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Working with Multiple Files

It is possible to open more than one file at a time.  Simply declare and use a separate stream variable name (fout, fin, fout2, fin2 -- file pointer) for each file.  You may need to use this multiple file process for situations where you need to compare file data, to use one set of file data to create a new set of adjusted data, or to simply use a combination of various files to accomplish your programming task.
EXAMPLE:
//Retrieve all of the zipcodes that are 13090 from an existing file which
//contains name, address and zipcode as three separate entries.
//Place the newly found names, addresses and zipcodes into a new file.
#include <iostream.h>
#include <fstream.h>
#include "apstring.cpp"
#include <stdlib.h>

int main(void)
{
     apstring name, address, dummy;
     int zipcode;
     ifstream fin;
     ofstream fout;

    system("CLS");

     fin.open("add.dat", ios::in);
     assert(! fin.fail( ) );
     fout.open("newZip.dat", ios::out);
     assert (! fout.fail( ) );

     getline(fin,name);       //priming loop
     getline(fin,address);
     fin>>zipcode;
     getline(fin,dummy);

     while(!fin.eof( ) )
     {
          if (zipcode==13090)
          {
               fout<<name<<endl;
               fout<<address<<endl;
               fout<<zipcode<<endl;
          }
         getline(fin,name);
         getline(fin,address);
         fin>>zipcode;
         getline(fin,dummy);
     }
     fout.close();
     fin.close();
     return 0;
}

 

If you need to open two or more files for input, simply give the stream variables different names, such as
fin, fin2, fin3, etc.