CPSC120B
Fundamentals of Computer Science

Moods Are Contagious

• Editorial Expression of Concern and Correction
• Facebook Tinkers With Users’ Emotions in News Feed Experiment, Stirring Outcry

Test Review

Reading Questions

Quiz

Writing Functions

• We know how to use functions… print, turtle.forward, math.cos
• Functions are named code, they do something

• We can define our own named code

  def draw_something():
      turtle.forward(100)
      turlte.left(90)

• Execute the function just like other functions, draw_something()

• Note, order matters, you can’t use it until you have defined it, just like a variable

Parameters and Arguments

• Note that the previous example creates a new turtle every time it is executed
• We can fix this if by having the function share the same turtle across multiple executions.
• To do this we need to give the function input, the turtle to use

• Parameters are variables with a value set outside of the function

  def draw_something(dist):
      turtle.forward(dist)
      turtle.left(90)

• Now, it can be called like this

  draw_something(10)

• This is much like the turtle functions we have already used… turtle.forward(100), the 100 is the input to the function, it changes the behavior of the function
• The names of the inputs, in the function definition, are called parameters
• The names of the inputs, in the function call, are called arguments

• If a function has multiple parameters, then the order of the arguments is the same

  def draw_something(dist, angle):
      turtle.forward(dist)
      turtle.left(angle)
  
  draw_something(10, 20)

• In this case, dist has the value 10 and angle has the value 20 because the parameter dist is before the parameter angle

Function Comments

• Functions allow you to write code that is isolated from other code
• This is great because it facilitates testing by making the amount of code to be tested small and managable
• It also facilitates code sharing and reuse
• It is easy to reuse a function by copying it from one file into another or importing it
• It also provides a distinction of responsibility for different people to contribute to a program
• To make using functions easier for other people, need to specify under what conditions it will succeed

• For example the above function draw_something will break if it is called with strings for input

  draw_something('a little bit')

• To inform the user of a function of its limitations, use a comment

  # dist (number) - distance the turtle will move
  def draw_something(dist):
      turtle.forward(dist)
      turtle.left(90)

• These limitation are often on type, but can also be on the values that are allowed for a particular type

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