Section 6

Sets + Tuples + Itemgetter + Sorting

Logistics

• HW4 Part 2 Due Monday (Nov. 11)
• Programming and Written Check-In 6 Due Today (Nov. 9)

Set Review

Sets!

• Sets are a type of data structure which is unordered and unindexed
• There can be no duplicates
• Imagine it as a bag of values

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You can imagine a set that contains the values 1 through 6 like this:

All of the following code would make a set called s with the elements 1-4 inside

s = set([1, 2, 3, 4])

s = {1, 1, 1, 1, 1, 1, 2, 3, 4}

s = {1, 2, 3, 4}

s = set()

s.add(1)

s.add(2)

s.add(3)

s.add(4)

Add, Remove, Discard

Say we have the set s that has elements 1, 2, 3, 4 inside

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• Add
• adds an element to the set
• s.add(5)

5

• Remove
• takes out an existing element from the set (Must exist in the set!)
• s.remove(5)

• Discard
• takes out an element from the set (doesn’t need to be in there already)
• s.discard(5)

• Pop
• Returns a random element
• s.pop()
• Could return 1, 2, 3, or 4

What can be added into a set?

• Although you can convert any data structure into a set, you can only add immutable types into a set (just like dict keys)
• Data types that can not go in a set (mutable types)
• Dictionaries
• Lists
• other sets
• Data types that can go in a set (immutable types)
• Integers
• Floats
• Booleans (but why would you do this?)
• Strings
• Tuples

Python

Python

TypeError: unhashable type: 'set'

Does a set of all sets contain itself?

Looping through a Set

To see all elements in a set, we can loop through it

Would print 1, 2, 3, 4 in some random order

for element in s:

print(element)

Checking if Something is in a Set: In

We can use in to see if an element is in a set

Returns True

Returns False

2 in s

6 in s

Set Operations

A | B

A & B

A - B

A ^ B

Example

Say we have two sets:

pets = {“dog”, “hamster”, “lizard”, “cat”}

felines = {“cat”, “puma”, “lion”, “tiger”}

Example

Say we have two sets:

pets = {“dog”, “hamster”, “lizard”, “cat”}

felines = {“cat”, “puma”, “lion”, “tiger”}

Set Operations

• pets | felines
• {“dog”, “hamster”, “lizard”, “cat”, “puma”, “lion”, “tiger”}

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• pets & felines
• {“cat”}

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• pets - felines
• {“dog”, “hamster”, “lizard”}

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• pets ^ felines
• {“dog”, “hamster”, “lizard”, “puma”, “lion”, “tiger”}

Tuples

Tuples

Tuple is a collection which is ordered and unchangeable.

Lists and tuples are similar, but have different properties

The table at the right shows what kind of things you can do with a tuple, but not a list.

Let the data structure be called name. A ✅ means you can do it, a 🚫 means it won’t work

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Making a Tuple

These are all ways to make tuples:

t = (1, 2, 3)

t = tuple([1, 2, 3])

t = (1,)

Note that to make a one element tuple you need to add a comma after the one value! (1) would not work!

Itemgetter

Itemgetter

Itemgetter returns a function.

from operator import itemgetter

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get_3rd_item = itemgetter(2)

get_3rd_item([7, 3, 8]) -> 8

# this is the same as

itemgetter(2)([7, 3, 8]) -> 8

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Sorting with Itemgetter in General

• Useful for when you have a list of tuples where every item inside those tuples corresponds to a particular feature
• ex: the element at index 0 of every tuple is a name, the element at index 1 of every tuple is an age)
• lst = [('Anne', 5), ('Bob', 6), ('Carl', 3), ('Elisa', 2), ('Diana', 2)]

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• If you want to sort with the feature in the tuple at the index i, do so like this:
• sorted_lst = sorted(lst, key = itemgetter(i))

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• Sort by the “least important” feature first (ie a tie breaker), the more important features last

Sorting with itemgetter

lst = [('Anne', 5), ('Bob', 6), ('Carl', 3), ('Elisa', 2), ('Diana', 2)]

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# sort alphabetically by name

alphabetical_lst = sorted(lst, key = itemgetter(0))

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# sort by the number (lowest number first)

sorted(lst, key = itemgetter(1))

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# sort by the number (highest number first)

sorted(alphabetical_lst, key = itemgetter(1), reverse = True)

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Sorting with itemgetter based on two criteria

lst = [('Anne', 5), ('Bob', 6), ('Carl', 3), ('Elisa', 2), ('Diana', 2)]

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# sort by the number (highest number first), break ties alphabetically

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# alphabetize first

sorted_lst = sorted(lst, key = itemgetter(0))

# then sort by number

sorted_lst = sorted(sorted_lst, key = itemgetter(1), reverse = True)

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Why do we care about itemgetter?

• Very easy way to sort values on multiple attributes
• Way to sort dictionaries into a list
• if my_dict is a dictionary, then list(my_dict.items()) will return a list of key value tuple pairs!
• Super important for hw5!

Kahoot

We’ll use this kahoot to go over problems 1, 2, and 4

No pressure to win (as always)

Section Problems

We’ll emphasize problems 5 and 6 (since they are the most relevant for the homework!)

Starter Code (Problem 5)

Programming Check In

''

Return true if subset_one contains all elements of subset_two.

Return false if subset_two contains an extra element besides

the ones found in subset_one except if the extra element is 1.

If the only additional element in subset_two is 1, return True.

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Arguments:

subset_one: a set of integers

subset_two: a set of integers

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Returns: False if subset_two contains additional elements besides

the ones found in subset_one except if the extra element is 1

'''

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