CPSC120B
Fundamentals of Computer Science

Reading Questions

Quiz

Variables

• Variables allow you to name values in a program
• They can be useful especially if you use a value multiple times

• For example, the star program from the previous lab

  import turtle
  turtle.forward(100)
  turtle.left(144)
  turtle.forward(100)
  turtle.left(144)
  turtle.forward(100)
  turtle.left(144)
  turtle.forward(100)
  turtle.left(144)
  turtle.forward(100)
  turtle.left(144)

• If you want to change the size of the star, you have to change 5 numbers

• With a variable, you only have to change 1

  import turtle
  size = 100
  turtle.forward(size)
  turtle.left(144)
  turtle.forward(size)
  turtle.left(144)
  turtle.forward(size)
  turtle.left(144)
  turtle.forward(size)
  turtle.left(144)
  turtle.forward(size)
  turtle.left(144)

• Using a varible for the angle

• Changing it makes the program no longer create a perfect star, but when you were creating the program you probably were annoyed that you had to go back and change all of those numbers multiple times

  import turtle
  size = 100
  turn_angle = 144
  turtle.forward(size)
  turtle.left(turn_angle)
  turtle.forward(size)
  turtle.left(turn_angle)
  turtle.forward(size)
  turtle.left(turn_angle)
  turtle.forward(size)
  turtle.left(turn_angle)
  turtle.forward(size)
  turtle.left(turn_angle)

• Using a variable would have made that easier
• Variables can also make your code easier to read and easier to debug

• Take the program from the last lab to draw a centered square

  import turtle
  turtle.up()
  turtle.forward(50)
  turtle.left(90)
  turtle.forward(50)
  turtle.left(90)
  turtle.down()
  turtle.forward(100)
  turtle.left(90)
  turtle.forward(100)
  turtle.left(90)
  turtle.forward(100)
  turtle.left(90)
  turtle.forward(100)
  turtle.left(90)

• We can use a variable to specify the size of the square

  import turtle
  size = 100
  turtle.up()
  turtle.forward(size / 2)
  turtle.left(90)
  turtle.forward(size / 2)
  turtle.left(90)
  turtle.down()
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)

• Note, we can use an expression, in this case a mathematical equation, anywhere you can use a value

• But it might make for sense to use another variable

  import turtle
  size = 100
  center_to_edge_dist = size / 2
  turtle.up()
  turtle.forward(center_to_edge_dist)
  turtle.left(90)
  turtle.forward(center_to_edge_dist)
  turtle.left(90)
  turtle.down()
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)

• This is a really simple example, but it has two advantages
• The variable name serves as an explanation for why the turtle is moving size / 2
• It makes the code more readable, easier to understand
• This is important for you, because it will reduce the chances of you making a silly mistake
• It can also make finding a fixing errors in your code easier
• Each variable you create is an opportunity to print and check the progress of your program when it is not behaving as you expect or want it to

• The fewer computations each line does are fewer places that there could be a mistake (and fewer things you have to check when there is an error on a line)

Style

• Variable names must begin with a letter or underscore and can contain numbers
• By convention we use all lowercase, and use underscores to separate words
• There are also a list of reserved words you can’t use as variable names
• Don’t worry about remembering these, as idle highlights them to let you know
• Meaninful names are important, they help make the code easier to read
• single letter variable names a, b, c, etc., are not meaningful
• Can also use comments and white space to make code easier to read
• Comments are lines of code that are ignored when running the program but are useful when reading the program

• Comments start with the # character

  import turtle
  size = 100
  center_to_edge_dist = size / 2
  
  # move the turtle to the upper right corner of the square
  turtle.up()
  turtle.forward(center_to_edge_dist)
  turtle.left(90)
  turtle.forward(center_to_edge_dist)
  turtle.left(90)
  turtle.down()
  
  # draw the square
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)
  turtle.forward(size)
  turtle.left(90)

• These can also help you debug your code

• When a program gets big it helps you find the part of your code that contains the problem

Expressions

• Expressions are code that evaluate to a value
• Mathematical equations for example are expressions

• For example,

  4 / 2 + 1

• Evaluates to the value 3.0

• As we saw above, variables can be used in expressions

  20 + size / 2

• Expressions behave much like mathematical expressions you know, there is precedence and order of operations

• One operation is performed at a time until a value is computed

  1 + 3 / 2 # division first
  1 + 1.5   # then addtion
  2.5       # final value

• You can use parentheses to change the order

  (1 + 3) / 2 # addition first 4 / 2 # then division 2.0 # final value

• These are all the rules you are already familiar with
• A couple operators may be new
• The ** is exponentiation
• The // operator is integer division, when the operands are integers the result is how many times the denominator evenly goes into the numerator
• The % operator is the mod operator, the remainder of integer division

• These operators together can be really useful for some types of unit conversion like minutes to hours, or cents into coins, or inches into feet.

  total_minutes = 121
  hours = total_minutes // 60
  minutes = total_minutes % 60
  print(total_minutes, "is", hours, "hours and", minutes, "minutes")

• Each of the above lines is a statement, they don’t evaluate to a value
• Statements are commands to the computer
• The first three are assignment statements, they create a variable that associates the name of the left of the = symbol to the value on the right, which may be an expression that evaluates to a value
• The print statement commands the computer to display some text
• Print statements can contain multiple values separated by a comma
• Note, each of the values is separated with a space, when printed

• If you want to print text, but it in quotes

Activity