Re: [Visualpython-users] strange math

[Lenore H. <lh...@si...>]

(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?

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.

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?

-----
Testing:
Nope, that's not true.
 >>> a = 1
 >>> print a
1
 >>> b = a
 >>> print a, b
1 1
 >>> b = 3
 >>> print a, b
1 3

Well maybe vectors behave differently than scalars.
Nope.
 >>> a = (0,0,1)
 >>> b = a
 >>> print b
(0, 0, 1)
 >>> b = (1,0,0)
 >>> print a, b
(0, 0, 1) (1, 0, 0)

Maybe a vector is different than a tuple.
Nope.
from visual import *
 >>> a = vector(0,0,1)
 >>> b = a
 >>> b = vector(1,0,0)
 >>> print a, b
<0, 0, 1> <1, 0, 0>

But the above isn't quite what I'm doing in my code.  Maybe component  
assignment works differently than all other types of assignment.
Yup!
 >>> a = vector(0,0,1)
 >>> b = a
 >>> c = a
 >>> print a, b, c
<0, 0, 1> <0, 0, 1> <0, 0, 1>
 >>> c.y = 5
 >>> print a, b, c
<0, 5, 1> <0, 5, 1> <0, 5, 1>

Ok, that's just plain spooky.

Can someone give me an example where that would be helpful?

Now I have to go figure out how to dodge this bullet.

Lenore


On Sep 12, 2009, at 06:36 , Steve Spicklemire wrote:

> Hi Lenore,
>
> It's about python assignments. You are assigning the *same*  
> acceleration vector to both objects, then changing the y component  
> of the same vector.
No.  I did not change the y component of the vector which was assigned  
to both of them.
>
> Instead of assigning both accelerations to the same vector (gravity)  
> try initializing them with different vectors:
>
> pixie.acceleration = vector(0,-9.8,0)
> air.acceleration = vector(0,-9.8,0)
>
> then when you fiddle with the y components, it will be the y  
> components of different vector objects.
>
> That should work!
> -steve
>
> On Sep 12, 2009, at 7:26 AM, Lenore Horner wrote:
>
>> Sorry.  I was in too big a hurry and forgot to paste the code.  Here
>> it is.  I've put arrows off to the right indicating the lines where
>> both air.acceleration and pixie.acceleration are being changed  
>> despite
>> the fact that only air.acceleration appears in the code.  I've
>> included all of the while loop.  I don't see anywhere that I'm
>> reassigning pixie.acceleration from its original value of gravity but
>> it changes as soon as it hits those to statements (and not before).
>>
>> Thanks,
>> Lenore
>>
>>
>> angle = math.pi/4
>> speed = 13.0
>>
>> power = 2
>> constant = 1
>>
>> gravity = vector(0,-9.8,0)
>>
>> launch_velocity =  
>> vector(speed*math.cos(angle),speed*math.sin(angle),0)
>>
>> pixie = sphere(pos=vector(-9,-3.55,0), radius=0.2, color=color.red)
>> pixie.velocity = launch_velocity
>> pixie.acceleration = gravity
>>
>> air = sphere(pos=vector(-9,-3.55,0.3), radius=0.15, color=color.blue)
>> air.velocity = launch_velocity
>> air.acceleration = gravity
>>
>> scene.mouse.getclick()    # Don't launch until we're watching
>>
>> dt = 0.01
>> while 1:
>>    rate(100)
>>    air.pos = air.pos + air.velocity*dt
>>    if air.y < -3.55:
>>        air.velocity = vector(0,0,0)
>>    else:
>>        air.velocity = air.velocity + air.acceleration*dt
>>        air.acceleration.y = gravity.y - sign(air.velocity.y) *
>> constant * air.radius * air.velocity.y**power    <-----
>>        print  "5", air.acceleration, pixie.acceleration
>>        air.acceleration.x = -sign(air.velocity.x) * constant *
>> air.radius * air.velocity.x**power              <-------
>>        print "6", air.acceleration, pixie.acceleration
>>
>>    pixie.pos = pixie.pos + pixie.velocity*dt
>>    if pixie.y < -3.55:
>>        pixie.velocity = vector(0,0,0)
>>    else:
>>        pixie.velocity = pixie.velocity + pixie.acceleration*dt
>>
>>
>> On Sep 11, 2009, at 18:20 , Bruce Sherwood wrote:
>>
>>> Can you explain the context of your question? I have no idea what
>>> program or
>>> documentation you're referring to.
>>>
>>> Bruce Sherwood
>>>
>>> Lenore Horner wrote:
>>>> Perhaps I'm just blind, but I can't figure out why the assignment
>>>> statements above print "5" and print "6" affect both acceleration
>>>> vectors, but they do.  How do I change the acceleration vector of  
>>>> one
>>>> object without touching the vector of other objects?
>>>>
>>>> Thanks,
>>>> Lenore
>>>
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