Basic Syntax

The general form of set comprehension is:

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{expression for var in iter [if condition][...set comp]}

Where:

• expression is the value to include in the set
• var is the variable that iterates over the iterable
• iter is the iterable (list, range, etc.)
• if condition is optional - filters elements
• Additional set comprehensions are optional - for nested iterations

Simple Set Comprehension

The simplest form creates a set by applying an expression to each element:

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s1 = {n * 2 for n in range(3)}
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print(s1)  # {0, 2, 4}

This is equivalent to:

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s1 = set()
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for n in range(3):
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    s1.add(n * 2)

Set Comprehension with Condition

You can add a condition to filter elements:

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s2 = {n * 2 for n in range(3) if n > 0}
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print(s2)  # {2, 4}

This only includes elements where n > 0, so it skips n = 0.

Copying a Set

You can use set comprehension to create a copy of a set:

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s1 = {0, 2, 4}
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s3 = {n for n in s1}
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print(s3)  # {0, 2, 4}

Nested Set Comprehension

You can use multiple for clauses to create nested iterations:

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s1 = {0, 2, 4}
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s2 = {2, 4}
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s4 = {(m, n) for m in s1 for n in s2}
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print(s4)  # {(0, 2), (0, 4), (2, 2), (2, 4), (4, 2), (4, 4)}

This creates all combinations of elements from s1 and s2 as tuples.

Multiple Conditions

You can have multiple conditions and multiple loops:

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s5 = {m + n for m in range(-4, 4) if m % 2 == 0 for n in range(-4, 4) if n % 2 == 1}
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print(s5)  # {-7, -5, -3, -1, 1, 3, 5}

Note that sets automatically remove duplicates, so even though the list comprehension version had duplicates, the set version only contains unique values.

Exercises

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squares = {n * n for n in range(10)}
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print(squares)

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evens = {n for n in range(21) if n % 2 == 0}
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print(evens)

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n = int(input())
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numbers = []
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for i in range(n):
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    numbers.append(int(input()))
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unique_evens = {num for num in numbers if num % 2 == 0}
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print(unique_evens)

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sums = {i + j for i in range(3) for j in range(3)}
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print(sums)

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numbers = {n for n in range(1, 51) if n % 3 == 0 and n % 5 == 0}
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print(numbers)