CPSC120B
Fundamentals of Computer Science I

Day 11 Notes

Functions

Cool Computer Science Thing of the Day

Reading Questions

Quiz

quiz 11

Emacs Tip of the Day

M-/ auto complete

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():
    alex = turtle.Turtle()
    alex.forward(100)
    alex.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(t):
    t.forward(100)
    t.left(90)
```

Now, it can be called like this

alex = turtle.Turtle()
draw_something(alex)

This is much like the turtle functions we have already used… alex.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

Pre and Post Conditions

What if when calling a function you give an argument that doesn’t make sense?
```
draw_something(100)
```
The function breaks, because it is expecting a turtle, not a number
This is a common mistake, to help prevent it we add comments above the function describing the conditions in which the function is guaranteed to work correctly

Also need to specify the effects of the function as a post condition

# PRE: t is a turtle object
# POST: draws a line and turns 90 degrees
def draw_something(t):
    t.forward(100)
    t.left(90)

Lab

lab11