Identify Unique & Shared Elements with Sets

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Problem to Solve

I want to keep track of a collection of unique values and perhaps compare the values present in different such collections.

A python set is a collection of elements whose properties are analogous to the mathematical concept of a set.

The key differences between a set and a list are:

• A set is a collection of unique elements, so it cannot contain duplicates.

• A set is unordered, so the order of elements is not guaranteed, and you cannot access its elements by index.

• A set is mutable, but it cannot contain mutable elements (e.g., lists or dictionaries).

• Set objects support several operations that are specific to set logic (union, intersection, etc.)

Some online introductions:

• W3 Schools: Python Sets

• Geeks for Geeks: Python Sets

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A Few Recipes

determining the number of elements in a set

set1 = {1, 2, 3, 4, 5}  # a set of integers
len(set1)  # yields 5

finding the intersection of two sets

set1 = {1, 2, 3, 4, 5}  # a set of integers
set2 = {3, 4, 5, 6, 7}  # another set of integers
set3 = set1.intersection(set2)  # the intersection of set1 and set2
print(set3)  # yields {3, 4, 5}

finding the union of two sets

set1 = {1, 2, 3, 4, 5}  # a set of integers
set2 = {3, 4, 5, 6, 7}  # another set of integers
set3 = set1.union(set2)  # the union of set1 and set2
print(set3)  # yields {1, 2, 3, 4, 5, 6, 7}

finding the elements in one set that aren't also in a second set

set1 = {1, 2, 3, 4, 5}  # a set of integers
set2 = {3, 4, 5, 6, 7}  # another set of integers
set3 = set1.difference(set2)  # the difference of set1 and set2
print(set3)  # yields {1, 2}

finding the elements in either set that aren't in both sets

set1 = {1, 2, 3, 4, 5}  # a set of integers
set2 = {3, 4, 5, 6, 7}  # another set of integers
set3 = set1.symmetric_difference(set2)  # the symmetric difference of set1 and set2
print(set3)  # yields {1, 2, 6, 7}

removing duplicate elements from a list

This is a useful trick!

list1 = [1, 4, 3, 4, 2, 1, 5, 1, 6, 4]  # a list of integers
list2 = list(set(list1))  # convert the list to a set and back to a list
print(list2)  # yields [1, 2, 3, 4, 5, 6] -- order is not guaranteed!