How to use Graphics to draw a cannon ball

Question

Hi currently have to write a class to draw the the flight of a cannon ball. I wrote this code but I cant seem to be able to draw the ball on the graph. I need to draw the ball given each interval of time.

from graphics import*
from math import sin, cos, radians

class Tracker:

    def __init__(self,window,objToTrack):
         self.objToTrack = objToTrack
         self.circ = Circle(Point(objToTrack.getX(), objToTrack.getY()), 2)
         self.circ.draw(window)

    def update(self):
         point = self.circ.getCenter()
         x = point.getX()
         y = point.getY()
        self.circ.move(self.objToTrack.getX() - x, self.objToTrack.getY() - y)




class Projectile:

    def __init__(self, angle, velocity, height):
         self.xpos = 0.0
         self.ypos = height
         theta = radians(angle)
         self.xvel = velocity * cos(theta)
         self.yvel = velocity * sin(theta)

    def update(self, time):
         self.xpos = self.xpos + time * self.xvel
         yvel1 = self.yvel - 9.8 * time
         self.ypos = self.ypos + time * (self.yvel + yvel1) / 2.0
         self.yvel = yvel1

    def getY(self):
         return self.ypos

    def getX(self):
         return self.xpos

    def getInputs():
         a = eval(input("Enter the launch angle (in degrees): "))
         v = eval(input("Enter the initial velocity (in meters/sec): "))
         h = eval(input("Enter the initial height (in meters): "))
         t = eval(input("Enter the time interval between position calculations: "))
         return a,v,h,t

    def main():
        angle, vel, h0, time = getInputs()
        cball = Projectile(angle, vel, h0)
        while cball.getY() >= 0:
           cball.update(time)        
        print("\nDistance traveled: {0:0.1f} meters.".format(cball.xpos))

       Tracker(GraphWin("Tracker",500,500),cball)




 if __name__ == "__main__": 
     main()

Answer 1

First and foremost take the following questions and hints into consideration:

• When does Tracker.update() ever get called?
• When is your window instantiated?
• When do you actually draw a circle onto the window?
• Check out the observer pattern

Your Tracker class is not getting informed about the updates of its object. Also you build the Tracker and Window after the simulation of the cannon ball flight. Right now your call Tracker(GraphWin("Tracker",500,500),cball) will lead to a single draw operation on the final position of the cannon ball.

To achieve a plot of the curve do:

def main():
    angle, vel, h0, time = getInputs()
    cball = Projectile(angle, vel, h0)

    win = GraphWin("Tracker",500,500,autoflush=False)
    while cball.getY() >= 0:
       cball.update(time)
       Tracker(win,cball)
    print("\nDistance traveled: {0:0.1f} meters.".format(cball.xpos))

    input("Ready to close? Type any key")
    win.close()

Now you create a Tracker after every update, which will draw a circle on the graphics window. This results in many circles being drawn on top of each other, plotting the curve. The drawing is still upside down, so think about your coordinate system again. Also create the window outside of the Tracker objects, otherwise it will be closed once the tracker object gets deleted.

For an animation of the cannon ball you need to change your main() so that the tracker knows that something has changed. For more complex situations the observer pattern is useful, but in this case simply call Tracker::update() after the cannon ball update:

import time as sleeper
def main():
    angle, vel, h0, time = getInputs()
    cball = Projectile(angle, vel, h0)

    win = GraphWin("Tracker",500,500)
    win.setCoords(0, 0, 500, 500) #fixed coordinates

    tracker = Tracker(win,cball)
    while cball.getY() >= 0:
        sleeper.sleep(0.1) # wait 0.1 seconds for simple animation timing
        cball.update(time)
        tracker.update()
    print("\nDistance traveled: {0:0.1f} meters.".format(cball.xpos))

    input("Ready to close? Type any key") #keep the window open
    win.close()