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Opening Multiple files, alternatives to fopen()

Labib666 (4)
Hello, I am a newbie programmer and am interested in competitive programming.
I made a grader for COCI problems recently. In a function of this code, I take input from input files using a loop. Its file opening part looks like this -

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int next(int id)
{
        // [[OPEN FILES]] -----------------------

        string name1 = probid+".in." + itoa(id);
        string name2 = probid + "OUTPUT" +".out." + itoa(id);

	FILE *fp1 = fopen(name1.c_str(), "r");   
	if(!fp1) return 0;                            // no file left?
	FILE *fp2 = fopen(name2.c_str(), "w");        
        
        // process data
}


"id" changes and opens the input files and writes results to output file.
The main problem is I have to read data using (fscanf) but I want to take input using cin, cout. (things freopen offers)

but when I run the loop, it fails to read input from more than one file using freopen. so I have to use fopen().

Is there anyway I can use cin, cout to take input from files using this function?
Last edited on
JLBorges (13770)
> but when I run the loop, it fails to read input from more than one file using freopen.
> so I have to use fopen().

Perhaps you should post the code for the loop.
Labib666 (4)
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int next(int id)
{
        // [[OPEN FILES]] -----------------------

        string name1 = probid+".in." + itoa(id);
        string name2 = probid + "OUTPUT" +".out." + itoa(id);

	FILE *fp1 = fopen(name1.c_str(), "r");   
	if(!fp1) return 0;                            // no file left?
	FILE *fp2 = fopen(name2.c_str(), "w");        
        
        // process data
        return 1;
}

int main()
{
        int i = 1;
        while (next (i++)) ;
        return 0;
}



The loop is like this ^^
Full code :
http://ideone.com/TMbx3s
Last edited on
JLBorges (13770)
The local variables FILE* fp1 and FILE* fp2 have automatic storage duration.; the life time of the objects are over when the function returns.

std::freopen() reassigns an existing file stream stream to a different file.
When next() is called, fp1 and fp2 from the previous invocation of the function no longer exist.

Change the storage duration to static?

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int next(int id)
{
        // [[OPEN FILES]] -----------------------

        string name1 = probid+".in." + itoa(id);
        string name2 = probid + "OUTPUT" +".out." + itoa(id);

	static FILE *fp1 = 0 ;
        fp1 = freopen(name1.c_str(), "r", fp1 );   
	if(!fp1) return 0;                            // no file left?
	
        static FILE *fp2 = 0 ;
        freopen(name2.c_str(), "w", fp2 );        
        
        // process data
        return 1;
}
Labib666 (4)
I edited my code, with stdin as third parameter, function read 1 input file, but now it doesn't even read 1 file. I double checked file name and directories. It doesn't read the input file. (returns "nope").
Tried it without fclose() at the bottom. still same result.. :(

My new function is like this

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int next(int id)
{
        // [[FILES]] --------------------------------------------------------------------------------------
		string name1 = probid+".in." + itoa(id);
		string name2 = probid + "OUTPUT2" +".out." + itoa(id);

		static FILE *fp1 = 0 ;
		fp1 = freopen (name1.c_str(), "r", fp1);       // input file opens
		if (!fp1) { cout << "nope\n"; return 0; }      // check


		static FILE *fp2 = 0 ;
		freopen (name2.c_str(), "w", fp2);             // output file opens


		// [[PROCESS INPUT]] --------------------------------------------------------------------------------


		// [local variables] ++++++++++++++++++++++++++++++++++++++++++++++++++++




        // [initialise globals] +++++++++++++++++++++++++++++++++++++++++++++++++





        //  [take input] +++++++++++++++++++++++++++++++++++++++++++++++++++++++++




        // [[PROCESS DATA FOR OUTPUT]] --------------------------------------------------------------------------




        // [[close files]] ----------------------------------------------------------------------------------------

        fclose(fp1);
        fclose(fp2);


	return 1;
}


JLBorges (13770)
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int next(int id)
{
    string name1 = probid+".in." + itoa(id);
    string name2 = probid + "OUTPUT2" +".out." + itoa(id);

    static FILE *fp1 = 0 ;
    if(fp1) fp1 = freopen( name1.c_str(), "r", fp1 );  // if it is already open, reopen it
    else fp1 = fopen( name1.c_str(), "r" ) ; // else open it

    if (!fp1) { cout << "nope\n"; return 0; } 

     // likewise for fp2

     // ...


     // do not close the files; we intend to use freopen them later
     // fclose(fp1);
     // fclose(fp2);
     
     // or if we do close them, reset fp1 and fp2 to null pointers.

}
Labib666 (4)
now it's just taking input from me manually (when using cin, cout) and giving output manually (on the screen, not in file)
closed account (o3hC5Di1)
Libib666 wrote:
now it's just taking input from me manually (when using cin, cout) and giving output manually (on the screen, not in file)


I'm afraid I don't follow - isn't that what std::cin and std::cout are supposed to do?
Also, is there a specific reason you're using the C file-handling functions as opposed to C++'s <fstream>?

All the best,
NwN
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