Teach Your Kids To Code: A Parent-friendly Guide to Python Programming

194 Chapter 8

Changing direction

“Bouncing” off the edge of the screen means going in the oppo-

site direction after hitting that edge. The direction our image is

moving is controlled by the updates to picx and picy. In our old

SmileyMove.py, we just added 1 pixel to picx and picy every time

through the while loop with these lines:

picx += 1

picy += 1

However, these lines kept our image moving right and down

1 pixel every time; there was no “bounce,” or changing direction,

because we never changed the number added to picx and picy.

Those two lines mean we’re guaranteed to move right and down

at a speed of 1 pixel per frame, every frame, even after the smiley

has left the screen.

Instead, we can change the constant value 1 to a variable that

will represent the speed, or number of pixels the image should

move each frame. Speed is the amount of movement in a period of

time. For example, a car that moves a lot in a short time is moving

at a high speed. A snail that barely moves in the same period of

time is moving at a low speed. We can define a variable called speed

in the setup portion of our program for the amount of movement in

pixels that we want for each frame:

speed = 5