Sets

# Creation:
courses = {"Java", "Python", "SQL", "Java"}  # duplicate removed silently
print(courses)  # order not guaranteed: {'Java', 'Python', 'SQL'}
print(len(courses))  # 3

# Empty set — MUST use set(), not {} (that creates a dict!):
empty_set  = set()
empty_dict = {}  # this is a dict, not a set

# From a list (deduplication):
marks_with_dupes = [65, 72, 80, 65, 72, 91]
unique_marks = set(marks_with_dupes)
print(unique_marks)  # {65, 72, 80, 91}

courses = {"Java", "Python", "SQL"}

# Adding and removing:
courses.add("C#")
courses.add("Java")    # ignored — already exists
courses.discard("Ruby")  # no error if not found
courses.remove("SQL")    # KeyError if not found
print(courses)  # {'Java', 'Python', 'C#'}

# Membership — O(1) average (much faster than list for large sets):
print("Python" in courses)  # True
print("Kotlin" in courses)  # False

morning   = {"Thandi", "Sipho", "Lerato", "Bongani"}
afternoon = {"Bongani", "Zanele", "Sipho", "Nomsa"}

# Union — all unique elements from both:
all_learners = morning | afternoon
# or: morning.union(afternoon)
print(f"All: {all_learners}")

# Intersection — elements in both:
both_sessions = morning & afternoon
# or: morning.intersection(afternoon)
print(f"Both sessions: {both_sessions}")  # {'Bongani', 'Sipho'}

# Difference — in morning but not afternoon:
morning_only = morning - afternoon
print(f"Morning only: {morning_only}")

# Symmetric difference — in one but not both:
exclusive = morning ^ afternoon
print(f"Exclusive: {exclusive}")

# Subset and superset:
a = {1, 2, 3}
b = {1, 2, 3, 4, 5}
print(a.issubset(b))    # True  — a <= b
print(b.issuperset(a))  # True  — b >= a