/* 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.c: Random walk simulation   
 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

//    if you don't have drand48 uncomment the following two lines 
    #define drand48 1.0/RAND_MAX*rand 
   #define srand48 srand                 
                        
#define SQRT2 1.4142135623730950488E0
#define max 10000
#define seed 68111

main() {
	
   int i, j;
   double x, y, r[max+1];
	 
   FILE *output;                             // save data in walk.dat
   output = fopen("walk.dat","w");
	 
   srand48(seed);                        // seed the number generator
	                                                     // clear array
   for (i=0; i<=max; i++) r[i]=0.0;                    
   for (j=1; j<=100; j++)  {               // average over 100 trials
     x = y= 0;       
		                                               // starting point
     for (i=1; i<=max; i++)  {
       x += (drand48()-0.5)*2*SQRT2;             // dx and dy between
       y += (drand48()-0.5)*2*SQRT2;          // -sqrt(2) and sqrt(2)
       r[i] += sqrt(x*x + y*y);               // distance from origin
     }
   }
   for (i=0; i<=max; i++)  
		                fprintf(output, "%f\t%f\n", sqrt(i), r[i]/100.);
   printf("data stored in walk.dat.\n");
   fclose(output);
}