## Collisions

Arcade games need to know when one Actor sprite has hit another Actor sprite. Most of this code is copied from Program 4.2 and Program 4.5.

WIDTH = 500
HEIGHT = 500

alien = Actor("alien")
alien.pos = (400, 50)
box = Rect((20, 20), (100, 100))

def draw():
screen.clear()
screen.draw.filled_rect(box, "red")
alien.draw()

def update():
if keyboard.right:
alien.x = alien.x + 2
elif keyboard.left:
alien.x = alien.x - 2
box.x = box.x + 2
if box.x > WIDTH:
box.x = 0
if alien.colliderect(box):
print("hit")


Program 5.1 Collisions

Exercise

Add vertical movement (as you did in Exercise Program 4.5).

Make the box chase the alien.

Print number of times the box hits the alien (i.e. the score).

## Chase

Instead of moving constantly to the right we can make the movement conditional with an if statement so the box chases the alien. Most of this code is copied from Program 5.1. New lines are highlighted. We have also changed what happens when the box catches the alien: the program now exits and you must run it again to play again. This may not be what you want in your game!

WIDTH = 500
HEIGHT = 500

alien = Actor("alien")
alien.pos = (400, 50)
box = Rect((20, 20), (100, 100))

def draw():
screen.clear()
screen.draw.filled_rect(box, "red")
alien.draw()

def update():
if keyboard.right:
alien.x = alien.x + 2
elif keyboard.left:
alien.x = alien.x - 2
if box.x < alien.x:
box.x = box.x + 1
if box.x > alien.x:
box.x = box.x - 1
if alien.colliderect(box):
exit()


Program 5.2 Alien chase

Exercise

Add vertical movement (as you did in previous exercise).

Draw a new enemy image. Save it as enemy.png in your mu_code/images folder. Load it as an Actor(‘enemy’) instead of the Rect().

## Powerup

Instead of an enemy the box here is a powerup that the player must collect. When he does it disappears and moves to a new location.

WIDTH = 500
HEIGHT = 500
import random

alien = Actor("alien")
alien.pos = (400, 50)
box = Rect((20, 20), (100, 100))
score = 0

def draw():
screen.clear()
screen.draw.filled_rect(box, "green")
alien.draw()

def update():
global score
if keyboard.right:
alien.x = alien.x + 2
elif keyboard.left:
alien.x = alien.x - 2
if alien.colliderect(box):
box.x = random.randint(0, WIDTH)
box.y = random.randint(0, HEIGHT)
score = score + 1
print("Score:", score)



Program 5.3 Collect the powerups

Exercise

## Sound and animation

Pygame Zero comes with one other image alien_hurt.png and one sound eep.wav. If you want more you will have to add them to the sounds and images folders.

Most of this code is copied from Program 5.1

WIDTH = 500
HEIGHT = 500
alien = Actor("alien")
alien.pos = (0, 50)
box = Rect((20, 20), (100, 100))

def draw():
screen.clear()
screen.draw.filled_rect(box, "red")
alien.draw()
def update():
if keyboard.right:
alien.x = alien.x + 2
elif keyboard.left:
alien.x = alien.x - 2
box.x = box.x + 2
if box.x > WIDTH:
box.x = 0
# PLAY SOUND AND SHOW IMAGE WHEN HIT
if alien.colliderect(box):
alien.image = 'alien_hurt'
sounds.eep.play()
else:
alien.image = 'alien'


Program 5.4 Sound and animation upon collision

Exercise

Add more boxes or sprites that move in different ways for the player to avoid.

Add a second alien controlled by different keys or gamepad for player 2.

## Mouse clicks

This uses a function call-back for event-based input. It is similar to Program 5.4 but:

• The box has been removed.
• There is an on_mouse_down() special function that is called automatically when the player click the mouse.
• The score is displayed.
alien = Actor("alien")
alien.pos = (0, 50)
score = 0

def draw():
screen.clear()
alien.draw()
screen.draw.text("Score "+str(score), (0,0))

def update():
if keyboard.right:
alien.x = alien.x + 2
elif keyboard.left:
alien.x = alien.x - 2
alien.image = 'alien'

def on_mouse_down(pos, button):
global score
if button == mouse.LEFT and alien.collidepoint(pos):
alien.image = 'alien_hurt'
sounds.eep.play()
score = score + 1


Program 5.5 Getting input from mouse clicks

## Mouse movement

# wiggle your mouse around the screen!

alien = Actor("alien")

def draw():
screen.clear()
alien.draw()

def on_mouse_move(pos):
alien.pos = pos


Program 5.6 Getting input from mouse movement

Exercise

What happens if you delete line 8 and replace it with this:

animate(alien, pos=pos, duration=1, tween='bounce_end')


Exercise

What happens if you change on_mouse_move to on_mouse_down?