Re: [Visualpython-users] using angle brackets for vectors

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

On Thu, 2005-02-17 at 15:06 -0500, Joe Heafner wrote:
> I'm running VPython 3.0 with Python 2.3.4 under Mac OS X 10.3.x. I just 
> noticed that this version of VPython uses angle brackets when printing 
> out the components of vectors, e.g. <1,2,3>. Previous versions used 
> square brackets. Since the M&I textbook uses angle brackets, I was 
> wondering whether it would be possible, 

To the best of my knowledge, the use of parenthesis in Python in this
way cannot be changed.  I changed the way that vectors are converted to
strings for the reasons you list below, but I'm pretty sure that there
isn't a way to introduce new syntactic elements to Python in this way.

> or even worthwhile, to change 
> the VPython syntax to use angle brackets to define the numerical 
> components of vectors. For example, instead of writing cart.pos = 
> vector(2,0,0)
> we could write cart.pos = <2,0,0> or instead of writing Earth = 
> sphere(pos=vector(1e11,0,0)) we could write Earth = 
> sphere(pos=<1e11,0,0>). Basically any time the components of a vector 
> quantity are given as a triplet of numbers the angle brackets could be 
> used.

> Pedagogically, I think this would make VPython syntax "look" more like 
> the notation in the textbook. While the "vector" preceding the 
> components in the pos attribute above is optional, 

It is optional only in the sense that any two- or three-sequence of
numbers is implicitly converted to a vector when calling any function in
visual itself.  The statement sphere( pos=(1e11, 0, 0)) creates a
temporary tuple (1e11, 0, 0) and passes it to the sphere object's
constructor, which is automatically changed to a vector by Visual.  For
similar reasons, the statement sphere( pos=[1e11, 0, 0]) works.  < 1e11,
0, 0 > isn't valid Python syntax by itself (although somewhat
perversely, "( vector < 1e11, 0, 0 > vector )" is).

> I tell my students 
> to include it. I think using angle brackets would reduce the list of 
> things to remember by one and still immediately draw attention to 
> vectors in VPython code.

-Jonathan