Monte Carlo method for Smoluchowski equation

[Feynman]

Hi every body,
i've a simple question in the programation for a certain methode called Monte Carlo method for Smoluchowski equation:
i programe this method in maple but the problem is that the positions of particules z[l]is not changing ( it most be changed ): can anyone tell me where is the problem?
this is the method:

//calculate natural random numbers
test:=proc(N)
local a,i,L;
L:=[];
a:=rand(1..N):
for i from 1 to N do L:=[op(L),a()] od;
L;
end:
\\z is the position of particules
z:=test(10);

pi:=test(10);
\\ calculate uniform random numbers U[0,1]

r:=stats[random, uniform](10):for i from 1 to 10
do
printf("r[%d]=%f\n",i,r);
od;

\\ position of particules at time t+dt=t+1/20

for i from 1 to 10
do
if
r>1/(20*z) or r=1/(20*z)
then
z(t+dt)=z:
end if;
if
r<1/(20*z)
then
z(t+dt)=z+z[pi]:
end if;
od:
thankx verry much

 

[techwonder]

I guess that r>1/(20*z) is "always" true. Then you have z[t+dt](i)=z[t] explicitly in your code.