Чтение из двух файлов и запись в один

    std::ifstream old_report("oldmast.dat");
    std::ifstream transaction("trans.dat");
    std::ofstream new_report("newmast.dat", std::ios::app);
 
    if( old_report.fail( ) || transaction.fail( ) )
    {
        std::cerr << "This file cann't open by PC" << std::endl;
        exit(1);
    }
 
    clientData client;
    transData trans;
 
    while( old_report >> client.accountNum >> client.surname >> client.name >> client.balance )
    {
        while( transaction >> trans.accountNum >> trans.balance )
        {
            if(client.accountNum == trans.accountNum)
                new_report << client.accountNum << '\t' << client.surname << '\t' << client.name << '\t' <<    (client.balance + trans.balance) << std::endl; 
        }
    }

typedef std::map<std::string, double> transaction_map_t;
 
transaction_map_t transactionMap;
for (transData trans; transaction >> trans.accountNum >> trans.balance; ) {
   transactionMap.insert(std::make_pair(trans.accountNum, trans.balance));
}
 
// ...
 
while( old_report >> client.accountNum >> client.surname >> client.name >> client.balance ) {
   transaction_map_t::const_iterator pos const = transactionMap.find(client.accountNum);
   if (pos != transactionMap.end()) {
      new_report << client.accountNum << '\t' << client.surname << '\t' << client.name << '\t' <<    (client.balance + trans.balance) << std::endl; 
   }
}

while( old_report >> client.accountNum >> client.surname >> client.name >> client.balance )
{
     while( transaction >> trans.accountNum >> trans.balance )
     {
         if(client.accountNum == trans.accountNum)
             new_report << client.accountNum << '\t' << client.surname << '\t' << client.name << '\t' <<    (client.balance + trans.balance) << std::endl; 
     }
     transaction.clear();
     transaction.seekg(0, ios::beg);
}

new_report << client.accountNum << '\t' << client.surname << '\t' << client.name << '\t' <<    (client.balance + it->second) << std::endl;