# Quiz 2¶

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This quiz is open notes, open internet. The only thing you can’t do is ask for help.

## Question 1¶

Suppose you have a function that takes a lot of options; some are required and some are optional.

Before you run the function, you might want to check that:

1. All required options are provided, and

2. No illegal options are provided.

For example, suppose this dictionary contains the provided options and their values:

options = dict(a=1, b=2)
options


And suppose that only a is required.

required = ['a']


And the optional arguments are b, and c:

optional = ['b', 'c']


An option is legal if it is required or optional. All other options are illegal.

Write a function called check_options that takes a dictionary of options and their values, a sequence of required options, and a sequence of options that are legal but not required.

1. It should check that all required options are provided and, if not, print an error message that lists the ones that are missing.

2. It should check that all provided options are legal and, if not, print an error message that lists the ones that are illegal.

For full credit, you must use set operations when they are appropriate rather than writing for loops.

The following test should display nothing because the dictionary contains all required options and no illegal ones.

options = dict(a=1, b=2)
check_options(options, required, optional)


The following test should print an error message because the dictionary is missing a required option.

options = dict(b=2, c=3)
check_options(options, required, optional)


The following test should display an error message because the dictionary contains an illegal option.

options = dict(a=1, b=2, d=4)
check_options(options, required, optional)


## Question 2¶

The set method symmetric_difference operates on two sets and computes the set of elements that appear in either set but not both.

s1 = {1, 2}
s2 = {2, 3}

s1.symmetric_difference(s2)


The symmetric difference operation is also defined for more that two sets. It computes the set of elements that appear in an odd number of sets.

The symmetric_difference method can only handle two sets (unlike some of the other set methods), but you can chain the method like this:

s3 = {3, 4}
s1.symmetric_difference(s2).symmetric_difference(s3)


However, for the sake of the exercise, let’s suppose we don’t have the set method symmetric_difference the equivalent ^ operator.

Write a function that takes a list of sets as a parameter, computes their symmetric difference, and returns the result as a set.

Use the following tests to check your function.

symmetric_difference([s1, s2])    # should be {1, 3}

symmetric_difference([s2, s3])     # should be {2, 4}

symmetric_difference([s1, s2, s3]) # should be {1, 4}


## Question 3¶

Write a generator function called evens_and_odds that takes a list of integers and yields:

• All of the elements of the list that are even, followed by

• All of the elements of the list that are odd.

For example, if the list is [1, 2, 4, 7], the sequence of values generated should be 2, 4, 1, 7.

Use this example to test your function.

t = [1, 2, 4, 7]

for x in evens_and_odds(t):
print(x)


As a challenge, JUST FOR FUN, write a version of this function that works if the argument is an iterator that you can only iterate once.

## Question 4¶

The following string contains the lyrics of a well-known song.

lyrics = """
Never gonna give you up
Never gonna let you down
Never gonna run around and desert you
Never gonna make you cry
Never gonna say goodbye
Never gonna tell a lie and hurt you
"""


The following generator function yields the words in lyrics one at a time.

def generate_lyrics(lyrics):
for word in lyrics.split():
yield word


Write a few lines of code that use generate_lyrics to iterate through the words only once and build a dictionary that maps from each word to the set of words that follow it.

For example, the first two entries in the dictionary should be

{'Never': {'gonna'},
'gonna': {'give', 'let', 'make', 'run', 'say', 'tell'},
...


because in lyrics, the word “Never” is always followed by “gonna”, and the word “gonna” is followed by six different words.