visualpython-users

[Visualpython-users] bug incrementing vector ?

[<gp...@ri...>]

I recently ran a simulation that I haven't looked at for a while.

It didn't run correctly:  the physics was wrong.  I can't be certain, but
I think it was doing the right thing last time I looked at it.

I'm trying to distill the program down to a small example that illustrates
the problem.  The problem is rather mischievous (or I'm missing something
obvious) and attempts to write a small broken program haven't yet met with
success.  Nonetheless ...

I discovered that if a replace the line

FMass.v = FMass.v + (Fext / FMass.m)*dt

with

FMass.v += (Fext / FMass.m)*dt

The behavior of the program changes.  FMass is a sphere.  FMass.v is a
visual vector, FMass.m is a scalar.  Fext is a vector.  Did I miss
something?  Should I expect that "+=" will not increment a vector?

Interactively, I *can* correctly
increment a vector using "+=".

While I'm trying to track down the problem and write a small example, I
thought I'd check with the group for a sanity check.

-gary

Re: [Visualpython-users] bug incrementing vector ?

[Jonathan B. <jbr...@ea...>]

On Tue, 2004-08-17 at 15:11, gp...@ri... wrote:
>  Should I expect that "+=" will not increment a vector?

It should work (and does, as far as I can tell) partially because
visual.vector provides the __iadd__() "special member function".  Worst
case, event if a visual.vector only provided __add__(), the interpreter
would synthesize the expression "x += y" as "x = x + y".  Python floats
are manipulated in this way.

Consider this toy example:

>>> class fwrap(object):
...     def __init__(self):
...         self.x = 1.0
...     def get(self):
...         return self.x
...     def __add__(self, other):
...         return self.x + other
...
>>> f = fwrap()
>>> f.get()
1.0
>>> f + 1.0
2.0
>>> f += 1.0
>>> f.get()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
AttributeError: 'float' object has no attribute 'get'
>>> f
2.0

HTH,
-Jonathan