Question

After adding records in the relative file, I am trying to update one field (the balance) of a given record(client) that the user provide the account number. The update happens in the file, but it is not properly done. The output shows that the update has affected other data, and it comes also with garbage. I cannot figure out the cause of the problem. Your help will be appreciated. I am using Dev-C++. The code followed by the output is below.

#include <iostream>   //   cin, cout
#include <iomanip>
#include <fstream>
#include <conio.h>
using namespace std;


#define SIZE 10

struct     client        // Client record
{   int    account;      // from 1 to SIZE
    char   name[20];
    double balance;
};


void show_file(char filename[])  // Sequential display of all records
{
   client c;
   int n=0;
   void *ptr;

   ifstream IS(filename, ios::in);  // Open for sequential read
   if(!IS) {cerr << filename<< " file open error." << endl; exit(1);}

   cout << "\n\nSHOW_FILE: The contents of file " << filename;

   while(ptr=IS.read((char *)&c, sizeof(c)))
   {
     cout <<'\n'<< setw(3)<< ++n << setw(6) << c.account <<setw(20)
       << c.name << setw(10) << c.balance;
   }

   IS.close();
}


int main(void)
{  client c;
   void *ptr; 
   int n=0, acc,number_of_records=SIZE, field1; 
   double new_balance, field3;
   char *fname = "credit.dat"; char field2;


   cout << "\nMAKE_FILE: Creating a blank relative file " << fname
     << " containing " << number_of_records << " records.";
   fstream iof(fname, ios:: in | ios::out | ios::binary );
   if(!iof) {cerr << "File open error." << endl; exit(1);}

   client blank={0, "", 0.0}; // Create an empty client record
   while(number_of_records--)
   iof.write((char *)&blank, sizeof(blank));
   cout << "\n\n\nFile has been succesfully created!";  //file is still empty, no records yet.

   cout<<"\n\nenter the 10 customers into the file: "<< fname<<endl<<endl;

   cout << "\nAccount[1.." << SIZE 
     << "], Name, Balance  (0 0 0 to exit)= ";
   cin >> c.account >> c.name >> c.balance;

   while(0 < c.account) // && c.account <= maxrec)
   {
     iof.seekp((c.account-1) * sizeof(client));  // position the pointer
     iof.write((char *)&c, sizeof(c));
     cout << "Account[1.."<< SIZE 
       << "], Name, Balance  (0 0 0 to exit)= ";
     cin >> c.account >> c.name >> c.balance;
  }

  cout << "\n\nAccount number to apply changes on balance(0 to exit) = "; 
  cin >> acc;
  /// while(0 < acc && acc <= SIZE)
  if (0<acc && acc <= SIZE)
  {
    //cout << "\nPositioning at " << (acc-1) * sizeof(client)<< endl;
    iof.seekg((acc-1) * sizeof(client));  // position the pointer
    iof.read((char *)&c, sizeof(c));
    if(c.account) 
     cout <<'\n'<< setw(6) << c.account <<setw(20)
                     << c.name << setw(10) << c.balance;

    new_balance=c.balance+0.05*(c.balance);  //calculation of the new balance by adding interests of 5%

    cout<<"\n\n\nnew balance after the 5% interest:"<<new_balance<<endl;
    c.balance=new_balance;
    cout<<"current new balance:  "<<c.balance<<endl;  //just to check if it will be displayed

    //WHERE THE PROBLEM IS...
    iof.seekg(0, ios::cur); //trying to stay in the current position to apply 
                          //change on current balance
    iof<<c.account << c.name << c.balance;  //trying to update record with new balance

  }
  else cout << "\nEmpty record";


  iof.close();
  cout<<"\n\nFILE after THE UPDATE: "<<endl;
  show_file (fname);



   cout << "\n\n"; 
   system("pause");
   return 0;

}


*********************output**************************

MAKE_FILE: Creating a blank relative file credit.dat containing 10 records.


File has been succesfully created!

enter the 10 customers into the file: credit.dat


Account[1..10], Name, Balance  (0 0 0 to exit)= 1 aaaa 2399
Account[1..10], Name, Balance  (0 0 0 to exit)= 2 bbbb 4000
Account[1..10], Name, Balance  (0 0 0 to exit)= 3 cccc 50
Account[1..10], Name, Balance  (0 0 0 to exit)= 4 dddd 5000
Account[1..10], Name, Balance  (0 0 0 to exit)= 5 eeee 180
Account[1..10], Name, Balance  (0 0 0 to exit)= 0 0 0


Account number to apply changes on balance(0 to exit) = 3

 3                cccc        50


new balance after the 5% interest:52.5
current new balance:  52.5


FILE after THE UPDATE:


SHOW_FILE: The contents of file credit.dat
  1     1                aaaa      2399
  2     2                bbbb      4000
  3     3                cccc        50
  41667457843             c52.5♫'      5000
  5     5                eeee       180
  6     0                             0
  7     0                             0
  8     0                             0
  9     0                             0
 10     0                             0

Press any key to continue . . .

Answer 1

Ha ha ha, we must be in the same class…

You use seekg() to position pointer to read and seekp() to position pointer to write.

Like this as you did earlier in your file:

iof.seekp((c.account-1) * sizeof(client)); // position the pointer
iof.write((char *)&c, sizeof(c));

seekg = seek to get
seekp = seek to put – requ

good luck!