STL

#include <iostream>
#include <fstream>
#include <iterator>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
 
typedef std::map<std::string, std::string> TableRow;
typedef std::vector<TableRow> Table;
 
void PrintRow(TableRow row) {
  std::cout << row["owner"] << " " << row["pet"] << " "
            << row["nick"] << std::endl;
};
 
int main(int argc, char *argv[]) {
  Table table;
  std::back_insert_iterator<Table> inserter(table);
 
  std::ifstream source_file("data.txt");
  while (source_file.good()) {
    TableRow row;
    source_file >> row["owner"] >> row["pet"] >> row["nick"];
    if (!source_file.eof()) inserter++ = row;
  }
 
  std::for_each(table.begin(), table.end(), PrintRow);
 
  std::map<std::string, std::map<std::string, int> > owners;
  for (Table::iterator i = table.begin(); i < table.end(); ++i)
    ++(owners[(*i)["owner"]][(*i)["pet"]]);
 
  for (std::map<std::string, std::map<std::string, int> >::iterator i =
       owners.begin(); i != owners.end(); ++i) {
    std::cout << (*i).first << ": " << std::endl;
    for (std::map<std::string, int>::iterator j = i->second.begin();
         j != i->second.end(); ++j)
      std::cout << (*j).first << " = " << (*j).second << std::endl;
  }
 
  std::map<std::string, std::map<std::string, int> > pets;
  for (Table::iterator i = table.begin(); i < table.end(); ++i) {
    ++(pets[(*i)["pet"]]["owners"]);
    ++(pets[(*i)["pet"]]["nicks"]);
  }
 
  for (std::map<std::string, std::map<std::string, int> >::iterator i =
       pets.begin(); i != pets.end(); ++i) {
    std::cout << i->first << ": owners = " << i->second["owners"]
              << ", nicks = " << i->second["nicks"] << std::endl;
  }
 
  std::map<std::string, std::set<std::string> > pet_owners;
  for (Table::iterator i = table.begin(); i < table.end(); ++i)
    pet_owners[(*i)["pet"]].insert((*i)["owner"]);
 
  for (std::map<std::string, std::set<std::string> >::iterator i =
       pet_owners.begin(); i != pet_owners.end(); ++i) {
    std::cout << i->first << ": ";
    for (std::set<std::string>::iterator j = i->second.begin(); j != i->second.end(); ++j)
      std::cout << *j << " ";
    std::cout << std::endl;
  }
 
  return 0;
}