Re: [Visualpython-users] strange math

[Bruce S. <Bru...@nc...>]

Thanks for the additional background, Craig. It's good to have a card-carrying 
computer scientist commenting on the issues!

Your point about updating a bunch of Visual objects however doesn't work, 
because the setters for Visual vector attributes like pos or axis make new 
vectors. This explains for example why you can write as a shorthand pos=(1,0,0) 
instead of pos=vector(1,0,0); a vector is constructed out of the triple.

Here is a routine that shows the (non)effect:

A = vector(0,1,0)
arrow(pos=(0,0,0), axis=A)
arrow(pos=(1,0,0), axis=A)
scene.mouse.getclick()
A = 3*A

After clicking, the arrows don't get longer, because axis=A is basically 
axis=vector(A).

In any case, it would be a rare circumstance where you wanted some vector 
attribute of lots of objects to be the same, and to change together.

Bruce Sherwood

Craig Struble wrote:
> It should be pointed out that many object oriented languages share  
> this behavior. Java behaves the same way, for example. Many others  
> have pointed out the difficulties in copying objects (deep vs. shallow  
> copies) and bigger problems for copying arise when there are circular  
> references, which are common for complex data structures (e.g. general  
> graphs).
> 
> The object reference behavior is useful when implementing some kinds  
> of data structures when you need a "trailing pointer" for updates.  
> Lots of linked list implementations use this approach. It's atypical  
> to do this in Python since it has a very rich collection of data  
> structures already implemented, but it can be useful.
> 
> I would guess in VPython, this behavior could make it easier to update  
> the positions or movements of a large group of visual objects. If they  
> all refer to the same point or vector, change that vector and all the  
> objects change accordingly. It would be nice to make a single update  
> instead of having to looping over each object, which would probably be  
> slower.
> 
>      Craig
>