Python Set (HashSet)

Introduction to Python set.

In Python, a set is an unordered collection of unique elements. It is defined using curly braces {} or the set() constructor. The set in Python is implemented as a hash table, making it functionally similar to Java's HashSet.

Characteristics:

  • Stores unique elements only (no duplicates)
  • Unordered collection (no indexing)
  • Supports fast lookup (O(1) average time complexity)
  • Mutable but elements must be hashable (e.g., lists are not allowed)

Here are some key properties and operations with sets:

Creating a Set

# Using curly braces
my_set = {1, 2, 3, 4, 5}

# Using set() constructor
another_set = set([3, 4, 5, 6, 7])

# Creating an empty set (must use set(), not {})
empty_set = set()

Basic Set Operations

# Adding an element
my_set.add(6)

# Removing an element (raises KeyError if not found)
my_set.remove(2)

# Removing an element safely
my_set.discard(10)  # No error if 10 is not in the set

# Checking membership
print(3 in my_set)  # True

# Set length
print(len(my_set))

Set Operations

A = {1, 2, 3, 4}
B = {3, 4, 5, 6}

# Union (A ∪ B)
print(A | B)  # {1, 2, 3, 4, 5, 6}

# Intersection (A ∩ B)
print(A & B)  # {3, 4}

# Difference (A - B)
print(A - B)  # {1, 2}

# Symmetric Difference (A Δ B)
print(A ^ B)  # {1, 2, 5, 6}

Other Set Methods

# Copying a set
new_set = my_set.copy()

# Clearing all elements
my_set.clear()

Iterating Over a Set

for item in hash_set:
    print(item)

FrozenSet (Immutable HashSet)

If you need an immutable version of a set (similar to Collections.unmodifiableSet() in Java), you can use frozenset.

frozen = frozenset([1, 2, 3, 4])
# frozen.add(5)  # This will raise an error

When to Use a Python Set?

  • When you need fast lookups (O(1) time complexity).
  • When duplicate values should be avoided.
  • When performing set operations (union, intersection, difference).
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