% DiffusionProcess1.m uses a coin-tossing process to simulate 
% a particle moving back and forth between two compartments 
%
% When the particle is in the left box, it has probability R
% of moving to the right. When it is in the right box, it has
% probability L of moving to the left. 

% parameters
R=0.3; L=0.1;

% time interval
tmax=250;

% initialize vectors storing the results over time
Boxnow=1; Boxt=1; 
tvals=0; t=0;

while t<tmax;
   if Boxnow==1
      m=rand(1,1);
	  if(m<R) 
	  Boxnow=2;
	end 
   else
   	m=rand(1,1);
	if(m<L)
	  Boxnow=1;
	end
   end  

   % store the results for plotting
   t=t+1;
   tvals=[tvals;t];
   Boxt=[Boxt;Boxnow];
end; 

% label the plot
plot(tvals,Boxt),axis([0 tmax 0.5 2.5]);
xlabel('Time')
ylabel('Particle location'); 
title('Two-compartment diffusion');

nleft=sum(Boxt==1); nright=sum(Boxt==2);
disp([nleft/(nleft+nright) nright/(nleft+nright)]);