How annoying it is to experiment this way:

      int num_rabbits, num_foxes, num_grass;
      double rabbit_birth_rate, rabbit_death_rate;
      double grass_per_rabbit, rabbits_per_fox;
      double fox_brith_rate, fox_death_rate;
      int length_of_simulation;
      
      void main(void)
      { cout << "How much grass? ";
        cin >> num_grass;
        cout << "How many rabbits? ";
        cin >> num_rabbits;
        cout << "How much grass does each rabbit want? ";
        cin >> grass_per_rabbit;
        cout << "Rabbit birth rate? ";
        cin >> rabbit_birth_rate;
        cout << "Rabbit death rate? ";
        cin >> rabbit_death_rate;
        cout << "How many foxes? ";
        cin >> num_foxes;
        cout << "How many rabbits does each fox want? ";
        cin >> rabbits_per_fox;
        cout << "Fox birth rate? ";
        cin >> fox_birth_rate;
        cout >> "Fox death rate? ";
        cin >> fox_death_rate;
        // now we can start the simulation.










This would be so much easier all round:

      void main(void)
      { ifstream rfdata("conditions.txt");
        rfdata >> num_grass;
        rfdata >> num_rabbits;
        rfdata >> grass_per_rabbit;
        rfdata >> rabbit_birth_rate;
        rfdata >> rabbit_death_rate;
        rfdata >> num_foxes;
        rfdata >> rabbits_per_fox;
        rfdata >> fox_birth_rate;
        rfdata >> fox_death_rate;










And this would be so much better to use:

      void main(void)
      { ifstream rfdata("conditions.txt");
        rfdata >> num_grass;
        rfdata >> num_rabbits;
        rfdata >> grass_per_rabbit;
        rfdata >> rabbit_birth_rate;
        rfdata >> rabbit_death_rate;
        rfdata >> num_foxes;
        rfdata >> rabbits_per_fox;
        rfdata >> fox_birth_rate;
        rfdata >> fox_death_rate;
        while (true)
        { cout << "1: amount of grass = " << num_grass << "\n";
          cout << "2: amount of rabbits = " << num_rabbits << "\n";
          cout << "3: grass per rabbit = " << grass_per_rabbit << "\n";
          // etc etc etc
          cout << "9: fox death rate = " << fox_death_rate << "\n";
          cout << "0: run simulation\n";
          cout << "Your selection: ";
          int select;
          cin >> select;
          if (select==0)
            break;
          else if (select==1)
          { cout << "How much grass? ";
            cin >> num_grass; }
          else if (select==1)
          { cout << "How many rabbits? ";
            cin >> num_rabbits; }
          // etc etc etc
          else if (select==9)
          { cout << "Fox death rate? ";
            cin >> fox_death_rate; }
          else
            cout << "Bad input, try again"; }
        // start simulation here.







Global variables are bad.

What if I want to perform two simulations?

      int num_rabbits1, num_foxes1, num_grass1;
      double rabbit_birth_rate1, rabbit_death_rate1;
      double grass_per_rabbit1, rabbits_per_fox1;
      double fox_brith_rate1, fox_death_rate1;
      int length_of_simulation1;

      int num_rabbits2, num_foxes2, num_grass2;
      double rabbit_birth_rate2, rabbit_death_rate2;
      double grass_per_rabbit2, rabbits_per_fox2;
      double fox_brith_rate2, fox_death_rate2;
      int length_of_simulation2;


This would be so much better:

      struct state
      { int num_rabbits, num_foxes, num_grass;
        double rabbit_birth_rate, rabbit_death_rate;
        double grass_per_rabbit, rabbits_per_fox;
        double fox_brith_rate, fox_death_rate;
        int length_of_simulation; };

      state sim1, sim2, sim3;