/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; & CC Bordeianu, Univ Bucharest, 2007
   Support by National Science Foundation
*/
//  exp-good.c: A good algorithm for calculating exponential     
// related programs: exp-bad.c.c       

#include <stdio.h>
#include <math.h>
#define min 1E-10                               // limit for accuracy
#define max 10.                                      // maximum for x
#define step 0.1                                          // interval

main () {
	
  double x, sum, element;         
  int n;          
	
  FILE *output;                                        // file output
  output = fopen("exp-good.dat", "w");                     
  for (x = 0.0; x<= max; x+= step)  {
    sum= element= 1.;                              // reset variables
    n  = 0;
		                                     // sum till accuracy reached
    do {                                     
      n++; 
      element *=  -x/n;                     // calculate next element
         sum +=  element;
     }
		 while ((sum ==  0) || (fabs(element/sum) > min));
       fprintf(output, "%f\t%e\n", x, sum);
   }
   printf("results saved in exp-good.dat\n");     
   fclose(output);
}