Please could someone fix my code

[Greatness]

#include <iostream>
 
void sizeYear(double,double,double ,int);
 
using namespace std;
 
int main()
{
 
   double population;
   double birthRate;
   double deathRate;
 
   int numYears;
 
   cout << "Please enter a Population" << endl;
   cin >> population;
 
   cout << "Please enter the Birth rate" << endl;
   cin >> birthRate;
 
   cout << "Please enter a Death rate" << endl;
   cin >> deathRate;
 
   cout << "Please enter the number of years to display" << endl;\
   cin >> numYears;
 
   sizeYear(population,birthRate,deathRate,numYears);
 
   while (numYears = 0)
   {
      if (numYears>0) population=1;
      else
         population=0;
   }
   return 0;
}

ok this is what i have came up with so far it should work but i have some errors could someone twink it a little

[Greatness]

g2.obj : error LNK2019: unresolved external symbol "void __cdecl sizeYear(double ,double,double, int)" (?sizeYear@@YAX NNNH@Z) referenced in function _main


that is the only error i am getting

[Ganon11]

Have you included a definition of sizeYears() yet?

[ilikepython]

   while (numYears = 0)
   {
      if (numYears>0) population=1;
      else
         population=0;
   }
   return 0;
}

ok this is what i have came up with so far it should work but i have some errors could someone twink it a little

You have while numYears = 0;. You want while numYears == 0;.
Also, I don't really get what you are trying to do with that loop. You are saying loop while numYears is equal to 0. Then, you check if numYears is greater than 0. It can't be both, so that if will never evaluate true. Also, the loop will run forever because you never change numYears so it will always be equal to 0.