факториал

#include <iostream>
 
int fact (int n)
{
    if (n == 0 || n == 1)
        return 1;
    return fact(n-1) * n;
}
 
int main()
{
    double result = 1;
    int n;
    std:: cin >> n;
 
    for (int i=0; i<n; ++i)
        result += 1.0/fact(i);
    
    std:: cout << std::fixed << result << std::endl;
    system ("pause");
    return 0;
}

  double sum;
  int n, i;
   
  n = 1000;
  
  sum = 1;
  for (i = 2; i <= n; i++)
      sum /= i;

#include <iostream>
 
int main()
{
   double sum = 1.0, first = 1.0;
   int n = 2;
   
   for (int i = 1; i <= n; i++)
   {
       first /= i;
       sum += first;      
   }
      
   std::cout << sum;
   return 0;
 }

cout << exp(1) << endl;

#include <iostream>
#include <ratio>
#include <cstdint>
 
template < int i >
struct Factorial
{
    static const std::uint64_t value = i * Factorial< i - 1 >::value;
};
 
template <>
struct Factorial< 1 >
{
    static const int value = 1;
};
 
template < int i >
class Sum
{
private:
    typedef std::ratio < 1, Factorial< i >::value > current;
 
public: 
    typedef std::ratio_add< current, typename Sum< i - 1 >::value > value;
};
 
template <>
class Sum< 1 >
{
public:
    typedef std::ratio< 1, 1 > value;
};
 
int main()
{
   constexpr int n = 10;
    
   typedef typename Sum< n >::value result_ratio;
    
    constexpr std::uint64_t num = result_ratio::num;
    constexpr std::uint64_t den = result_ratio::den;
    std::cout << static_cast< double > (num) / den + 1 << std::endl;
}

#include <iostream>
 
using namespace std;
 
int main()
{
    double sum, yi;
    int n, i;
   
    n = 1000;
  
    sum = yi = 1;
    for (i = 1; i <= n; i++) {
        yi /= i;
        sum += yi;
    }
    
    cout << sum << endl;
    
    return 0;
}