c  From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
c  by RH Landau, MJ Paez, and CC Bordeianu 
c  Copyright Wiley-VCH, 2007.
c  Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
c  MJ Paez, Univ Antioquia, 2007; and CC Bordeianu, Univ Bucharest, 2007.
c  Support by National Science Foundation                             
c    		
c  walk.f: random walk simulation                               
c	        If your compiler complains about drand48, seed48,         
c         replace drand48 with rand(seed) and remove call to seed48	                                             
c         Data output as sqrt(steps), distance                      
c
   	  Program walk
   	    Implicit none
      	Real*8 drand48, root2, theta, x, y, r(1:10000)
        Integer i, j, max, seed
c                                         set parameters (# of steps)
	      max = 10000
	      seed = 11168
	      root2 = 1.4142135623730950488E0
c                                           open file, seed generator
	      Open(6, file = 'walk.dat', Status = 'Unknown')
	      Call seed48(seed)
c                                                         clear array
	      Do 1 j = 1, max
	        r(j) = 0
 1	    Continue
c                                             average over 100 trials
	      Do 10 j = 1, 100
          x = 0
          y = 0
c                                                      take max steps
          Do 20 i = 1, max
            x = x + (drand48()-0.5)*2.*root2
            y = y + (drand48()-0.5)*2.*root2
             r(i) = r(i)+ Sqrt(x * x + y * y)
 20       Continue
 10	    Continue 
c                               output data for plot of r vs. sqrt(N)
  	    Do 30 i = 1, max
          Write (6, *) Sqrt(Real(i)), ' ', r(i)/100
 30    	Continue
 	      Close(6)
	      Stop 'data saved in walk.dat'
	    End