! 	  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; and CC Bordeianu, Univ Bucharest, 2007.
! 		Supported by the US National Science Foundation															 
! 		
!  exp-good.f90: calculate e^ - x as a finite sum, good algorithm

Program expgood

  Implicit none
  Real*8 :: element, min, max, step, sum, x   
	Integer :: n
	
	min  = 1E - 10
	max  = 10.
	step = 0.1
	open(6, File = 'exp-good.dat', Status = 'Unknown')
	                                                        ! summation
  Do x = 0, max, step                                            
    sum     = 1
    element = 1
    n = 0
		                                      ! while loop may never stop
    Do while ((abs(element/sum)  >  min) .or. (sum .eq. 0))
	    n = n + 1
      element = element*(-x)/n
	    sum     = sum + element
	  End Do
    write (6, *) x, sum
  End Do
  close(6)
  Stop 'data saved in exp-good.dat'
End Program expgood