L4.5: More on Sets

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Learning outcomes

• use set methods confidently
• apply union, intersection, and difference
• explain why sets are useful in duplicate removal and membership

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Set methods

s = {1, 2, 3}
s.add(4)
s.remove(2)
print(s)

• add(x)
• remove(x)
• discard(x)
• pop()
• clear()

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Set operations

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

print(a | b)  # union
print(a & b)  # intersection
print(a - b)  # difference
print(a ^ b)  # symmetric difference

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remove vs discard

s = {1, 2, 3}
# s.remove(9)   # KeyError
s.discard(9)    # no error

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Subset and superset checks

a = {1, 2}
b = {1, 2, 3}
print(a <= b)  # True
print(b >= a)  # True

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Trap: list inside set

# s = {[1, 2], [3, 4]}  # TypeError

• lists are mutable, so they cannot be set elements

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Exam hints and traps

• pop() removes arbitrary element from set
• order-based answers are unsafe for sets
• use discard if uncertain whether element exists

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Quick practice

1. Find common elements of {1, 2, 3} and {2, 3, 4}.
2. Remove duplicates from [5, 5, 6, 7, 7].
3. Explain why sets do not support indexing.

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Answer key

1. {2, 3}
2. set([5, 5, 6, 7, 7])