CPSC120A
Fundamentals of Computer Science

Reading Questions

Quiz

Aliasing

• The mutability of lists makes some list operations faster
• It also introduces some potential errors

• With strings

  a_string = 'spam'
  b_string = a_string
  print(a_string, b_string)
  b_string = a_string + ' is delicious'
  print(a_string, b_string)

• Changing one string does not affect the other because the concatenate creates a new string

• Lists on the other hand

  a_list = [1, 2, 3]
  b_list = a_list
  print(a_list, b_list)
  b_string.append(-1)
  print(a_list, b_list)

• Changing on list affects another list because append does not make a copy, it modifies
• This is called aliasing, and would be simple to avoid if it only occured when you assign on list to another
• Note, if you did need one list to be equivalent to another, without aliasing you can make a copy with a slice

• This also happens every time you call a function with a list as a parameter

  def a_func(b_list):
      b_list.append(-1)
  
  a_list = [1, 2, 3]
  a_func(a_list)
  print(a_list)

• This is desirable sometimes

• A function can modify a list instead of returning a list, which can be more efficient than copying the entire list to make a small change

Sprites

• One other example of mutability, sprites

• I also created a module to make doing some of common things with the graphics module easier

  import graphics
  import sprite
  
  a_sprite = sprite.Sprite(0, 0, 'sprite.gif')
  
  while graphics.window_open():
      graphics.clear()
  
      graphics.draw_sprite(a_sprite)
  
      graphics.wait()

• Similar to the turtle module, you create sprite objects
• The graphics module can draw a sprite
• Note, the location of the sprite is an attribute of the sprite

• You can change the attributes of a sprite using the dot notation

  a_sprite.y += 1

• Sprites are mutable, which means that functions can modify them

• This makes it much easier to organize you code

  def update_sprite(a_sprite):
      a_sprite.y += 1

• You can also add your own attributes

• For example the speed of a sprite could be an attribute

  def update_sprite(a_sprite):
      a_sprite.y += a_sprite.speed_y
  
  a_sprite.speed_y = 1
  a_sprite = sprite.Sprite(0, 0, 'sprite.gif')

• This is useful for adding gravity, an acceleration

  def update_sprite(a_sprite):
      a_sprite.speed_y += GRAVITY
      a_sprite.y += a_sprite.speed_y
  
  
  a_sprite = sprite.Sprite(0, 0, 'sprite.gif')
  a_sprite.speed_y = 0
  
  while graphics.window_open():
      graphics.clear()
  
      update_sprite(a_sprite)
      graphics.draw_sprite(a_sprite)
  
      graphics.wait()

• Also have useful methods for testing for and handling collision

  if a_sprite.collides(b_sprite):
      a_sprite.speed_y = 0
      a_sprite.uncollide(b_sprite)

Lab