[Visualpython-users] passing vectors as function arguments

Hi.
I have the following Python code:

from visual import *

def derivative(t,x,xp,xpp):

    xmu = 1.407645794e16
    r = mag(x)
    r3 = r**3
    xpp[0] = -xmu*x[0]/r3
    xpp[1] = -xmu*x[1]/r3
    xpp[2] = -xmu*x[2]/r3

t = 2451545.0
x=vector(1.,1.,1.)
xdot=vector(1.,1.,1.)
xmu = 1.407645794e16
#xddot = -xmu*x/(mag(x)**3)
#print xddot
xddot=vector(0,0,0)

x=vector(2.,2.,2.)
derivative(t,x,xdot,xddot)

print xddot

When executed, I get the expected result for xddot. When I modify the code to read:

from visual import *

def derivative(t,x,xp,xpp):

    xmu = 1.407645794e16
    r = mag(x)
    r3 = r**3
    #xpp[0] = -xmu*x[0]/r3
    #xpp[1] = -xmu*x[1]/r3
    #xpp[2] = -xmu*x[2]/r3
    xpp = -xmu*x/r3

t = 2451545.0
x=vector(1.,1.,1.)
xdot=vector(1.,1.,1.)
xmu = 1.407645794e16
#xddot = -xmu*x/(mag(x)**3)
#print xddot
#xddot=vector(0,0,0)

x=vector(2.,2.,2.) 
derivative(t,x,xdot,xddot)

print xddot

I get the error message:

[localhost:~] joe% python testrk4.py
Visual-2002-07-22
Traceback (most recent call last):
  File "testrk4.py", line 22, in ?
    derivative(t,x,xdot,xddot)
NameError: name 'xddot' is not defined
[localhost:~] joe% 

Why does this not work when simply writing

xddot = -xmu*x/(mag(x)**3) 

outside my function works perfectly? I'm lost here, and Lutz and Ascher's book has not helped me. I'm trying to port some QuickBASIC numerical analysis code and I need the capability to pass a vector (a VPython vector, not an ordinary array) into a function, potentially modify that vector's components, and then have the same vector accessible outside the function. I can make it work with arrays but not with vectors. I'd appreciate any help.


Cheers,
Joe Heafner - Instructional Astronomy and Physics
Home Page http://users.vnet.net/heafnerj/index.html
I don't have a Lexus, but I do have a Mac. Same thing.