Set (Python)

Intro

• Python’s set is a hash table-based implementation of the Set ADT (each element is stored as a key with no associated values)

Sets (Python)

https://docs.python.org/3/tutorial/datastructures.html#sets

basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
print(basket)                      # show that duplicates have been removed
 
'orange' in basket                 # fast membership testing
'crabgrass' in basket
 
# Demonstrate set operations on unique letters from two words
 
a = set('abracadabra')
b = set('alacazam')
a                                  # unique letters in a
 
a - b                              # letters in a but not in b
a | b                              # letters in a or b or both
a & b                              # letters in both a and b
a ^ b                              # letters in a or b but not both

Creating Sets

• Using curly braces

fruits = {"apple", "banana", "cherry"} # Note: fruits = ("apple", "banana", "cherry") creates a tuple
 
mixed_set = {4,3,2,3,5,1} # Duplicates removed -> {1, 2, 3, 4, 5}

• Using the set() constructor

# Create an empty set 
empty_set = set() # Note: {} creates an empty dictionary
 
# Convert an iterable to a set
numbers_list = [1,1,2,3,4,4,5]
unique_numbers_set = set(numbers_list) # Duplicates removed -> {1, 2, 3, 4, 5}

Common Operations

s = {1,2,3}
s.add(4) # {1, 2, 3, 4}
s.remove(2) # {1, 3, 4}
print(3 in s) # True

• Since sets are unordered, they have no indices, so you cannot access or assign by index

s[0] # TypeError: 'set' object is not subscriptable
s[0] = 4 # TypeError: 'set' object does not support item assignment

Mathematical Operations

A = {1, 2, 3}
B = {3, 4, 5}
 
A | B   # union → {1, 2, 3, 4, 5}
A & B   # intersection → {3}
A - B   # difference → {1, 2}