! 	  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				 
! 
! fit.f95: Least square fit											
  
Program fit

  Implicit none
  Integer :: i
	Real*8 :: s, sx, sy, sxx, sxy, delta, inter, slope
	Real*8 :: x(12), y(12), d(12)
	
	Data y /328, 187, 821, 78, 88, 6, 5, 82, 2, 0.1, 84, 1/  ! y values
	Do	i = 1, 12                                            ! values x
	  x(i) = i*10 - 5
	End Do
	                                                    ! input delta y
  Do	i = 1, 12                                              
	  d(i) = 1.
	End Do
  s = 0.0;   sx = 0.;   sy = 0.
  sxx = 0.
  sxy = 0.
                                                    ! calculate  sums
	Do	i = 1, 12
	  s	 = s	 + 				 1 / (d(i)*d(i))
		sx	 = sx	 + 			x(i) / (d(i)*d(i))
		sy	 = sy	 + 			y(i) / (d(i)*d(i))
		sxx = sxx + x(i)*x(i) / (d(i)*d(i)) 
		sxy = sxy + x(i)*y(i) / (d(i)*d(i))
	End Do
                                             ! calculate coefficients
  delta =  s*sxx - sx*sx
	slope = 	(s*sxy - sx*sy) / delta		
	inter = (sxx*sy - sx*sxy) / delta
	write(*, *) 'intercept = ', inter
	write(*, *) 'slope = ', slope
	write(*, *) 'correlation = ', - sx/sqrt(sxx*s)		
	Stop 'fit'
End Program fit