File Help

[Malloc]

Hello everyone...

I am new in this forum so please forgive me if i'm doing this wrong.

Lets get to the problem now.I am trying to read a file and put data in a specific record,witch means that i should propably move the pointer of my file to the record that i want and then write the data that i want.The problem is that i don't know how to do that.I've just started learning c++ and so far i've been searching the net to find informations about creating files etc.

Thanks in advance
Malloc.....

P.S : My english aren't very good so just ignore any mistake you notice.

[khajeddin]

File Help

Hello everyone...

I am new in this forum so please forgive me if i'm doing this wrong.

Lets get to the problem now.I am trying to read a file and put data in a specific record,witch means that i should propably move the pointer of my file to the record that i want and then write the data that i want.The problem is that i don't know how to do that.I've just started learning c++ and so far i've been searching the net to find informations about creating files etc.

Thanks in advance
Malloc.....

P.S : My english aren't very good so just ignore any mistake you notice.

hi:
you have to use function "fseek" for moving the pointer of file ,and use function "fwrite "to write every record in the file.if you have to save several records you can use a WHILE loop ,with condition (!feof(pt)).
pt is a FILE pointer the you have been define it before.

#include<stdio.h>
FILE *pt;
pt=fopen("file name","condion of opneing the file")
while(!feof(pt))
    {
     fwrite ( void *buffer, int num_byte,int count ,FILE *pt );
     fseek(FILE *pt ,long num_byte ,int origin);
    }

[Malloc]

hi:
you have to use function "fseek" for moving the pointer of file ,and use function "fwrite "to write every record in the file.if you have to save several records you can use a WHILE loop ,with condition (!feof(pt)).
pt is a FILE pointer the you have been define it before.

#include<stdio.h>
FILE *pt;
pt=fopen("file name","condion of opneing the file")
while(!feof(pt))
    {
     fwrite ( void *buffer, int num_byte,int count ,FILE *pt );
     fseek(FILE *pt ,long num_byte ,int origin);
    }

Thanks a lot.I'll try it.

[horace1]

if you are coding in C++ you will probably use seekg() and seekp()

http://www.cplusplus.com/istream::seekg

http://www.cplusplus.com/reference/iostream/ostream/seekp.html

[Malloc]

Hi again

I tryed the seekp but i still have a problem.When i try to put a record to the file it puts it to the begining of it.At first i thought that the seekp didn't work but when i used tellp() to check it out i saw that the pointer was where i told it to be.So,i must be doing something wrong with the way i insert the record to the file.
Here is my code

myfile.open("Fi le.dat",ios::ou t | ios::app);
if (products.is_op en())
{
products.seekp( meg*kod,ios::be g);
products << proion.code ;
}
else
cout << "Couldn't open file......." << endl;

Please help.
Thanks......

PS: meg=the size of every record
kod = the number of the record that i want to go.

[horace1]

here is a simple example using seekp() and seekg()

#include <fstream>
#include <iostream>
using namespace std;

struct  Test{          // test structure
       int i;
       float x;
       };
       
int main () {
  long pos;
  Test test;
  fstream outfile;
  // create file with some data
  outfile.open ("test.txt",  ios::binary | ios::out);
    if (! outfile)                                                 // open OK ?
        {
        cout << "\nUnable to open file "  << strerror(errno);
        return 1;                                                 // open failed
        }
  // write 10 records
  for(int i=0; i< 10; i++)
      {
       test.i=i;
       test.x=i*1000.;
       outfile.write ((char *) &test,sizeof(Test));
       }
  outfile.close();  
  
  // now, open file for read/write
  outfile.open ("test.txt",  ios::binary | ios::out | ios::in);
  if (! outfile)                                                 // open OK ?
        {
        cout << "\nUnable to open file "  << strerror(errno);
        return 1;                                                 // open failed
        }
  // print the contents 
  cout << "\nfile contents\n";
  while(outfile.good())
      {
       outfile.read((char *) &test,sizeof(Test));
       if(outfile.good()) cout << "i " << test.i << " x " << test.x << endl;
       }
  outfile.clear();    // cklear error condition from last read
  
  // rewrite record 5 and add record on the end 
  outfile.seekp (sizeof(Test)*5);
  test.i=10000;
  test.x=20000.0;
  outfile.write ((char *) &test,sizeof(Test));
  outfile.seekp (0, ios::end);
  test.i=100000;
  test.x=200000.0;
  outfile.write ((char *) &test,sizeof(Test));

   // print the updated contents 
  cout << "\nupdated file contents\n";
  outfile.seekg (0);
  while(outfile.good())
      {
       outfile.read((char *) &test,sizeof(Test));
       if(outfile.good()) cout << "i " << test.i << " x " << test.x << endl;
       }
  outfile.clear();    // cklear error condition from last read
  outfile.close();
  cin.get();
  return 0;
}

when run it gives

file contents
i 0 x 0
i 1 x 1000
i 2 x 2000
i 3 x 3000
i 4 x 4000
i 5 x 5000
i 6 x 6000
i 7 x 7000
i 8 x 8000
i 9 x 9000

updated file contents
i 0 x 0
i 1 x 1000
i 2 x 2000
i 3 x 3000
i 4 x 4000
i 10000 x 20000
i 6 x 6000
i 7 x 7000
i 8 x 8000
i 9 x 9000
i 100000 x 200000

[Malloc]

here is a simple example using seekp() and seekg()

#include <fstream>
#include <iostream>
using namespace std;

struct  Test{          // test structure
       int i;
       float x;
       };
       
int main () {
  long pos;
  Test test;
  fstream outfile;
  // create file with some data
  outfile.open ("test.txt",  ios::binary | ios::out);
    if (! outfile)                                                 // open OK ?
        {
        cout << "\nUnable to open file "  << strerror(errno);
        return 1;                                                 // open failed
        }
  // write 10 records
  for(int i=0; i< 10; i++)
      {
       test.i=i;
       test.x=i*1000.;
       outfile.write ((char *) &test,sizeof(Test));
       }
  outfile.close();  
  
  // now, open file for read/write
  outfile.open ("test.txt",  ios::binary | ios::out | ios::in);
  if (! outfile)                                                 // open OK ?
        {
        cout << "\nUnable to open file "  << strerror(errno);
        return 1;                                                 // open failed
        }
  // print the contents 
  cout << "\nfile contents\n";
  while(outfile.good())
      {
       outfile.read((char *) &test,sizeof(Test));
       if(outfile.good()) cout << "i " << test.i << " x " << test.x << endl;
       }
  outfile.clear();    // cklear error condition from last read
  
  // rewrite record 5 and add record on the end 
  outfile.seekp (sizeof(Test)*5);
  test.i=10000;
  test.x=20000.0;
  outfile.write ((char *) &test,sizeof(Test));
  outfile.seekp (0, ios::end);
  test.i=100000;
  test.x=200000.0;
  outfile.write ((char *) &test,sizeof(Test));

   // print the updated contents 
  cout << "\nupdated file contents\n";
  outfile.seekg (0);
  while(outfile.good())
      {
       outfile.read((char *) &test,sizeof(Test));
       if(outfile.good()) cout << "i " << test.i << " x " << test.x << endl;
       }
  outfile.clear();    // cklear error condition from last read
  outfile.close();
  cin.get();
  return 0;
}

when run it gives

file contents
i 0 x 0
i 1 x 1000
i 2 x 2000
i 3 x 3000
i 4 x 4000
i 5 x 5000
i 6 x 6000
i 7 x 7000
i 8 x 8000
i 9 x 9000

updated file contents
i 0 x 0
i 1 x 1000
i 2 x 2000
i 3 x 3000
i 4 x 4000
i 10000 x 20000
i 6 x 6000
i 7 x 7000
i 8 x 8000
i 9 x 9000
i 100000 x 200000

Thanks a lot.That really helped me...