# Problem :surprise:

1. Jul 1, 2004

### 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 any one 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

2. Jul 1, 2004

### Gokul43201

Staff Emeritus
I couldn't help myself...

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Inta SCOTEEEEEEEEEEEEEEEEEEEEEEEEEEEE

3. Jul 1, 2004

### robphy

Code (Text):

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

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

These lines look strange to me:
z(t+dt)=z:
z(t+dt)=z+z[pi]:

First, the := means assignment.
So, you might have meant:
z(t+dt) := z+z[pi]:

Second, what kind of maple object is z(t+dt)?
Do you mean to "update" the z[] array, e.g.
z := z+z[pi]:
?

4. Jul 7, 2004