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Random walking on a torus

/ Published in: C
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% gcc -lm rndwalk-torus.c
% ./a.out 1000 > rnd1.data
% gnuplot
gnuplot> splot "rnd1.data" with line

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  1. #include <stdio.h>
  2. #include <stdlib.h>
  3. #include <time.h>
  4. #include <math.h>
  5.  
  6. #define R			(M_PI*4)
  7. #define r			(M_PI*2)
  8.  
  9. int main(int argc, char **argv)
  10. {
  11. 	int i, step;
  12. 	double x=R+r, y=0.0, z=0.0;
  13. 	double theta=0.0, phi=0.0, omega;
  14.  
  15. 	if(argc!=2 || (step=atoi(argv[1]))<=0){
  16. 		fprintf(stderr, "Usage: %s <step-number>\n", argv[0]);
  17. 		exit(EXIT_FAILURE);
  18. 	}
  19.  
  20. 	srand((unsigned)time(NULL));
  21. 	printf("%.3f\t%.3f\t%.3f\n", x, y, z);
  22. 	for(i=0; i<step; i++){
  23. 		omega=(double)rand() / (double)RAND_MAX * M_PI*2;
  24. 		theta+=cos(omega)/R;
  25. 		if(theta<0)				theta+=M_PI*2;
  26. 		else if(theta>M_PI*2)	theta-=M_PI*2;
  27. 		phi+=sin(omega)/r;
  28. 		if(phi<0)				phi+=M_PI*2;
  29. 		else if(phi>M_PI*2)		phi-=M_PI*2;
  30.  
  31. 		x=(R+r*cos(phi))*cos(theta);
  32. 		y=(R+r*cos(phi))*sin(theta);
  33. 		z=r*sin(phi);
  34. 		printf("%.3f\t%.3f\t%.3f\n", x, y, z);
  35. 	}
  36.  
  37. 	return 0;
  38. }

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