Loop to change spring lengths doesn't change anything when I run it

Question

The project I'm working on is supposed to model a pulse moving down a set of spheres connected by springs. I am trying to decrease the spring length as the pulse moves down the chain, but when I run it, nothing happens.

Here's my code:

    from visual import *

one = sphere(pos=(-10,0,0), radius = 0.5, color = color.red)
two = sphere(pos=(-8,0,0), radius = 0.5, color = color.orange)
three = sphere(pos=(-6,0,0), radius = 0.5, color = color.yellow)
four = sphere(pos=(-4,0,0), radius = 0.5, color = color.green)
five = sphere(pos=(-2,0,0), radius = 0.5, color = color.blue)
six = sphere(pos=(0,0,0), radius = 0.5, color = color.cyan)
seven = sphere(pos=(2,0,0), radius = 0.5, color = color.magenta)
eight = sphere(pos=(4,0,0), radius = 0.5, color = color.white)
nine = sphere(pos=(6,0,0), radius = 0.5, color = color.red)
ten = sphere(pos=(8,0,0), radius = 0.5, color = color.orange)

spring1 = helix(pos = (-10, 0, 0), length = 2, radius = 0.3,
   thickness = 0.05, color = color.red)
spring2 = helix(pos = (-8, 0, 0), length = 2, radius = 0.3,
   thickness = 0.05, color = color.orange)
spring3 = helix(pos = (-6, 0, 0), length = 2, radius = 0.3,
   thickness = 0.05, color = color.yellow)
spring4 = helix(pos = (-4, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.green)
spring5 = helix(pos = (-2, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.blue)
spring6 = helix(pos = (0, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.cyan)
spring7 = helix(pos = (2, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.magenta)
spring8 = helix(pos = (4, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.white)
spring9 = helix(pos = (6, 0, 0), length = 2.0, radius = 0.3,
   thickness = 0.05, color = color.red)

masses = [one, two, three, four, five, six, seven, eight, nine, ten]
springs = [spring1, spring2, spring3, spring4, spring5, spring6, spring7,
           spring8, spring9]

while True:
    n=0
    deltax=.2
    while n < 10:
        rate(30)
        masses[n].pos.x = masses[n].pos.x + deltax
        if n < 9:
            springs[n].pos = masses[n].pos
            springs[n].axis = masses[n+1].pos-masses[n].pos
        n=n+1

    n = n-1
    while n >= 0:
        rate(30)
        masses[n].pos.x = masses[n].pos.x - deltax
        if n < 0:
            springs[n-1].pos = masses[n-1].pos - deltax
            springs[n-1].axis = masses[n].pos-masses[n-1].pos
        n = n-1


while True:
    m=0
    deltat=.2
    while m<9:
        rate(30)
        springs[m].length = springs[m].length - deltat
        springs[m].length = springs[m].length + deltat
        m=m+1

    m=m-1
    while n>=0:
        rate(30)
        springs[m].length = springs[m].length - deltat
        springs[m].length = springs[m].length + deltat
        m=m-1

Answer 1

Make the masses have opacity = 0.3 and replace the first rate statement with scene.waitfor('click'). You'll see that when you move the nth mass to the right, the spring does in fact shorten, and when the (n+1)th mass is moved, the spring to its right shortens, leaving a gap between the nth spring and the (n+1)th spring.

Incidentally, I don't understand where there is a second "while True". It will never be reached.

I'll advertise that a better place to pose VPython questions is in the VPython forum at

https://groups.google.com/forum/?fromgroups&hl=en#!forum/vpython-users