Arrays and Lists

Announcements

• Midterm Exam now on Friday this week.
• Problem Set 4 is posted and will be due next week Tuesday at 10pm
• Section Leaders are working on Project 1 feedback
• TechByte talk tomorrow at 11:30 in Ford 102
  • I will be talking on Broadening Participation in Computing for Adults
• Polling continues https://www.polleverywhere.com/agbofred203

Review Question

Which of the below blocks of code will create the image to the right? The window measures 500 x 200 pixels and the value of d is 150.

x, y = 250 - d / 2, 100 - d / 2
a1 = GArc(x, y, d, d, 90, -180)
gw.add(a1)

x, y = 250 - d, 100 - d
a1 = GArc(x, y, d, d, -180, 90)
gw.add(a1)

x, y = 250 - d / 2, 100 - d / 2
a1 = GArc(x, y, d, d, 90, 180)
gw.add(a1)

x, y = 250 - d / 2, 100 - d / 2
a1 = GArc(x, y, d, 180, -90)
gw.add(a1)

Arrays and Lists

• From the earliest days, programming languages have supported the idea of an array, or an ordered sequence of values.
• Individual values in an array are called elements, and the number of elements is the length of the array.
• Each element’s position in the array is given by its index, with index numbers starting at 0 and extending up to 1 less than the length of the array
• Python implements the array concept in a bit more general form called a list.

Reminder: Making Lists

• Created using square brackets with elements separated by commas:

  COIN_VALUES = [1, 5, 10, 25, 50, 100]
  COIN_NAMES = [ "penny", "nickle", "dime",
                 "quarter", "half-dollar", "dollar" ]

• Lists are commonly represented visually or conceptually as a series of numbered boxes:

What we already know

• Can retrieve the value of any element in a list by writing the index of that element in square brackets after the list name

  • COIN_VALUES[3] ⟶ 25
  • COIN_NAMES[2] ⟶ "dime"

• Can concatenate two lists to form a new list with elements from both

  • [1,2,3] + [4,5,6]⟶ [1,2,3,4,5,6]

• Can loop through each of the list elements in turn

  for elem in my_list:
    # do stuff with elem

  where elem is the name of the variable that will sequentially get assigned the value of every element in my_list

Sequences

• Sound familiar? Lists are just like more general strings!
• Strings and lists are examples of a more general class of object in Python called sequences

Understanding Check!

What would the below expression evaluate to?

Mutants

• The most important difference between strings and lists is one of mutability
  • Strings we have already identified as being immutable: you can not change the individual elements
  • Lists, in contrast, are mutable, which means that we can change or assign new values to the elements of a list
• Immutable objects have many advantages in programming:
  • You don’t have to worry about if the values will change
  • Immutable values can be more easily shared
  • Immutable objects are easier to use with concurrent programs
• In some situations though, mutable objects are the perfect tool for the job

A Tutorial on Lists

cool = ['blue', 'violet']
warm = ['red', 'orange']

colors = [cool, warm]
other_colors = [['blue', 'violet'],
                ['red', 'orange']]

print(colors == other_colors)
print(colors is other_colors)

cool[0] = 'indigo'
warm = ['orange', 'yellow']

print(colors)
print(other_colors)

For Reference

• When working with mutable objects, it is better to think of the variable as holding a reference to the object, rather than the actual contents of the object
• I find it useful to think of a reference as the “address” in memory where that object’s contents can be found
• This undeniably complicates things, as referencing a mutable object lets you change it, which will immediately be reflected in anything else that referenced that object
• Mutable objects can be terrific to work with, as their mutability makes them very flexible, but be wary of unexpected behavior

Project 2: Breakout!

Breakout History

• The popular Breakout arcade game was released by Atari in 1976
• Atari founder Nolan Bushnell wanted a new game that would build on the success of the earlier game Pong. He assigned Steve Jobs to develop the game, promising a bonus if the game required a small number of chips.
• As Wikipedia tells the story, “Jobs had little specialized knowledge of circuit board design but knew [his friend Steve] Wozniak was capable of producing designs with a small number of chips. He convinced Wozniak to work with him, promising to split the fee evenly between them.”
• Wozniak completed the game design in four days, but Jobs never told him about the bonus offer. Jobs was paid \(\$5,000\), but Wozniak received only \(\$350\).
• Jobs and Wozniak co-founded Apple Computer the following year, which has grown to be the largest corporation in the world by market capitalization.

Breakout Basics

Breakout Milestones

• Breakout is broken up over 5 milestones
• You have already seen or written pieces of similar code to many of the milestones!
  • Milestone 1: PS4 brick pyramid
  • Milestone 3: Section bouncy ball problem (this week)

Other Milestones

Check the desciption here