Re: [Visualpython-users] strange math

[Steve S. <st...@sp...>]

Hi Lenore...

On Sep 12, 2009, at 8:38 AM, Lenore Horner wrote:

> (Sorry, forgot to cc the list.)
>
> Hi Steve,
>
> You're saying the following?
> a = 1
> b = a
> b = 3
> Now print a would give 3?

No... not really. Assignments in python just add references to  
existing objects. So...

a=1

Makes the label 'a' refer to the integer object that represents the  
integer '1'.

b=a

Makes the label 'b' refer to the same object that 'a' refers to....  
in this case it happnes to be an integer: '1' (but python doesn't  
really care about that).

b=3

just makes 'b' now refer to the integer object '3', 'a' still refers  
to '1' as it did before.


>
> I could almost understand if the following were true.
> air.acceleration = gravity
> gravity = gravity + (1,0,1)
> print acceleration gives (1,-9.8,1)
>
> That would mean the assignment was somehow permanent, which is to my
> mind quite weird but potentially useful.

This case is more subtle...

gravity = gravity + (1,0,1)

First the right hand side... "gravity + (1,0,1)" creates a *new*  
vector object. The assignment then makes the label 'gravity' refer to  
that new object.

>
> But it sounds like you're saying that assignment in python makes two
> variables identical rather than giving the one on the left the current
> value of the one on the right?  That seems to me to be incredibly
> awkward.  How is one supposed to initialize several things identically
> (no possibility of typos or slider misadjustments) and then let them
> evolve differently?

You can use the vector constructor to make multiple identical vectors:

x = vector(1,2,3)
y = vector(x)

The 'vector(x)' creates a new, independent vector object that is  
initialized to have the same components as the existing vector object  
referred to by 'x'.

As Bruce points out... check out the document:

http://vpython.org/contents/experienced.html

it has some great tips and may avert other problems ;-).

Hope that's helpful....

-steve