Why dictionary comprehension is used in Python - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dictionary comprehension with condition

What is the output of this Python code using dictionary comprehension with a condition?

Python

result = {x: x*x for x in range(5) if x % 2 == 0}
print(result)

A {0: 0, 2: 4, 4: 16}

B SyntaxError

C {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

D {1: 1, 3: 9}

💡 Hint

Look at the condition after the for loop inside the comprehension.

The comprehension creates key-value pairs for x and x squared only when x is even (x % 2 == 0). So keys 0, 2, and 4 are included.

🧠 Conceptual

intermediate

Why use dictionary comprehension instead of a loop?

Why is dictionary comprehension preferred over a traditional for-loop when creating dictionaries?

A It always runs faster than any for-loop.

B It is shorter and easier to read, making code cleaner.

C It can only create dictionaries with string keys.

D It does not allow conditions inside the comprehension.

💡 Hint

Think about code length and readability.

Dictionary comprehension lets you write dictionary creation in one line, making code shorter and easier to understand.

❓ Predict Output

advanced

Output of nested dictionary comprehension

What is the output of this nested dictionary comprehension code?

Python

matrix = {i: {j: i*j for j in range(3)} for i in range(2)}
print(matrix)

A {0: {0: 0, 1: 0, 2: 0}, 1: {0: 0, 1: 1, 2: 2}}

B {0: {0: 0, 1: 1, 2: 2}, 1: {0: 0, 1: 1, 2: 2}}

C {0: {0: 0, 1: 0, 2: 0}, 1: {0: 1, 1: 1, 2: 1}}

D TypeError

💡 Hint

Look carefully at how i and j are multiplied inside the inner comprehension.

The outer loop runs for i in 0 and 1. For each i, the inner loop creates a dictionary with keys 0 to 2 and values i*j.

🔧 Debug

advanced

Identify the error in dictionary comprehension

What error does this code raise?

Python

result = {x: x*2 if x > 2 for x in range(5)}
print(result)

A TypeError

B No error, prints {3: 6, 4: 8}

C KeyError

D SyntaxError

💡 Hint

Check the syntax of the comprehension carefully.

The syntax is invalid because the if condition is used incorrectly inside the comprehension without else.

🚀 Application

expert

Count character frequency using dictionary comprehension

Which option correctly creates a dictionary counting each character's frequency in the string 'banana'?

Python

text = 'banana'

A {char: len(char) for char in text}

B {char: text.index(char) for char in text}

C {char: text.count(char) for char in text}

D {char: char*2 for char in text}

💡 Hint

Think about how to count occurrences of each character.

Option C counts how many times each character appears using text.count(char). Others do not count frequency.