[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