Sets

Code sample link: https://replit.com/@jjoco/python-sets

Sets are unordered (ie does not remember the insertion order) and unindexed (ie cannot read or write an element via an index) records of values. Python sets can be utilized like mathematical sets, as operations between sets are similar to that of mathematical sets (eg. intersections, unions, etc.). Each item in a set is unique, and fortunately, one can iterate through a set and can add items into it.

Why use sets

Sets in Python are highly optimized for reading and writing speed. When you check if a list contains some certain value, Python, at worst, would need to search through the entire list. On other hand, Python, at worst, would need to use a lookup table to figure out if an item exists in the set. To be more technical, time complexities for reading and writing values in a set is O(1).

Creating a set

To create an empty set, you can use the set() function, or use curly braces like dictionaries. Keep in mind that Python will interpret empty curly braces as a dictionary; however, populated curly braces with items (that are not key-value pairs) will be interpreted as sets.

#Create empty set
init_set = set()
print(init_set)
'''
Prints out:
set()
'''

#Create a set using curly braces
set_with_curly_braces = {"hi", "there", "hello"}

#Create a set from an initial list
set_from_list = set(["hello", "world", "set"])
print(set_from_list)
'''
Prints out:
{'set', 'world', 'hello'}
'''

Checking if a set contains an element

Use keyword in to check to see if a value exists in a set:

#Check if element exists in set
target = "hello"
if target in set_with_curly_braces:
  print("target is seen")
'''
Prints out:
target is seen
'''

Pushing an element into a set

Use the .add(elem) method to add an item into a set:

set_with_curly_braces.add("added_elem")
print(set_with_curly_braces)
'''
Prints out:
{'there', 'hi', 'hello', 'added_elem'}
'''

Getting the length of a set

You can use len to get the number elements in a set (ie. its length) .

set_length = len(my_set)

So, if my_set = {'set', 'world', 'hello'}, then len(my_set) would be 3.

Iterating

You can interate through a set like any other collection:

for elem in set_with_curly_braces:
  print(elem)
'''
Prints out:
there
hi
hello
added_elem
'''

Since sets are unordered, elements may be printed in a different order each time the for-loop is run.

Clear set

Use .clear() to remove all items in the set

set_with_curly_braces.clear()
print(set_with_curly_braces)
'''
Prints out:
set()
'''

Set operations

Like I alluded to before, set operations can be used between set operands. The following subsections will use the following sets in their examples:

set_a = {5, 7, 3, 6 ,2 ,4, 1, 0}
set_b = {5, 7, 3, 6 , 1, 0, 19}
set_c = {5, 7, 3, 6 ,2 , 0}

Union

Given sets A and B, use | or A.union(B) to get the union of the two sets (ie a set of all distinct elements common or not common in the two sets)

print(set_a | set_b)
print(set_a.union(set_b))
# Both print {0, 1, 2, 3, 4, 5, 6, 7, 19}

Intersection

Given sets A and B, use & or A.intersection(B) to get the intersection of the two sets (ie a set of all distincts elements common in the two sets)

print(set_a & set_b)
print(set_a.intersection(set_b))
# Both print {0, 1, 3, 5, 6, 7}

Difference

Given sets A and B, use - or A.difference(B) to get the union of the two sets (ie a set of all elements in A, but not in B).

print(set_a - set_b)
print(set_a.difference(set_b))
# Both print {2, 4}

Is Subset

Given sets A and B, use <= or A.issubset(B) to check if A is a subset of B (ie. all elements in A are contained in B)

print(set_c <= set_a)
print(set_c.issubset(set_a))
# Both print True

Is Superset

Given sets A and B, use >= or A.issuperset(B) to check if A is a superset of B (ie. A contains all elements in B)

print(set_a >= set_c)
print(set_a.issuperset(set_c))
# Both print True