Writing functions

functions encapsulate complexity so that we can treat it as a single thing
functions enable re-use: write one time, use many times

First define:

def greeting():
    print('Hello!')

and then we can run it:

greeting()

def printDate(year, month, day):
    joined = str(year) + '/' + str(month) + '/' + str(day)
    print(joined)
printDate(1871, 3, 19)

Every function returns something, even if it’s None.

a = printDate(1871, 3, 19)
print(a)

How do we actually return a value from a function?

def average(values):   # the argument is a list
    if len(values) == 0:
        return None
    return sum(values) / len(values)
print('average of actual values:', average([1, 3, 4]))

Here is an example of a more complex calendar function returning an alphabetical day of the week:

def dayOfTheWeek(year, month, day):
    import datetime
    week = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    return week[datetime.datetime(year, month, day).weekday()]
dayOfTheWeek(2022, 11, 10)   # 'Thu'

Question 8.1

Write a function to convert from Fahrenheit to Celsius, e.g. typing celsius(77) would produce 25.

Question 8.2

Write a function to convert from Celsius to Fahrenheit. Test it with celcius(), e.g. by converting Fahrenheit → Celsius → Fahrenheit, or Celsius → Fahrenheit → Celsius.

Question 8.3

Now modify celsius() to take a list of Fahrenheit temperatures, e.g., celcius([70,80,90,100]), to return a list of Celsius temperatures.

Function arguments in Python can take default values becoming optional:

def addNumber(a, b=1):
    return a+b
print(addNumber(5))
print(addNumber(5,3))

With several optional arguments it is important to be able to differentiate them:

def modify(a, b=1, coef=1):
    return a*coef + b
print(modify(10))
print(modify(10, 1))   # which argument did we add?
print(modify(10, coef=2))
print(modify(10, coef=2, b=5))

Any complex python function will have many optional arguments, for example:

?print