More on Functions (Part 2)
Intro to Computer Science
CS1510, Section 2
Dr. Sarah Diesburg
What are functions?

Little pieces of code that we can write and
invoke by name
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Reusable
General
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Functions
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Take in any number of parameters


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Including no parameters!
Possibly compute something
Return a value
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Can be None
If no return is specified, returns None
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Function that takes in no
parameters
def answerToEverything():
return 42

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This function takes in no parameters
It does no computation
Just returns a number
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Function that returns nothing
Let’s take a look at print()
>>> result = print(“hi”)
>>> print(result)
None
>>>
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None

None is a special value in Python that
represents nothing

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The first letter of None must be capitalized – it will
turn orange
Use it when you have nothing to return
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Like if one of the parameters was invalid
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Functions

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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
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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
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prime.py (continued)
if isPrime:
print("That number is prime")
else:
print("That number is NOT prime")
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primeAsFunction.py
def isPrime(number):
for divisor in range(2,number):
if number%divisor==0:
return False
return True

Much smaller!
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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

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Step 1: Run your python file (F5)
At interpreter, type your function name with
parameters
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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)
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Revisiting Design Documents
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Now that our future programming
assignments will involve functions, I will
expect more from your design documents
and comments in your code
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Code Comments
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Still do what you are already doing
Before each function, list
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Function name
Inputs – inputs parameters to your function and
their types
Output – what your function returns and its type
Description – what your function does
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Example Program

Let’s say that we are writing a computercomputer rock/paper/scissors game with 4
functions
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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
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Example Program

Let’s say that we are writing a computercomputer rock/paper/scissors game with 4
functions
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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
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Remember…
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Write your worksheet and function comments
first
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Then fill in the code
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Example
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Take a look at RPS.py to see what I am
expecting.
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