Python: while loops

The following program finds the average of several test scores entered by the user.

num_students = input('Number of students? ')
sum = 0
for i in range(num_students):
    score = input('Student score: ')
    sum = sum + score
print 'Average is', float(sum) / num_students

This works fine, but it requires the user to tell us how many students there are. We do this because we need to know how many times to repeat the for loop. But it would be much more convenient for a user if the user started by entering scores, and then said I'm done once there were no more scores to enter. To allow this to happen, we'll us a while loop.

num_students = 0
sum = 0
score = raw_input('Student_score: ')
while score != "I'm done":
    sum = sum + int(score)
    num_students = num_students + 1
    score = raw_input('Student_score: ')
print 'Average is', float(sum) / num_students

We can also use this in a program to show a moving ball. In this case, we want the animation to continue until the window is closed. We can ask the window whether it is closed using its isClosed method.

from graphics import *
import time

win = GraphWin('Bouncing')   # Create a window
center = Point(100, 180)     # Add a red circle at its bottom
ball = Circle(center, 20)
ball.setFill('red')
ball.draw(win)

x_vel = 1               # Track the ball's horizontal speed
y_vel = -6              # Track the ball's vertical speed
while not win.isClosed():
    ball.move(x_vel, y_vel)
    center = ball.getCenter()
    y_vel += 0.2
    if center.getY() > 180:
        ball.move(0, 180 - center.getY()) # move it up to 180
        y_vel = -0.9 * y_vel
    if center.getX() < 20 or center.getX() > 180:
        x_vel = -x_vel
    click = win.checkMouse()
    if click != None:
        y_vel = -6
    time.sleep(0.05)         # Wait before drawing next animation frame

This uses the checkMouse method, which returns the mouse location if the user has pressed the mouse button but None if the user hasn't. This is different from getMouse, which will always wait until the user presses the mouse button before returning its location. If we had used getMouse instead of clickMouse above, then the computer would stall there each time through the loop, and the computer would only continue downward to the line telling to the ball to move once the user clicks the mouse. In other words, it would perform very badly!