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11. Sets

A set is an unordered, mutable collection of unique items (no duplicates allowed). Sets are very fast for checking if an item exists (in operator) and for performing mathematical operations like union, intersection, etc.

11.1 What is a Set? Unique Values

Key Features of Sets:

• Unordered → No indexing (cannot use [0], [1], etc.)

• No duplicates → Adding the same item multiple times has no effect

• Mutable → You can add/remove items (but items themselves must be immutable: numbers, strings, tuples — not lists or dictionaries)

• Very fast membership testing (in is O(1) average time)

Creating a Set:

1. Using curly braces {}

Python

fruits = {"apple", "banana", "mango", "apple"} # duplicate "apple" ignored print(fruits) # {'apple', 'banana', 'mango'} (order may vary)

2. Using set() function

Python

empty_set = set() # Important! {} is empty dict, not empty set numbers = set([1, 2, 2, 3, 4]) # {1, 2, 3, 4} from_string = set("hello") # {'h', 'e', 'l', 'o'} (unique characters)

Remove duplicates from a list (very common use)

Python

my_list = [10, 20, 10, 30, 20, 40] unique = set(my_list) print(unique) # {10, 20, 30, 40}

Check membership

Python

print("apple" in fruits) # True print("grape" in fruits) # False

Tip: Use sets when order doesn't matter and duplicates are unwanted (e.g., unique users, tags, IDs).

11.2 Set Operations (union, intersection, difference, symmetric difference)

Sets support mathematical operations — very useful in data analysis, filtering, etc.

OperationSymbolMethodDescriptionExample (A = {1,2,3}, B = {3,4,5})Union``A.union(B)All elements from both setsIntersection&A.intersection(B)Elements common to both{3}Difference-A.difference(B)Elements in A but not in B{1,2}Symmetric Difference^A.symmetric_difference(B)Elements in A or B but not both{1,2,4,5}

Examples with code:

Python

A = {1, 2, 3, 4} B = {3, 4, 5, 6} # Union print(A | B) # {1, 2, 3, 4, 5, 6} print(A.union(B)) # same # Intersection print(A & B) # {3, 4} print(A.intersection(B)) # same # Difference print(A - B) # {1, 2} print(B - A) # {5, 6} # Symmetric Difference print(A ^ B) # {1, 2, 5, 6} print(A.symmetric_difference(B)) # same

Real-life Example – Find common friends

Python

my_friends = {"Rahul", "Priya", "Ankit", "Sneha"} her_friends = {"Sneha", "Vikram", "Priya", "Rohan"} common = my_friends & her_friends print("Common friends:", common) # {'Priya', 'Sneha'} only_mine = my_friends - her_friends print("Only my friends:", only_mine) # {'Rahul', 'Ankit'}

Tip: You can chain operations: A | B & C - D

11.3 Set Methods (add, remove, discard, update, etc.)

Sets have useful methods for modification.

MethodDescriptionExampleEffect / Returnsadd(item)Add a single items.add(10)Adds if not presentupdate(iterable)Add multiple items from list/tuple/sets.update([5,6,7])Adds all unique itemsremove(item)Remove item (raises KeyError if missing)s.remove(3)Removes itemdiscard(item)Remove item (no error if missing)s.discard(99)Safe removepop()Remove & return arbitrary items.pop()Returns removed itemclear()Remove all itemss.clear()Becomes empty setcopy()Shallow copynew = s.copy()Independent copyissubset()Check if all elements are in another setA.issubset(B)True/Falseissuperset()Check if contains another setB.issuperset(A)True/Falseisdisjoint()No common elementsA.isdisjoint(B)True/False

Practical Examples:

Python

s = {1, 2, 3} s.add(4) # {1, 2, 3, 4} s.update([3, 5, 6, 4]) # {1, 2, 3, 4, 5, 6} (duplicates ignored) s.remove(2) # {1, 3, 4, 5, 6} s.discard(99) # No error print(s.pop()) # Removes random item, e.g., 1 print(s) # e.g. {3, 4, 5, 6} # Set relations A = {1, 2, 3} B = {1, 2, 3, 4} print(A.issubset(B)) # True print(B.issuperset(A)) # True print(A.isdisjoint({5,6})) # True

Mini Project – Unique Visitors & Common Interests

Python

visitors_today = ["user1", "user2", "user1", "user3", "user4"] unique_visitors = set(visitors_today) print("Unique visitors today:", len(unique_visitors), unique_visitors) # Interests group_A = {"cricket", "coding", "music", "football"} group_B = {"coding", "reading", "music", "chess"} print("Common interests:", group_A & group_B) # {'coding', 'music'} print("All interests:", group_A | group_B) print("Only group A:", group_A - group_B)

This completes the full Sets section — short, powerful, and super useful for uniqueness and operations!