Newbie question about updating multiple objects ...

[donkeyboy]

Awesome Diez -- I'll give this a go tomorrow, and hopefully it will all
work!!

Diez B. Roggisch wrote:
> fivenastydisco at hotmail.com wrote:
> > I've copied in the code I'm using below, as it's not very long -- can
> > anyone help me?
>
> I'll have some remarks:
>
>
> > <--START-->
> >
> > from visual import *
> > from random import randrange # to create random numbers
> >
> > numballs = 5 # number of balls
> > balls = [] # a list to contain the balls
> >
> > # set up a box to collide with
> > side_length = 100
> > minx = side_length * -10
> > maxx = side_length * 10
> > miny = side_length * -4
> > maxy = side_length * 4
> >
> > # create a wireframe of space
> > left = curve(pos=[(minx, miny),(minx, maxy)], color = color.white)
> > top = curve(pos = [(minx, maxy),(maxx, maxy)], color = color.white)
> > right = curve(pos = [(maxx, maxy),(maxx, miny)], color = color.white)
> > bottom = curve(pos = [(maxx, miny),(minx,miny)], color = color.white)
> >
> > #create some balls
> > for b in range(numballs): #  for each ball
> >     x = randrange(minx, maxx)
> >     y = randrange(miny, maxy)
>
>
> Don't use range - use xrange. range actually creates a list of numbers,
> where xrange creates a iterator that will just return the subsequent
> numbers. The difference: the first solution consumes memory linear to the
> number of numbers!
>
> >     balls.append(sphere(pos = (x,y), radius = 50, color = color.red)) #
> > add balls
> >
> > dt = 0.05 # time interval
> > velocity = vector(2,0.2,1) # velocity of the balls
> >
> > while (1==1): # forever ...
> >     rate(50)
> >     for b in range(numballs): # for each ball ...
>
> This is a C-ism (or JAVA-ism if you like). The for-loop in python allows you
> to iterate over objects in iterables. And list is one of those. So
>
> for ball in balls:
>     ball.x = ...
>
> does the trick. And if you need an index, the commonly used idiom is
>
> for i, ball in enumerate(balls):
>     ...
>
> >         if balls[b].x < minx: # check it hasn't overrun a boundary
> >             balls[b].x = maxx
> >         if balls[b].x > maxx:
> >             balls[b].minx
> >         if balls[b].y < miny:
> >             balls[b].y = maxy
> >         if balls[b].y > maxy:
> >             balls[b].y = miny
>
> You should do that checks above _after_ you changed the coordinates, not
> before!
>
> >         # NB betting this is where it's going wrong ...
> >         balls[b].pos = balls[b].pos + (velocity * dt) # update the
> > position of the ball
>
> This could be the problem: you are using a python tuple and a vector object
> and add them. Are you sure that works? The python tuples themselves
> concatenate under the +-operator. So you'd create a large tuple of
> coordinate differences here.
>
> I suppose that using a vector for the coordinates or doing the calculation
> component-wise will solve the problem.
> 
> HTH,
> 
> Diez