Could not open file.\n";
return 0;
}

It would be a good idea to make your main():
int main()

Thanks swoopy. I caught all my init stuff. That was just a dumb mistake. I know that the file is being found because it actually performs the first loop and outputs to the file. The other loop is when it crashes. Of course, if I comment out the first loop and just run the second then that one works. Sorry everyone. I am just trying to figure out file i/o.

*tears hair out*

No one has any other ideas?

Don't have my compiler available at this time to run your code. If you haven't figured it out by time I have it available I will compile it and let you know.

For now I would try placing a series of cout statements after each critical step to see exactly where things are getting hung up when you try to run the entire program.

PHP:--------------------------------------------------------------------------------
#include <iostream>
#include <fstream>

using namespace std;

int main()
{
int count = 0;
int num = 0;
int total = 0;
int avg = 0;

ifstream inData;
ofstream outData;

inData.open("datfile1.txt");
outData.open("outdata.txt");

// check if the open calls actually succeeded !!!
if(!infile)
{
cout << "unable to open file on first attempt" << endl;
}

while(inData >> num)
{
outData << num << " ";
cout << num << " ";
}

outData << endl;
cout << endl;

inData.close();

// lets get the avg of all ints in the file

inData.open("datfile1.txt");

if(!infile)
{
cout << "unable to open file on second attempt" << endl;
}


while(inData >> num)
{
outData << num << " ";

total += num;
cout << num << endl;
count++;
cout << count << endl;
}

avg = total/count;

outData << endl << avg << endl;
cout << avg << endl;

inData.close();
outData.close();

return 0;
}

I also might try using a second ifstream to read in the second time or commenting out the first call to inData.close() and the second call to inData.open().

I might also try making sure inData closed when it should have by commenting out the second call to inData.open() and see if you get the appropriate cout statement. You probably should comment out the second while loop if you do this though.

Good luck. I'll try to check back later.

Kinda new at file io but doesn't the first instance of a stream being attached to a file make it read and write protected from other streams unless specified not to be?

guest, thx for the code help.

I tried outputting to the screen to see exactly where the failure was and it is indeed upon 'opening' the file the second time. Now, why it is not closing/opening properly is beyond me. I just dont see it. Do I need to set a delay or something?

*shakes head*

Im telling you guys, you just dont have this kind of trouble with microcontrollers. :p

I pasted and compiled the code with all the couts listed. It ran fine. Then I took all the couts out and just ran the basic program. It ran and gave correct calculation of file (well, I did change avg to type float, but that doesn't matter much.) I use B4.52 as my IDE. I looked up the member functions for ifstreams. open() is listed, but close() is not. I looked up close() in my reference text, and it is clearly used in programs there in conjunction with ifstreams. I commented out the second call to inData.open() and it crashed. I used tellg() to find out the file cursor position at the end of the first while loop and then again after opening the file for the second time. The file positions were appropriate (at least the file pointer position went from 33 after the first loop and before closing inData to 0 after opening the file for the second time).

Maybe you can try retyping the lines with close() and the second call to inData.open(). Otherwise I am flumuxed, too. The code looks correct.

Well, I am getting a runtime error when I try to run the code above. In fact, I get the error for not being able to open the file the second time. Then the division by zero error crashes it since the file was never read the second time.

I didnt think file i/o was this complicated.

If you know anybody with the same compiler you are using you can try your source code there. If your source code runs on a different compiler, then I suspect you have an error in the software of your compiler.

As I said, your code closing and reopening the ifstream seems to work fine on my compiler.


You could try resetting the file pointer using seekg() rather than closing and reopening the file, although I have to say I had trouble trying to do that when I was goofing around with the code. If you want to try it, look up what header seekg() is in and #include it. then comment out the call to close() and the second call to inData.open(). Place the line:

inData.seekg(0);

immediately after the commented out call to open(). And try compiling/running.

Are you sure the code you initially posted is symbol for symbol the same as the code you are trying to run?

Being self_taught I used to run into these sorts of snafu's all the time. Getting better now, though still a lot to learn.

Idn't life wunerful?

Well, how about this.

*shakes head*

After playing some more I finally figured it out. The file stream was going into failstate, so just closing and reopening a file isnt going to work in this instance. So, I used the magical:

file.clear();

And now...it works like a charm. Amazing, the simplest things can be the hardest to figure out.

Thanks for all your help everyone!

Here is the code for example:

PHP Code:

---

#include <iostream>
#include <iomanip>
#include <fstream>

using namespace std;

void main()
{
    int count;
    float num;
    float total;
    float avg;

    ifstream inData;
    ofstream outData;

    // lets get them all on one line
    inData.open("datfile1.txt");
    outData.open("outdata.txt");

    while(inData >> num)
    {
        outData << num << " ";
    }

    outData << endl;

    inData.clear();
    inData.seekg(0);

    count = 0;
    avg = 0;
    total = 0;

    outData << fixed << showpoint
    << setprecision(3);
    
    while(inData >> num)
    {
        total += num;
        count++;
    }
    
    avg = total / count;
    outData << avg << endl;

    inData.close();
} 


---

I'm glad you got it to work.

Here's a post-mortem evaluation. How much truth there is in it is debateable, but in an effort to understand what is happening, it's worth the effort, at least to me.

If close() truly isn't a member function ifstreams then calling close() could cause the stream to fail and remain unavailable until the the stream is "cleared" with clear(). Finding EOF may also send flip the fail bit, necessitating the call to clear() when end of file has been reached and before resetting the file pointer, but who knows.

My compiler, may not be as persnickity as others and allows the call to close() without putting up a stink allowing the code to run as initially posted, whereas mol's may not let that happen.

>>>If close() truly isn't a member function ifstreams then calling close() could cause the stream to fail and remain unavailable until the the stream is "cleared" with clear().

First, close() is a member of ifstream and ofstream. fstream is derived from both ifstream and ofstream, therefore it also has the close method. If it's not in your docs, its an oversite (if you're using the MSVC docs, they don't list many of the fstream members if they've been defined in one of the base classes, which in the case of fstream, is just about all of its functions).

Second, if you try to call a function of a class that is not really a member function of the class, your compiler should choke on it. The exception would be when you're calling a function that's implemented in a seperate .dll. There are some cases there where you could actually call a function that didn't exist.