Python in 30 Days: Day 7 – Sets

Day 7

Sets

A set is a collection of unordered and unindexed distinct elements. In Python, sets are used to store unique items, and it is possible to find the union, intersection, difference, symmetric difference, subset, superset, and disjoint set among sets.

Creating a Set

We use curly brackets, {} to create a set or the set() built-in function.

Creating an empty set

# syntax
st = {}
# or
st = set()

Creating a set with initial items

# syntax
st = {'item1', 'item2', 'item3', 'item4'}

Example:

# syntax
fruits = {'banana', 'orange', 'mango', 'lemon'}

Getting Set’s Length

We use len() method to find the length of a set.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
len(set)

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
len(fruits)

Accessing Items in a Set

We use loops to access items. We will see this in the loop section

Checking an Item

To check if an item exists in a list we use in membership operator.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
print("Does set st contain item3? ", 'item3' in st) # Does set st contain item3? True

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
print('mango' in fruits ) # True

Adding Items to a Set

Once a set is created we cannot change any items and we can also add additional items.

Add one item using add()

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
st.add('item5')

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
fruits.add('lime')

Add multiple items using update() The update() allows to add multiple items to a set. The update() takes a list argument.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
st.update(['item5','item6','item7'])

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
vegetables = ('tomato', 'potato', 'cabbage','onion', 'carrot')
fruits.update(vegetables)

Removing Items from a Set

We can remove an item from a set using remove() method. If the item is not found remove() method will raise errors, so it is good to check if the item exist in the given set. However, discard() method doesn’t raise any errors.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
st.remove('item2')

The pop() method removes a random item from a list and returns the removed item.

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
fruits.pop()  # removes a random item from the set

If we are interested in the removed item.

fruits = {'banana', 'orange', 'mango', 'lemon'}
removed_item = fruits.pop()

Clearing Items in a Set

If we want to clear or empty the set we use the clear method.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
st.clear()

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
fruits.clear()
print(fruits) # set()

Deleting a Set

If we want to delete the set itself we use the del operator.

# syntax
st = {'item1', 'item2', 'item3', 'item4'}
del st

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
del fruits

Converting List to Set

We can convert list to set and set to list. Converting the list to set removes duplicates and unique items will be reserved.

# syntax
lst = ['item1', 'item2', 'item3', 'item4', 'item1']
st = set(lst)  # {'item2', 'item4', 'item1', 'item3'} - the order is random, because sets in general are unordered

Example:

fruits = ['banana', 'orange', 'mango', 'lemon','orange', 'banana']
fruits = set(fruits) # {'mango', 'lemon', 'banana', 'orange'}

Joining Sets

We can join two sets using the union() or update() method.

Union This method returns a new set

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item5', 'item6', 'item7', 'item8'}
st3 = st1.union(st2)

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
vegetables = {'tomato', 'potato', 'cabbage','onion', 'carrot'}
print(fruits.union(vegetables)) # {'lemon', 'carrot', 'tomato', 'banana', 'mango', 'orange', 'cabbage', 'potato', 'onion'}

Update This method inserts a set into a given set

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item5', 'item6', 'item7', 'item8'}
st1.update(st2) # st2 contents are added to st1

Example:

fruits = {'banana', 'orange', 'mango', 'lemon'}
vegetables = {'tomato', 'potato', 'cabbage','onion', 'carrot'}
fruits.update(vegetables)
print(fruits) # {'lemon', 'carrot', 'tomato', 'banana', 'mango', 'orange', 'cabbage', 'potato', 'onion'}

Finding Intersection Items

Intersection returns a set of items that are in both sets. See the example

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item3', 'item2'}
st1.intersection(st2) # {'item3', 'item2'}

Example:

whole_numbers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
even_numbers = {0, 2, 4, 6, 8, 10}
whole_numbers.intersection(even_numbers) # {0, 2, 4, 6, 8, 10}

python = {'p', 'y', 't', 'h', 'o','n'}
dragon = {'d', 'r', 'a', 'g', 'o','n'}
python.intersection(dragon)     # {'o', 'n'}

Checking Subset and Super Set

A set can be a subset or superset of other sets:

Subset: issubset()
Super set: issuperset

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item2', 'item3'}
st2.issubset(st1) # True
st1.issuperset(st2) # True

Example:

whole_numbers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
even_numbers = {0, 2, 4, 6, 8, 10}
whole_numbers.issubset(even_numbers) # False, because it is a super set
whole_numbers.issuperset(even_numbers) # True

python = {'p', 'y', 't', 'h', 'o','n'}
dragon = {'d', 'r', 'a', 'g', 'o','n'}
python.issubset(dragon)     # False

Checking the Difference Between Two Sets

It returns the difference between two sets.

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item2', 'item3'}
st2.difference(st1) # set()
st1.difference(st2) # {'item1', 'item4'} => st1\st2

Example:

whole_numbers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
even_numbers = {0, 2, 4, 6, 8, 10}
whole_numbers.difference(even_numbers) # {1, 3, 5, 7, 9}

python = {'p', 'y', 't', 'o','n'}
dragon = {'d', 'r', 'a', 'g', 'o','n'}
python.difference(dragon)     # {'p', 'y', 't'}  - the result is unordered (characteristic of sets)
dragon.difference(python)     # {'d', 'r', 'a', 'g'}

Finding Symmetric Difference Between Two Sets

It returns the symmetric difference between two sets. It means that it returns a set that contains all items from both sets, except items that are present in both sets, mathematically: (A\B) ∪ (B\A)

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item2', 'item3'}
# it means (A\B)∪(B\A)
st2.symmetric_difference(st1) # {'item1', 'item4'}

Example:

whole_numbers = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
some_numbers = {1, 2, 3, 4, 5}
whole_numbers.symmetric_difference(some_numbers) # {0, 6, 7, 8, 9, 10}

python = {'p', 'y', 't', 'h', 'o','n'}
dragon = {'d', 'r', 'a', 'g', 'o','n'}
python.symmetric_difference(dragon)  # {'r', 't', 'p', 'y', 'g', 'a', 'd', 'h'}

Joining Sets

If two sets do not have a common item or items we call them disjoint sets. We can check if two sets are joint or disjoint using isdisjoint() method.

# syntax
st1 = {'item1', 'item2', 'item3', 'item4'}
st2 = {'item2', 'item3'}
st2.isdisjoint(st1) # False

Example:

even_numbers = {0, 2, 4 ,6, 8}
even_numbers = {1, 3, 5, 7, 9}
even_numbers.isdisjoint(odd_numbers) # True, because no common item

python = {'p', 'y', 't', 'h', 'o','n'}
dragon = {'d', 'r', 'a', 'g', 'o','n'}
python.isdisjoint(dragon)  # False, there are common items {'o', 'n'}

Now do some exercises for your brain and muscles.

Exercises: Python in 30 Days: Day 7 – Sets Day 7

# sets
it_companies = {'Facebook', 'Google', 'Microsoft', 'Apple', 'IBM', 'Oracle', 'Amazon'}
A = {19, 22, 24, 20, 25, 26}
B = {19, 22, 20, 25, 26, 24, 28, 27}
age = [22, 19, 24, 25, 26, 24, 25, 24]

Exercises: Level 1

Find the length of the set it_companies
Add ‘Twitter’ to it_companies
Insert multiple IT companies at once to the set it_companies
Remove one of the companies from the set it_companies
What is the difference between removing and discard

Exercises: Level 2

Join A and B
Find A intersection B
Is A subset of B
Are A and B disjoint sets
Join A with B and B with A
What is the symmetric difference between A and B
Delete the sets completely

Exercises: Level 3

Convert the ages to a set and compare the length of the list and the set, which one is bigger?
Explain the difference between the following data types: string, list, tuple, and set
I am a teacher and I love to inspire and teach people. How many unique words have been used in the sentence? Use the split method and set it to get the unique words.

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