Basic Syntax

The general form of dictionary comprehension is:

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{exp1: exp2 for var in iter [if condition][...dict comp]}

Where:

• exp1 is the expression for the key
• exp2 is the expression for the value
• var is the variable that iterates over the iterable
• iter is the iterable (list, range, etc.)
• if condition is optional - filters elements
• Additional dictionary comprehensions are optional - for nested iterations

Simple Dictionary Comprehension

The simplest form creates a dictionary by mapping keys to values:

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d1 = {n: n * 2 for n in range(3)}
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print(d1)  # {0: 0, 1: 2, 2: 4}

This creates a dictionary where each number maps to its double.

Dictionary Comprehension with Condition

You can add a condition to filter key-value pairs:

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d2 = {n: n * 2 for n in range(3) if n > 0}
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print(d2)  # {1: 2, 2: 4}

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

Copying a Dictionary

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

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d1 = {0: 0, 1: 2, 2: 4}
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d3 = {k: v for k, v in d1.items()}
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print(d3)  # {0: 0, 1: 2, 2: 4}

Nested Dictionary Comprehension

You can use multiple for clauses to create nested iterations:

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d1 = {0: 0, 1: 2, 2: 4}
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d2 = {1: 2, 2: 4}
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d4 = {m: m + n for m in d1.values() for n in d2.values()}
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print(d4)  # {0: 4, 2: 6, 4: 8}

This creates a dictionary where keys come from d1.values() and values are the sum of a key from d1.values() and a value from d2.values().

Multiple Conditions

You can have multiple conditions and multiple loops:

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d5 = {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(d5)  # {-4: 3, -2: 3, 0: 3, 2: 3}

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. Creates key-value pairs where the key is m and the value is n

Note: If multiple n values satisfy the condition for the same m, only the last one is kept (dictionaries can’t have duplicate keys).

Exercises

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

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halves = {n: n // 2 for n in range(21) if n % 2 == 0}
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print(halves)

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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 = {word: len(word) for word in words}
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print(lengths)

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products = {(i, j): i * j for i in range(3) for j in range(3)}
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print(products)

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n = int(input())
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original = {}
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for i in range(n):
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    key = i
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    value = int(input())
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    original[key] = value
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filtered = {k: v for k, v in original.items() if v > 10}
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print(filtered)