/* 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.
   Support by National Science Foundation                              
*/
// Walk.java Random walk simulation	 

import java.io.*;													// Location of PrintWriter
import java.util.*;														 // Location of Random
import java.lang.*;																// Location of Math

public class Walk {

	public static void main(String[] argv) 
											throws IOException, FileNotFoundException {
												
		PrintWriter q = new PrintWriter(
										new FileOutputStream("Walk.dat"), true); 
		int imax = 1999, jmax = 10000, i = 0, j = 0;					
		double x = 0., y = 0., root2 = Math.sqrt(2.); 
		double r[] = new double[imax + 1]; 
		
		long seed = 3008157;		             // New seeds => new sequence
		Random randnum = new Random(seed); 
		                                                  // Initialize r
		for ( i=0;	i <= imax;	i++ ) r[i] = 0. ;		
		                                           // Average over trials
		for ( j=0;	j <= jmax;	j++ ) {								
			x = 0. ;	y = 0. ;													 // Start at origin
				for ( i=0;	i <= imax;	i++ ) {			// -root2<dx, dy< +root2
					        // Access next random double: randnum.nextDouble()
					x += (randnum.nextDouble()-0.5)*2.*2; 
					y += (randnum.nextDouble()-0.5)*2.*root2; 
					r[i] += Math.sqrt((x*x) + (y*y));								  // Radius
				} 
		}
		                                                        // Output
		for ( i=0;	i <= imax;	i++ )	 q.println((i + 1) 
				 + "	" + Math.sqrt(i) + "	" + r[i]/((double)jmax)); 
		System.out.println(" "); 
		System.out.println("Walk Program Complete."); 
		System.out.println("Data stored in Walk.dat"); 
		System.out.println("Data Format: k, sqrt(k), r[k]"); 
		System.out.println("k = step number, k = 1 to " + (imax + 1)); 
		System.out.println("r[k] = distance from origin at step k"); 
		System.out.println("data averaged over " + jmax + " trials"); 
	} 
}