Sets

Set Variables

A set is an unordered collection of variables. By unordered it is meant a set that contains a and b is considered the same as a set that contains b and a.

A set is defined in Python using the familiar List Notation,

pets = { "dog", "cat", "fish", "hamster", "snake" }

four_legs = { "dog", "cat", "hamster" }

swims = { "dog", "fish", "snake" }

warm_blooded = { "dog", "cat", "hamster" }

poets = { "byron", "shakespeare", "eliot" }

The examples on this page will refer to the above sets.

Set Operations

Note

Before executing these commands, try working them out by hand first and see if your work agrees!

Cardinality

The Cardinality of a set is found by calculating its length,

total_pets = len(pets)
print(total_pets)

Cardinality Solution

Output:

5

Union

The Union of two sets is found by,

pets_or_poets = pets.union(poets)
print(pets_or_poets)

Union Solution

Output:

{‘snake’, ‘byron’, ‘shakespeare’, ‘eliot’, ‘fish’, ‘cat’, ‘dog’, ‘hamster’}

Important

Take note: set operations do not preserve the order of the sets. In technical terms, sets are not indexed. Notice the order of the set in the output is random.

Intersection

The Intersection of two sets is found by,

four_legs_and_swims = four_legs.intersect(swims)
print(four_legs_and_swims)

Intersection Solution

Output:

{‘dog’}

Difference

The Difference of two sets is found by,

swims_but_not_warmblooded = swims - warm_blooded
print(swims_but_not_warmblooded)

Difference Solution

Output:

{‘snake’}