// Program to calculate function exp(-x)
// using straightforward summation with differing  precision
using namespace std;
#include <iostream>
#include <cmath>
// type float:  32 bits precision
// type double: 64 bits precision
#define   TYPE          double
#define   PHASE(a)      (1 - 2 * (abs(a) % 2))
#define   TRUNCATION    1.0E-10
// function declaration 
TYPE factorial(int);

int main()
{
  int   n;
  TYPE  x, term, sum;
  for(x = 0.0; x < 100.0; x += 10.0)  {
    sum  = 0.0;                //initialization
    n    = 0;
    term = 1;
    while(fabs(term) > TRUNCATION)  {
      term =  PHASE(n) * (TYPE) pow((TYPE) x,(TYPE) n) / factorial(n);
      sum += term;
      n++;
    }  // end of while() loop 
    cout << " x = " << x << " exp = " << exp(-x) << " series = " << sum;
    cout  << " number of terms = " << n << endl;
  } // end of for() loop 
  return 0;
} // End: function main() 


//     The function factorial()
//     calculates and returns n!
 
TYPE factorial(int n)
{
  int  loop;
  TYPE fac;
  for(loop = 1, fac = 1.0; loop <= n; loop++)  {
    fac *= loop;
  }
  return fac;
} // End: function factorial()