[Matplotlib-users] Problem with simple use of draw() in animations of arrays

[David S. <dps...@gm...>]

Hi,

I have a problem with draw() to do simple animations of the contents of
arrays in matplotlib.

I was trying to use the idea in the animations cookbook (
http://www.scipy.org/Cookbook/Matplotlib/Animations)
to animate some "random walkers", but found that the animation did not work.

A minimal example of the problem is given by changing the first recipe in
the cookbook to just draw random arrays.
This version correctly animates as expected:

from pylab import *
import time

ion()

tstart = time.time()               # for profiling
x = rand(100)
y = rand(100)
line, = plot(x,y, ',')
for i in arange(1,200):
    x = rand(100)
    y = rand(100)
    line.set_data(x,y)
    draw()                         # redraw the canvas

Now, however, changing the "x =" and "y =" lines as follows:
    x[:] = rand(100)
    y[:] = rand(100)

so that they are modified in place, rather than creating new arrays, no
longer animates anything.
I am using version 0.99 on linux (Kubuntu 9.10).
The same behaviour is found from within ipython -pylab  or from the command
line with python.

In my real application, I wish to use this as a simple way to animate a
collection of random walkers.
I thus have an array of positions which is updated at every step, and this
is what I want to animate,
which is the reason why I tried the array updating step above.

So far, my code is as follows:

from pylab import *

ion()

N = 1000
pos = zeros((N,2))

figure(figsize=(8,8))
points, = plot(pos[:,0], pos[:,1], ',')
axis([-20,20,-20,20])

for t in range(1000):

    pos += uniform(-1,1,N*2).reshape(N,2)
    points.set_data(pos[:,0].copy(), pos[:,1].copy())
    draw()

The ".copy()" are an attempt at creating new arrays. Nonetheless, there is
no animation.
And if I put
points._x
in ipython, then it still has all zeros! Apparently the .set_data() hasn't
done anything.

Any help at getting this animation to work are greatly appreciated!

Thanks and best wishes,
David.