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// calculate PI by throwing darts

#include <iostream>
using namespace std;

#include <cstdlib>
#include <ctime>
#include <iomanip>

double darts(int); //prototype

int main()
{
   int num_darts;
   double PI;

   cout << "Enter the number of darts to be thrown: ";
   cin >> num_darts;

   srand(time(NULL)); // random number seed

   PI = darts(num_darts);

   cout << "For " << num_darts << " darts thrown the value of PI is " << PI << endl;

}

double darts(int num_darts)
{
  int i, p=0;

  for (i = 1; i <= num_darts; i++)
  {
     double x, y;
     x = static_cast< double >( rand() )/RAND_MAX;
     y = static_cast< double >( rand() )/RAND_MAX;

     if ( (x*x + y*y) <= 1.0)
        p++;
  }

  return 4.0*static_cast< double >( p )/num_darts;
}

% g++ ran_pi.cpp
% a.out
Enter the number of darts to be thrown: 500
For 500 darts thrown the value of PI is 3.176