CSE 160

Sets

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Hannah Cheung

Announcements

• Due Monday (7/25) @ 11:59 PM:
• HW4 Part 1 Due
• Machine learning (K-means clustering), Dictionaries
• Submit programming + reflection survey
• Due Wednesday (7/27) @ 11:59 PM:
• HW4 Part 2 Due
• Submit programming + reflection survey

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Sets

• A set is a data structure that contains values
• Order does not matter
• {1, 2, 3} == {3, 2, 1}

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Sets

• A set is a data structure that contains values
• Order does not matter
• No duplicates
• {3, 1, 4, 1, 5} == {5, 4, 3, 1}

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Sets

• A set is a data structure that contains values
• Order does not matter
• No duplicates
• For every data structure, ask:
• How to create
• How to query (look up) and perform other operations
• How to modify

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Set Creation

• Creating an empty set
• Do not use {} for empty set; it is empty dictionary

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• Creating a set with initial values

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• Creating a set from a list

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s = set()

odd = {1, 3, 5}

prime = {2, 3, 5}

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odd = set([1, 3, 5])

prime = set([2, 3, 5])

empty = set([]) # or set()

Set Operations

Iteration (but no indexing) over sets

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odd = {1, 3, 5}

prime = {2, 3, 5}

4 in prime # membership

odd | prime # union

odd & prime # intersection

odd - prime # difference

for i in odd:

print(i)

Set Operations Practice

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z = {5, 6, 7, 8}

y = {1, 2, 3, 1, 5}

k = z & y

j = z | y

m = y - z

n = z - y

See in Python Tutor

Set Modification

• Add element to set
• my_set.add(element)
• my_set = my_set | {element}
• Remove element from set
• my_set.remove(element)
• Element must be in set or error occurs
• my_set.discard(element)
• No errors occur
• my_set = my_set - {element}
• my_set.pop()
• Removes and returns arbitrary element

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Note: add remove, and discard all return None

Set Modification Practice

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z = {5, 6, 7, 8}

y = {1, 2, 3, 1, 5}

p = z

q = set(z) # makes a copy of set z

z.add(9)

q = q | {35}

z.discard(7)

q = q – {6, 1, 8}

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See in Python Tutor

List vs Set Operations

• Find common elements in both list1 and list2

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• Find common elements in both set1 and set2

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out = []

for elem in list2:

if elem in list1:

out.append(elem)

set1 & set2

List vs Set Operations

• Find elements in either list1 and list2 (without duplicates)

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• Find elements in either set1 and set2

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out = list(list1)

for elem in list2:

if elem not in list1:

out.append(elem)

set1 | set2

List vs Set Operations

• Find elements in either list (but not both)

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• Find elements in either set (but not both)

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out = []

out2 = list1 + list2

for elem in out2:

if elem not in list1 or elem not in list2:

out.append(elem)

set1 ^ set2

Set Limitations

• Not every value may be placed in a set
• Set elements must be immutable values
• int, float, bool, string, tuple
• not: list, set, dictionary
• Set itself is mutable
• We can add and remove elements
• Aside: frozenset must contain immutable values and is immutable (cannot add or remove elements)

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