1. Week 7 - Friday
CS 113

2. Last time What did we talk about last time? Software engineering
Testing
Programming languages

3. Questions?

4. Project 2

5. Functions

6. Idea of a function
Functions allow you to package up some code to run over and over
Functions usually take some input (like numbers or text) so that they can be customized
Functions often give back an answer (like the square root of a number)
Also called methods in some other languages
The customizable purple blocks played the role of functions in Scratch

7. Advantages of functions
More modular programming
Break a program into separate tasks
Each task could be assigned to a different programmer
Code reusability
Use code over and over
Even from other programs
Less code duplication
Improved readability
Each function can do a few, clear tasks

8. Defining a function Name of 1st argument Function name
def name( arg1, … , argn ):
statement1
statement2 …
statementm
Required syntax
Name of
last argument
Code done by function
(must be indented)

9. Simple function example
Given two numbers, find the smaller:
def smaller(a, b):
if a < b:
return a
else:
return b

10. Value returning functions
It is possible to divide functions into two types:
Functions that return values
Functions that don't return values
Functions that do return values give an answer:
It's as if the function is replaced by whatever answer is returned
x = 3
y = 4
small = smaller(x, y) #small contains 3

11. Functions that don't return values
A function doesn't have to return an answer:
This function only prints things to the screen
A function that doesn't return anything does some task but doesn't give back an answer
def callForHelp(times):
for i in range(times):
print("Help!")

12. return statements
Like most code in Python, the code inside of a function executes line by line
Of course, you are allowed to put if statements and loops inside methods
You can also put in return statements
A function will stop executing and jump back to wherever it was called from when it hits a return
The return statement is where you put the value that will be given back to the caller

13. Calling functions Defining a function is only half the game
You have to call functions to use them
Calling a function means giving it the parameters (or arguments) it needs and then waiting for its answer
By now, you have done many function calls
print()
You can call your own functions the same way

14. Calling functions callForHelp(3) #calls for help 3 times
You type the name of the function and then its arguments in parentheses
If it's a value returning function, you can store the answer it gives back
The arguments you give can be values or variables
You have to give the function the right number of arguments or the program will have an error
callForHelp(3) #calls for help 3 times
result = smaller(9, 2)

15. Key syntax Every function starts with def
Then comes the name of the function
Followed by the names of the arguments in parentheses
Followed by a colon (:)
Like loops and if statements, the code inside a function is indented one level
Call the function in other code by typing the name and the correct number of arguments in parentheses

16. Function example
Lets say we want to make a function that will draw three balls stacked on top of each other
The bottom one is green, the middle is orange, and the top is red
We want the function to take a radius and a position
The bottom ball will be drawn at the position with the radius given
The middle ball will be above it with half the radius
The top ball will be above that with one quarter the radius

17. Function code
Here's the function stacks() which takes a radius and a position and draws the balls
def stacks(radius, position):
sphere(radius=radius, pos=position, color=color.green)
newPosition=vector(position.x, position.y + 3*radius/2,position.z)
sphere(radius=radius/2, pos=newPosition, color=color.orange)
newPosition=vector(position.x, position.y + 9*radius/4, position.z)
sphere(radius=radius/4, pos=newPosition, color=color.red)

18. Calling the function Now we can call the function several times
Each time, it will draw a stack with the size and location we specify
Result shown to the right
stacks(1, vector(0,0,0))
stacks(.5, vector(2,3,1))
stacks(.2, vector(-2,1,2))

19. Lab 7

20. Upcoming

21. Next time…
Moore's law
Multicore computers
Complex decisions

22. Reminders Finish Project 2 Reading Python Chapter 7
Due tonight before midnight!
Reading Python Chapter 7
Think about what you want to do for your Final Project
Proposal due by 3/31