[Valmont]

Something like this?

#include <iostream>
#include <cstring>
#include <cmath> //floor()

using namespace std;

const double BOOM_FACTOR = 1.15;
const double BAD_FACTOR = 0.8;

int main()
{
  double population = 0;
  char userChoice = 'y';
  
  cout << "Enter a starting population size: ";
  cin >> population;
  
  double PredictBoomPopulation;
  double PredictBadPopulation;
  
  //std::floor: 0.67 humans don't exist.
//Let's add 0.5 because of rounding errors during conversion.

  PredictBoomPopulation = floor(population* BOOM_FACTOR);
  PredictBadPopulation = floor(population *BAD_FACTOR);
  cout << "After a boom year, your population will grow to: "
    << PredictBoomPopulation << endl;
  cout << "After a bad year, your population will shrink to: "
    << PredictBadPopulation << endl;
  
  do
  {
    cout<<endl;        
    cout << "Was it a boom or BAD year (o/a)? ";
    cin >> userChoice;
    if (userChoice == 'o')
    {
      population = floor(PredictBoomPopulation);
    }
    if (userChoice == 'a')
    {
      population = floor(PredictBadPopulation);
    }

    cout<<"Population: "<<population<<endl<<endl;
    
    PredictBoomPopulation = floor(population* BOOM_FACTOR+.5);
    PredictBadPopulation = floor(population *BAD_FACTOR+.5);
    cout << "After a boom year, your population will grow to: "
        << PredictBoomPopulation << endl;
    cout << "After a bad year, your population will shrink to: "
        << PredictBadPopulation << endl;

    cout<<"Do you wish to continue program? (yes = y): ";
    cin>>userChoice;
  } while(userChoice == 'y');
  
  cout<<"Program terminated."<<endl;
   return 0;
}