Basic Syntax

The general form of list comprehension is:

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[expression for var in iter [if condition][...list comp]]

Where:

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

Simple List Comprehension

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

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list1 = [n * 2 for n in range(3)]
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print(list1)  # [0, 2, 4]

This is equivalent to:

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list1 = []
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for n in range(3):
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    list1.append(n * 2)

List Comprehension with Condition

You can add a condition to filter elements:

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list2 = [n * 2 for n in range(3) if n > 0]
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print(list2)  # [2, 4]

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

Copying a List

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

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list1 = [0, 2, 4]
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list3 = [n for n in list1]
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print(list3)  # [0, 2, 4]

Nested List Comprehension

You can use multiple for clauses to create nested iterations:

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list1 = [0, 2, 4]
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list2 = [2, 4]
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list4 = [[m, n] for m in list1 for n in list2]
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print(list4)  # [[0, 2], [0, 4], [2, 2], [2, 4], [4, 2], [4, 4]]

This creates all combinations of elements from list1 and list2.

Multiple Conditions

You can have multiple conditions and multiple loops:

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list5 = [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(list5)  # [-7, -5, -3, -1, -5, -3, -1, 1, -3, -1, 1, 3, -1, 1, 3, 5]

This:

1. Iterates over m from -4 to 3, but only when m % 2 == 0 (even numbers)
2. For each m, iterates over n from -4 to 3, but only when n % 2 == 1 (odd numbers)
3. Adds m + n for each valid combination

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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words = []
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for i in range(n):
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    words.append(input())
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lengths = [len(word) for word in words]
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print(lengths)

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pairs = [(i, j) for i in range(3) for j in range(3)]
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print(pairs)

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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)