C++ Primer 7.4

7.4.cpp

#include <iostream>

long double probability(unsigned numbers, unsigned picks, unsigned mega);

int main()
{
  using namespace std;
  // cout.setf(ios_base::fixed, ios_base::floatfield);

  double total, choices, mega;
  cout << "Enter the total number of choices on the game card and the number of picks allowed and the mega number:\n";
  while ((cin >> total >> choices>> mega) && choices <= total)
  {
    cout << "Your chance to win is: ";
    cout << probability(total, choices, mega) << endl; // compute the odds
    cout << "Next two numbers (q to quit):";
  }
  cout << "bye\n";

  return 0;
}

long double probability(unsigned numbers, unsigned picks, unsigned mega)
{
  long double result = 1.0;
  long double n;
  unsigned p;
  for (n = numbers, p = picks; p > 0; n--, p--) {
    result = result * n / p;
  }

  return 1 / (result * mega);
}