Question

I get this error:

Traceback (most recent call last):
  File "D:/Python26/PYTHON-PROGRAMME/049 bam", line 9, in <module>
    ball[i][j]=sphere()
NameError: name 'ball' is not defined

when I run this code. But the ball is defined ( ball[i][j]=sphere() ). Isn`t it?

#2D-wave
#VPython

from visual import *

#ball array #ready
for i in range(5):
    for y in range(5):
        ball[i][j]=sphere()

timer = 0
dt = 0.001
while(1):
    timer += dt

    for i in range(5):
        for y in range(5):
            #wave equation
            x = sqrt(i**2 + j**2) # x = distance to the source
            ball[i][j].pos.y = amplitude * sin (k * x + omega * timer)
    if timer > 5:
        break

Answer 1

When you say ball[i][j], you have to already have some object ball so that you can index (twice) into it. Try this segment instead:

ball = []    
for i in range(5):
    ball.append([])
    for y in range(5):
        ball[i].append(sphere())