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.
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