! 	  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-bad.f90: calculating exp(- x) as a finite sum, bad algorithm
      
Program expbad

  Implicit none
         ! min =  accuracy, x step, max in x, up numer, down denomin.
  Real*8 :: down, min, max, step, sum, up, x
	Integer :: i, j
	
	min  = 1E - 10
	max  = 10.
	step = 0.1
	open(6, File = 'exp-bad.dat', Status = 'Unknown')
	                                                        ! summation
  Do x = 0, max, step                                           
    sum = 1
    i = 0
	  down = 1 
    up = 1
		                                      ! while loop may never stop
    Do while((sum == 0).or.(abs((up/down)/sum)  >  min))   
	    i = i + 1
      down = 1 
      up = 1
        Do  j = 1, i
	        up   = - up*x
	        down = down*j
        End Do
        sum = sum + up/down
    End Do
    write (6, *) x, sum
  End Do
  close(6)
  Stop 'data saved in exp-bad.dat'
End Program expbad