#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>

#define R			(M_PI*4)
#define r			(M_PI*2)

int main(int argc, char **argv)
{
	int i, step;
	double x=R+r, y=0.0, z=0.0;
	double theta=0.0, phi=0.0, omega;

	if(argc!=2 || (step=atoi(argv[1]))<=0){
		fprintf(stderr, "Usage: %s <step-number>\n", argv[0]);
		exit(EXIT_FAILURE);
	}

	srand((unsigned)time(NULL));
	printf("%.3f\t%.3f\t%.3f\n", x, y, z);
	for(i=0; i<step; i++){
		omega=(double)rand() / (double)RAND_MAX * M_PI*2;
		theta+=cos(omega)/R;
		if(theta<0)				theta+=M_PI*2;
		else if(theta>M_PI*2)	theta-=M_PI*2;
		phi+=sin(omega)/r;
		if(phi<0)				phi+=M_PI*2;
		else if(phi>M_PI*2)		phi-=M_PI*2;

		x=(R+r*cos(phi))*cos(theta);
		y=(R+r*cos(phi))*sin(theta);
		z=r*sin(phi);
		printf("%.3f\t%.3f\t%.3f\n", x, y, z);
	}

	return 0;
}

% gcc -lm rndwalk-torus.c<br>
% ./a.out 1000 > rnd1.data<br>
% gnuplot<br>
gnuplot> splot "rnd1.data" with line

Random walking on a torus

C