! 	  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															 
! 		
! walk.f90:RanDom walk simulation				

Program walk

  Implicit none
	Real*8 :: ranDom, root2, x, y, r(1:10000)
	Integer :: i, j, max
	
  max		 = 10000                        ! set parameters (# of steps)
	root2 = 1.4142135623730950488E0
  open(6, FILE = 'walk.dat', Status = 'Unknown')          ! open file
	                                                      ! clear array
	Do	j = 1, max                                              
	  r(j) = 0
	End Do
                                            ! average over 100 trials
	Do	j = 1, 100
	  x = 0.
		y = 0.
		Do	i = 1, max
		  x = x + (ranDom() - 0.5)*2.0*root2
			y = y + (ranDom() - 0.5)*2.0*root2
			r(i) = r(i) + Sqrt(x*x + y*y)
    End Do
  End Do 
	                            ! output data for plot of r vs. sqrt(N)
  Do	i = 1, max                    
	  Write (6, *) Sqrt(Real(i)), ' ', r(i)/100
	End Do
	close(6)
	Stop 'data saved in walk.dat'
End Program walk