help in Vpython needed...

[paiseh]

Below is a source code for Vpython. I need to do some modification 
and sincerely hope you all can help....

Now, there are 2 compartments (upper and lower) and there are 5 
green balls in random motion.

I need to modify such that the 5 balls move only in the upper 
compartment. Also, i need another 20 balls that only move in the 
lower compartment.

Pls help......

With regards
Wee Nam

#########################################
# Import the library(s)
#########################################
from visual import * 
from random import uniform 
from visual.graph import *

##########################################
# Create Wall(s)
##########################################
thk = 0.3 
side = 4.0  
s2 = 2*side - thk 
s3 = 2*side + thk

wallR = box (pos=vector( side, 0, 0), length=thk, height=s2, 
width=s3, color = color.red) 
wallL = box (pos=vector(-side, 0, 0), length=thk, height=s2, 
width=s3, color = color.red) 
wallB = box (pos=vector(0, -side, 0), length=s3, height=thk, 
width=s3, color = color.blue) 
wallT = box (pos=vector(0, side, 0), length=s3, height=thk, 
width=s3, color = color.blue) 
wallBK = box(pos=vector(0, 0, -side), length=s2, height=s2, 
width=thk, color = (0.7,0.7,0.7)) 
wallM = box (pos=vector(0, -0.25*side, 0), length=s3, height=thk, 
width=s3, color = color.blue) 

##########################################
# Create Ball(s)
##########################################
no_particles=5
ball_radius=0.5
maxpos=side-.5*thk-ball_radius
maxv=1.0
D2=(2*ball_radius)**2 

ball_list=[]
p_list=[] 
pos_list=[] 
for i in arange(no_particles): 
    ball = sphere(color = color.green, radius=ball_radius)  
    p = maxv*vector(uniform(-1,1),uniform(-1,1),uniform(-1,1))
    p_list.append(p) 
    position=maxv*vector(uniform(-1,1),uniform(-1,1),uniform(-1,1))
    pos_list.append(position) 
    ball.pos=vector(position) 
    ball_list.append(ball) 

p_array=array(p_list) 
pos_array=array(pos_list)  

###########################################
# Setup Histogram Plot
###########################################
graphwindow=gdisplay(xtitle='v_x',ytitle='N',ymax=no_particles/2)
velocity_dist=ghistogram(bins=arange
(0,2*maxv,maxv/5),accumulate=1,average=1)
expected_distribution=gcurve(color=color.green)
for vx in arange(0,2*maxv,maxv/20):
       expected_distribution.plot(pos = (vx,.27*no_particles*exp(-
vx**2/maxv**2*3/2)))

##########################################
# Time loop for moving Ball(s)
########################################### 
ltriang=fromfunction(lambda i,j: less_equal(j,i), 
[no_particles,no_particles])

dt = 0.05

while 1: 
    rate(100)
    
    ######################################
    # Move Particles
    ######################################     
    pos_array=pos_array+p_array*dt 

    ######################################
    # Check wall collisions
    ######################################   
    putmask(p_array,greater(pos_array,maxpos),-p_array) 
    putmask(pos_array,greater(pos_array,maxpos),2*maxpos-pos_array) 
    putmask(p_array,less_equal(pos_array,-maxpos),-p_array) 
    putmask(pos_array,less_equal(pos_array,-maxpos),-2*maxpos-
pos_array) 

    ######################################
    # Check particle collisions
    ######################################  
    separation=pos_array-pos_array[:,NewAxis] 
    sepmag2=add.reduce(separation*separation,-1) 
    putmask(sepmag2,ltriang,4*D2) 
    hit=less_equal(sepmag2,D2) 
    hit_list=sort(nonzero(hit.flat)) 

    for ij in hit_list: 
        i, j = divmod(ij,no_particles) 
        sepmag=sqrt(sepmag2[i,j]) 
        direction=separation[i,j]/sepmag 
        pi=dot(p_array[i],direction) 
        pj=dot(p_array[j],direction) 
        exchange=pj-pi 
        p_array[i]=p_array[i]+exchange*direction 
        p_array[j]=p_array[j]-exchange*direction 

        overlap=2*ball_radius-sepmag 
        pos_array[i]=pos_array[i]-overlap*direction 
        pos_array[j]=pos_array[j]+overlap*direction
        
    ######################################
    # Update position and velocity 
    # of balls for plotting
    ######################################  
    for i in arange(len(ball_list)): 
        ball_list[i].pos=vector(pos_array[i])
        ball_list[i].velocity=vector(p_array[i]) 

    #######################################
    # Plot the x velocity histogram
    #######################################
    velocity_dist.plot(data=abs(p_array[:,0]))