Debugging short exercise

Objective: Practice debugging techniques

This program is supposed to break once the ball has been caught, but it doesn't work. Your job is to work out why, and then fix it.

1. Copy the program at the bottom of this page, and open it in Dr. Python
2. Add print statements to trace program execution, like this:

    print "Trace: At start"
   

   Add at least five statements:
   1. Start of the program
   2. Once the game window has been created
   3. Inside of the main game update loop
   4. Inside of the if statement that checks if the ball has been caught
   5. End of program
3. Run the program and stop it as soon as the buggy behavior has occurred.
4. Analyse the console output (the debug 'trace').
   What would you expect to see if the program was running properly?
5. Is the problem that the distance isn't being properly calculated?
   Add another print statement that prints it, like this:

    print "Distance: "+ distance(ball_x, ball_y, mitt_x, mitt_y)
   

6. Once you've worked out what the real problem is, fix it.
7. Submit your code to school web locker

   
from gasp import *

def distance(x1, y1, x2, y2):
    return ((x2 - x1)**2 + (y2 - y1)**2)**0.5

begin_graphics(800, 600, title="Catch", background=color.YELLOW)
set_speed(120)

ball_x = 10
ball_y = 300
ball = Circle((ball_x, ball_y), 10, filled=True)
dx = 4
dy = random_between(-4, 4)

mitt_x = 740
mitt_y = 300
mitt = Circle((mitt_x, mitt_y), 60)

while True:
    # move the ball
    if ball_y >= 590 or ball_y <= 10:
        dy *= -1
        
    ball_x += dx
    if ball_x >= 810 or ball_x <= 10:
        dx *= -1
    
    ball_y += dy    
    ball_x += dx
    move_to(ball, (ball_x, ball_y))

    # check on the mitt
    if key_pressed('k') and mitt_y <= 580:
        mitt_y += 5
    elif key_pressed('j') and mitt_y >= 20:
        mitt_y -= 5

    if key_pressed('escape'):
        break

    move_to(mitt, (mitt_x, mitt_y))

    if distance(ball_x, ball_y, mitt_x, mitt_y) <= -60:  # ball is caught
        remove_from_screen(ball)
        break

    update_when('next_tick')

end_graphics()