# With curly braces
fruits = {"apple", "banana", "cherry"}

# With set() constructor
numbers = set([1, 2, 3, 4, 5])
chars = set("hello")  # {'h', 'e', 'l', 'o'} - duplicates removed

# Empty set (NOT {} which creates a dict!)
empty = set()    # Correct
# empty = {}     # This creates a dict!

# Duplicates are automatically removed
nums = {1, 2, 2, 3, 3, 3, 4}
print(nums)  # {1, 2, 3, 4}

# From list (remove duplicates)
items = [1, 2, 2, 3, 3, 4]
unique = set(items)
print(unique)  # {1, 2, 3, 4}

colors = {"red", "green", "blue"}

# add() - single element
colors.add("yellow")
print(colors)  # {'red', 'green', 'blue', 'yellow'}

# update() - multiple elements
colors.update(["purple", "orange"])
colors.update({"pink", "cyan"})
print(colors)

# discard() - remove (no error if missing)
colors.discard("red")
colors.discard("black")  # No error

# remove() - remove (raises error if missing)
colors.remove("green")
# colors.remove("black")  # KeyError!

# pop() - remove and return arbitrary element
item = colors.pop()
print(f"Removed: {item}")

# clear() - remove all
colors.clear()
print(colors)  # set()

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

print(a | b)         # {1, 2, 3, 4, 5, 6}
print(a.union(b))    # {1, 2, 3, 4, 5, 6}

print(a & b)               # {3, 4}
print(a.intersection(b))   # {3, 4}

print(a - b)             # {1, 2}
print(a.difference(b))   # {1, 2}
print(b - a)             # {5, 6}

print(a ^ b)                        # {1, 2, 5, 6}
print(a.symmetric_difference(b))    # {1, 2, 5, 6}

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

A | B  = {1, 2, 3, 4, 5, 6}    Union
A & B  = {3, 4}                  Intersection
A - B  = {1, 2}                  Difference
B - A  = {5, 6}                  Difference
A ^ B  = {1, 2, 5, 6}           Symmetric Difference

a = {1, 2, 3}
b = {1, 2, 3, 4, 5}
c = {1, 2, 3}

# Subset: all elements of a are in b
print(a <= b)          # True
print(a.issubset(b))   # True

# Proper subset: subset but not equal
print(a < b)    # True
print(a < c)    # False (they're equal)

# Superset: b contains all elements of a
print(b >= a)            # True
print(b.issuperset(a))   # True

# Disjoint: no common elements
x = {1, 2, 3}
y = {4, 5, 6}
print(x.isdisjoint(y))   # True

# Create frozen set
frozen = frozenset([1, 2, 3, 4, 5])
print(frozen)  # frozenset({1, 2, 3, 4, 5})

# Cannot modify
# frozen.add(6)  # AttributeError

# Can use as dictionary key
permissions = {
    frozenset(["read"]): "viewer",
    frozenset(["read", "write"]): "editor",
    frozenset(["read", "write", "admin"]): "admin",
}

user_perms = frozenset(["read", "write"])
print(permissions[user_perms])  # editor

# Set operations work on frozen sets
a = frozenset([1, 2, 3])
b = frozenset([2, 3, 4])
print(a | b)  # frozenset({1, 2, 3, 4})

# Basic comprehension
squares = {x**2 for x in range(10)}
print(squares)  # {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}

# With condition
even_squares = {x**2 for x in range(10) if x % 2 == 0}
print(even_squares)  # {0, 4, 16, 36, 64}

# From string - unique characters
text = "mississippi"
unique_chars = {c for c in text}
print(unique_chars)  # {'m', 'i', 's', 'p'}

items = [1, 2, 2, 3, 3, 3, 4, 4, 5]
unique = list(set(items))
print(unique)  # [1, 2, 3, 4, 5]

# Preserve order (Python 3.7+)
unique_ordered = list(dict.fromkeys(items))
print(unique_ordered)  # [1, 2, 3, 4, 4, 5] - wrong, should be:
# Actually dict.fromkeys preserves order and removes dupes
print(unique_ordered)  # [1, 2, 3, 4, 5]

# Sets have O(1) lookup vs O(n) for lists
valid_codes = {"A100", "B200", "C300", "D400", "E500"}

user_code = "B200"
if user_code in valid_codes:  # Very fast!
    print("Valid code")

# Students in multiple courses
math_students = {"Alice", "Bob", "Charlie", "Diana"}
science_students = {"Bob", "Diana", "Eve", "Frank"}
english_students = {"Alice", "Charlie", "Eve", "Grace"}

# Students in all three courses
all_three = math_students & science_students & english_students
print(f"All three courses: {all_three}")  # set()

# Students in at least one course
any_course = math_students | science_students | english_students
print(f"Any course: {any_course}")

# Only in math
only_math = math_students - science_students - english_students
print(f"Only math: {only_math}")  # {'Bob'} or similar