how to clear gcurve or gdots?

[Jennifer Hoffman]

How can I clear gcurve or gdots from a gdisplay, so that I can recompute & display it with new parameters?

Here is the abbreviated snippet of code:

(I didn't originally define rst as a separate function, but it was suggested on another thread... it seems to have made no difference.)

gd = gdisplay(width=400, height=scene1.height, xmax=tmax, xmin=0, ymax=L+br, ymin=-L-br)

line1 = gcurve(color=color.blue)

dots1 = gdots(color=color.red)

def rst():

    global line1, dots1

    # line1.clear()  # if I include this line, the curve will never draw in the first place

                          # but if I don't include this line, it will just keep redrawing curves without ever erasing

    # dots1.clear()

    line1 = gcurve(color=color.blue)

    dots1 = gdots(color=color.red)

 # I want to be able to call this many times, with different parameters, but clear the graph each time

def bounceball(): 

    global ball, line1, dots1

    rst()

    t=0.0

    while t<tmax:

        rate(10)

        line1.plot(pos=(t,ball.pos.y))

        dots1.plot(pos=(t,ball.pos.y))

        # some more code to update & plot the position of ball vs. time 

        t=t+dt

More specifically, I'd be really grateful for some more detailed feedback from Bruce. I'm teaching for the first time out of Matter & Interactions. I'm also totally new to python and vpython, so I'm sure my code is really clunky. But I'm trying to implement the "momentum principle in update form" to demonstrate two effects:

 (1) effect of the time interval

 (2) effect of using vi, vf, or vavg to update the momentum

I wrote the code below, and it has 2 big problems:

 (1) no matter what I try, the slider bar & dropdown menu stubbornly overlay themselves on top of the scene1 instead of beneath it, so they obscure the animation

 (2) I can't erase the old gcurve & gdots, so each time the user changes a parameter, it just plots a new curve on top of the old and clutters up the whole gdisplay

##################################################################

GlowScript 1.1 VPython

# Written by Jenny Hoffman, University of British Columbia.

# Fall 2015, Physics 107

# Illustrates the Momentum Principle in Update Form.

# Inspired by Chabay & Sherwood, "Matter & Interactions", 4th ed

k=10  # spring constant in N/m

L=1 # spring rest length in m

br=0.1 # ball radius in m

bm=0.1 # ball mass in kg

anchor=vector(0,L+br,0) # anchor point of top of spring

stretch=1.2  # starting stretch of the spring

dt=0.1

tmax=4 # 4 seconds should be 4 periods of the spring

val="vf" # whether to update using vi, vf, or vavg

Nth=0 # just for debugging: how many times did we run the bounceball loop?

sz=200

scene1=canvas(x=0,y=0,width=sz,height=sz,range=L+br,userzoom=False,userspin=False)

spring = helix(canvas=scene1, pos=anchor, axis=vector(0,-stretch*L,0), radius=br, color=color.green, coils=10)

ball = sphere(canvas=scene1, pos=spring.pos+spring.axis-vector(0,br,0), radius=br, color=color.red)

ball.p = vector(0,0,0)  # initial momentum

ball.m = bm

# here we make a slider to change the time interval dt

scene1.caption.text("Vary the time interval: ")

def setdt():

    global dt

    dt = $("#dt_slider").slider("value")

    bounceball()

$('<div id="dt_slider"></div>').appendTo(scene1.caption).css(width="200px")

$(def ():

    $("#dt_slider").slider(value=dt, min=0.01, max=0.5, range="min", slide=setdt, change=setdt)

)

# here we choose between using vi, vf, or vavg to update

choose = $("<p>Change the velocity update method: </p>").appendTo(scene1.caption)

# Create a drop-down menu (a "select" object). Set up the options to appear:

s = ""

s += "<option select=selected>vf</option>"

s += "<option>vavg</option>"

s += "<option>vi</option>"

$('<select/>').html(s).css(font="sans").change(def (): # come here when a change is made in the menu choice

    global val

    val = $(this).val()

    bounceball()

).appendTo(choose) # place drop-down menu beside the text, "Change the v update method: "

    

gd = gdisplay(width=400, height=scene1.height, 

#      xtitle='t', ytitle='y', 

      foreground=color.black, background=color.white, 

      xmax=tmax, xmin=0, ymax=L+br, ymin=-L-br)

line1 = gcurve(color=color.blue)

dots1 = gdots(color=color.red)

scene1.wrapper.css("float", "left")

gd.wrapper.css("float", "right")

bounceball()

while True:  # this should just wait for the user to change dt or vupdate method

    rate(10)

    pass

def rst():

    global spring, ball, line1, dots1

    spring.axis=vector(0,-stretch*L,0)

    ball.pos=spring.pos+spring.axis-vector(0,ball.radius,0)

    ball.p = vector(0,0,0)

    line1.clear()

    dots1.clear()

    line1 = gcurve(color=color.blue)

    dots1 = gdots(color=color.red)

def bounceball():

    global ball, spring, line1, dots1, Nth

    print("Entering function bounceball: ", Nth)

    rst()

    t=0.0

    while t<tmax:

        rate(10)

        line1.plot(pos=(t,ball.pos.y))

        dots1.plot(pos=(t,ball.pos.y))

        ball.vi = ball.p/ball.m    

        F=-k*ball.pos

        ball.p = ball.p+F*dt

        ball.vf = ball.p/ball.m

        ball.vavg = (ball.vi+ball.vf)/2

        if val == "vi": 

            vupdate=ball.vi

        elif val == "vf": 

            vupdate=ball.vf

        elif val == "vavg": 

            vupdate=ball.vavg

        ball.pos = ball.pos+vupdate*dt

        spring.axis = ball.pos+vector(0,ball.radius,0)-anchor

        t=t+dt

    Nth=Nth+1

[Bruce Sherwood]

For someone "totally new to python and vpython" you sure are ambitious!

The GlowScript program will run if you delete these 4 statements:

scene1.wrapper.css("float", "left")

gd.wrapper.css("float", "right")

    line1.clear()

    dots1.clear()

The wrapper stuff is pretty subtle. You might look at the program 13-fields at tinyurl.com/MI-physics-demos, where I did manage to get a slider to the right of the display. If I remember correctly, a key element was to put all the right-hand stuff inside a <div>.....</div>.

There is no clear option for graphing objects. There should be, but there isn't. I haven't really looked into it, but conceivably if you changed the color of the object (pale, or white, or something), that would change the color used in the graph.

With these changes the program won't display quite the way you want, but it does the right things. You might consider using different line and dot colors for each of the three situations.

A nit: In the sequence

scene1=canvas(x=0,y=0,width=sz,height=sz,range=L+br,userzoom=False,userspin=False)

spring = helix(canvas=scene1, pos=anchor, axis=vector(0,-stretch*L,0), radius=br, color=color.green, coils=10)

it's unnecessary to say helix(canvas=scene1....) because objects automatically go into the most recently defined canvas.

Even smaller nit: In GlowScript canvas and display are synonyms, but since classic VPython only uses "display" I would change canvas -> display.

I'll contact you about the very extensive instructor resources that would be of great use to you.