1. More on Functions (Part 2)
Intro to Computer Science
CS1510, Section 2
Dr. Sarah Diesburg

2. What are functions?
Little pieces of code that we can write and invoke by name
Reusable
General

3. Functions Take in any number of parameters Possibly compute something
Including no parameters!
Possibly compute something
Return a value
Can be None
If no return is specified, returns None

4. Function that takes in no parameters
def answerToEverything():
return 42
This function takes in no parameters
It does no computation
Just returns a number

5. Function that returns nothing
Let’s take a look at print()
>>> result = print(“hi”)
>>> print(result)
None
>>>

6. None None is a special value in Python that represents nothing
The first letter of None must be capitalized – it will turn orange
Use it when you have nothing to return
Like if one of the parameters was invalid

7. Functions
Remember, a function stops running when it hits a return statement
Often times it is easier to write something as a function than as a script by itself
Let’s look at our old script to find a prime number

8. prime.py
number = int(input("Please enter a number greater than 2. ")) isPrime = True #break out condition divisor = 2 while divisor<number and isPrime: if number%divisor==0: print("The number",number,"is divisible by",\ divisor) isPrime=False # Going to break out divisor=divisor+1

9. prime.py (continued)
if isPrime: print("That number is prime") else: print("That number is NOT prime")

10. primeAsFunction.py def isPrime(number):
for divisor in range(2,number):
if number%divisor==0:
return False
return True
Much smaller!

11. Future Programming Assignments
I will ask you to write python files with only functions in them
That’s ok – you can call functions directly from IDLE
Step 1: Run your python file (F5)
At interpreter, type your function name with parameters

12. Example Take functionFun.py and deleted all the code starting at
# Running the main part of the program
Save
Run the file
Type
>>> mySum = addNumToSelf(4)
>>>print(mySum)

13. Revisiting Design Documents
Now that our future programming assignments will involve functions, I will expect more from your design documents and comments in your code

14. Code Comments Still do what you are already doing
Before each function, list
Function name
Inputs – inputs parameters to your function and their types
Output – what your function returns and its type
Inside the function using a docstring (triple quotes)
Description – what your function does

15. Example Program
Let’s say that we are writing a computer- computer rock/paper/scissors game with 4 functions
Function getWeapon(): no inputs, gets random weapon from “r”, “p”, “s”, returns “r”,”p”,”s”
Function isTie(one,two): inputs weapons from both players, returns true if tie and false otherwise
Function playerOneWins(one,two): inputs weapons from both players, returns true if player one wins and false otherwise

16. Example Program
Let’s say that we are writing a computer- computer rock/paper/scissors game with 4 functions
Function rps(numberOfGames): input is the number of games (int), plays rock paper scissors with two computer players, and returns the result of the number of player1 wins, player2 wins, and ties in a string

17. Remember… Write your worksheet and function comments first
Then fill in the code

18. Example
Take a look at RPS.py to see what I am expecting.