L4.2: Lists and Sets

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Learning outcomes

• compare list and set behavior
• choose list or set for a given task
• understand duplicate handling and ordering

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List vs set at a glance

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List example

nums = [1, 2, 2, 3]
print(nums)  # [1, 2, 2, 3]

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Set example

nums = {1, 2, 2, 3}
print(nums)  # {1, 2, 3}

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Use cases

• order matters
• duplicates matter
• indexing is needed

• uniqueness matters
• membership checking is primary
• duplicate removal is required

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Duplicate removal pattern

vals = [1, 2, 2, 3, 3, 4]
unique_vals = set(vals)
print(unique_vals)

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Trap: empty set

a = {}
print(type(a))  # dict

a = set()

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Exam hints and traps

• set order should not be trusted in answers
• set removes duplicates automatically
• {} is empty dictionary, not empty set
• list preserves insertion order

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Quick practice

1. Convert [1, 1, 2, 3, 3] to unique values.
2. Which structure is better for roll numbers in attendance check?
3. Why can you use nums[0] with list but not set?

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Answer key

1. set([1, 1, 2, 3, 3])
2. Set, if only uniqueness and membership matter.
3. Set is unordered and has no index positions.