opening files sequentially

hi,

i have a problem with my following code. i'm trying to open 15 files sequentially named file01.txt to file15.txt. the very first file is opening and the program displays the contents perfectly. however i'm having problems opening file02.txt to file15.txt even though these files exist and the contents are the same as file01.txt. any help with debugging is greatly appreciated.

thanks.

Code:

---

void open_files()
{
string newfilename = "", extension = ".txt", filename = "", numstr = "", line = "";
char buffer[258] = "", numchr[3] = "";
ifstream file;
int num_of_files = 15;

        for(int file_number = 1; file_number < num_of_files + 1; file_number++)
        {
                if(file_number < 10)
                        newfilename = "file0";
                if(file_number > 9)
                        newfilename = "file";

                itoa(file_number, numchr, 10);
                numstr = numchr;
                newfilename = newfilename + numstr + extension;

                file.open(newfilename.c_str());

                if(file.fail())
                        cout << "file open failed!" << endl;

                while(!file.eof())
                {
                        file.getline(buffer, 258-1);
                        line = buffer;
                        cout << line << endl;
                }
        }
}

---

I had the same problem when I tried to re-use my fstream objects.. all you have to do is call the .clear( ) member each time before you want to open an additional file.

hi,

thanks for your response.

i tried file.clear() at the beginning and the end of the for-loop and i'm still receiving the same output and issues as before. i forgot to clear my fstream object and now i am. i have a feeling that the problem is in converting the number to character/string. but i'm creating 1 the same way 2 to 15 are created and 1 is fine.

what about closing file after you finish with it?

and also you should read Why it's bad to use feof() to control a loop

ok. now i have:

Code:

---

        file.close();
        file.open(newfilename.c_str());

---

and still the same.

Code:

---

        const int num_of_files = 2;
        for(int file_number = 1; file_number < num_of_files + 1; file_number++)
        {
                string newfilename ("c:\\test");
                newfilename += file_number/10 + '0';
                newfilename += file_number%10 + '0';
                newfilename += ".txt";
               
                ifstream file;
                file.open(newfilename.c_str());
               
                if(file.fail())
                        cout << "file open failed!" << endl;
                else
                        while(!file.eof())
                        {
                                char buffer[258];
                                file.getline(buffer, sizeof (buffer));
                                string line(buffer);
                                cout << line << endl;
                        }
        }

---

That's what I've tried and it IS reading 2 files...
Maybe you should just debug your app and see where is the problem?

>> and still the same.
Where is the call to clear()? That should be there as well, probably after the close and before the open.

A better solution would be to just declare your ifstream variable inside the for loop. It is better practice to restrict your variables to the smallest scope you need. In this case you will get the added benefit of automatic closing of the stream and clearing of any error bits.

hi everyone,

the following works fine after the .clear() and .close() suggestion. the .clear() is completely new to me. i also debugged the heck out of it and found out that when my num_of_files is greater than the files found, it goes in an infinite loop but that can be fixed by sending num_of_files to the open_files() function.

Code:

---

void open_files()
{
string newfilename = "", extension = ".txt", filename = "", numstr = "", line = "";
char buffer[258] = "", numchr[3] = "";
ifstream file;
int num_of_files = 5;

        for(int file_number = 1; file_number < num_of_files + 1; file_number++)
        {
                if(file_number < 10)
                        newfilename = "file0";
                if(file_number > 9)
                        newfilename = "file";

                itoa(file_number, numchr, 10);
                numstr = numchr;
                newfilename = newfilename + numstr + extension;

                file.close();                                        // new addition
                file.clear();                                        // new addition
                file.open(newfilename.c_str());

                if(file.fail())
                {
                        cout << "file open failed!" << endl;
                        break;
                }

                while(!file.eof())
                {
                        file.getline(buffer, 258-1);
                        line = buffer;
                        cout << line << endl;
                }
        }
}

---

> while(!file.eof())
Why are you still doing this?

> file.getline(buffer, 258-1);
Since file.getline(line) also works, why mess with a char array at all?

In fact,
while ( file.getline(line) ) cout << line << endl;
is all you need.

Here is how I'd do it. Many minor changes particular to how I typically do things. Changing all of your formating code to a single line using a stringstream object and opening the file using that stringstream object's str member function to turn it into a string and then of course the string's c_str member function directly after that to get a character pointer that the file stream's constructor expects. The declaration of the file stream object in the scope of the loop as opposed to being outside the loop means that it will automatically close and be cleared of any error flags within each run through the loop so you don't need to use the clear and close functions.

One important point is there is a problem with how you are doing the loop through the file to read data. You shouldn't use eof to control looping, it can be problematic, you may end up seeing the last line of data from the file twice. Better is to use the return value from the getline call itself to determine whether the read succeeds or not. Also, there is a string version of getline that I've used below as opposed to the char array version. This cleans up things a bit, you don't need the character array and the extra line of code that converts the array to a string before output... actually you don't need to convert at all, you could have just output the array directly.

Code:

---

#include <sstream>
#include <iomanip>

...

void open_files()
{
    string extension = ".txt", line;
    int num_of_files = 5;

    for(int file_number = 1; file_number <= num_of_files; file_number++)
    {
        // Declare the stringstream and format the file name
        stringstream sstr;
        sstr << "file" << setw(2) << setfill('0') << file_number << extension;

        // Declare and open the file
        ifstream file(sstr.str().c_str());

        if(file.fail())
        {
            cout << "file open failed!" << endl;
            continue;
        }

        // This is a better way to loop through a file
        while( getline(file,line) )
        {
            cout << line << endl;
        }
    }
}

---