09_sets.md

🐍 09 – Sets

🎯 What You'll Learn

• Create and use sets
• Set operations (union, intersection, difference)
• When to use sets vs lists

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📌 JS vs Python — Side by Side

Concept	JavaScript	Python
Create	new Set([1,2,3])	{1, 2, 3}
Add item	s.add(x)	s.add(x)
Remove item	s.delete(x)	s.remove(x)
Check member	s.has(x)	x in s
Size	s.size	len(s)
Union	new Set([...a,...b])	a | b
Intersection	manual loop	a & b
Difference	manual loop	a - b

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💡 Creating Sets

# Sets store UNIQUE values only — duplicates are ignored
s = {1, 2, 3, 3, 2, 1}
print(s)             # {1, 2, 3}

# From a list — great for removing duplicates!
lst = [1, 2, 2, 3, 3, 3, 4]
unique = set(lst)    # {1, 2, 3, 4}

# Empty set — MUST use set(), not {} (that creates empty dict)
empty = set()        # ✅
empty = {}           # ❌ this is an empty DICT

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💡 Set Methods

s = {1, 2, 3}

s.add(4)          # {1, 2, 3, 4}
s.add(2)          # {1, 2, 3, 4} — no duplicate added
s.remove(1)       # {2, 3, 4} — raises KeyError if missing
s.discard(99)     # no error if 99 not in set
s.pop()           # remove and return arbitrary element
s.clear()         # empty the set

2 in s            # True — O(1) lookup (much faster than list!)
len(s)            # count

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💡 Set Operations

a = {1, 2, 3, 4, 5}
b = {4, 5, 6, 7, 8}

# Union — all items from both
a | b                    # {1,2,3,4,5,6,7,8}
a.union(b)               # same

# Intersection — only items in BOTH
a & b                    # {4, 5}
a.intersection(b)        # same

# Difference — in a but NOT in b
a - b                    # {1, 2, 3}
a.difference(b)          # same

# Symmetric difference — in either but NOT both
a ^ b                    # {1,2,3,6,7,8}
a.symmetric_difference(b) # same

# Subset / Superset
{1,2}.issubset({1,2,3})     # True
{1,2,3}.issuperset({1,2})   # True
{1,2}.isdisjoint({3,4})     # True — no common elements

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💡 Common Use Cases

# 1. Remove duplicates from a list
lst = [1, 2, 2, 3, 3, 3]
unique = list(set(lst))      # [1, 2, 3]

# 2. Fast membership check
valid_roles = {"admin", "editor", "viewer"}
user_role = "admin"
if user_role in valid_roles:    # O(1) — instant!
    print("Access granted")

# 3. Find common items
users_a = {"Alice", "Bob", "Charlie"}
users_b = {"Bob", "Diana", "Charlie"}
both    = users_a & users_b     # {"Bob", "Charlie"}

# 4. Find unique to one group
only_a  = users_a - users_b    # {"Alice"}

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⚠️ Common Mistakes

Mistake	Wrong	Right
Empty set	{}	set()
Ordered set	sets are unordered	use list if order matters
Duplicate handling	check manually	sets handle it automatically

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🔗 Next Lesson

👉 10 – Functions