Basic dictionary comprehension in Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of dictionary comprehension with condition

What is the output of this Python code?

Python

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

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

B {1: 1, 3: 9}

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

D SyntaxError

💡 Hint

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

The comprehension includes only even numbers (x % 2 == 0). So keys are 0, 2, 4 with their squares as values.

❓ Predict Output

intermediate

Dictionary comprehension with ternary expression

What does this code print?

Python

result = {x: ('even' if x % 2 == 0 else 'odd') for x in range(4)}
print(result)

A {0: 'even', 1: 'odd', 2: 'even', 3: 'odd'}

B {0: 'odd', 1: 'even', 2: 'odd', 3: 'even'}

C {0: True, 1: False, 2: True, 3: False}

D TypeError

💡 Hint

Check how the ternary operator assigns values based on even or odd keys.

The ternary expression assigns 'even' if x is divisible by 2, else 'odd'.

❓ Predict Output

advanced

Output of nested dictionary comprehension

What is the output of this code?

Python

result = {x: {y: x*y for y in range(2)} for x in range(3)}
print(result)

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

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

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

D SyntaxError

💡 Hint

Look carefully at the inner dictionary comprehension and how multiplication works.

For each x, the inner dictionary maps y to x*y for y in 0 and 1.

❓ Predict Output

advanced

Result of dictionary comprehension with string keys

What will this code output?

Python

keys = ['a', 'b', 'c']
result = {k: ord(k) for k in keys}
print(result)

A TypeError

B {'a': 97, 'b': 98, 'c': 99}

C {'a': 'a', 'b': 'b', 'c': 'c'}

D {'a': 1, 'b': 2, 'c': 3}

💡 Hint

The ord() function returns the Unicode code point of a character.

Each key maps to its Unicode number using ord().

🧠 Conceptual

expert

Number of items in dictionary comprehension result

How many items are in the dictionary created by this comprehension? {n: n//2 for n in range(10) if n % 3 != 0}

A 3

B 7

C 10

D 6

💡 Hint

Count numbers from 0 to 9 excluding multiples of 3.

Numbers 0,3,6,9 are multiples of 3 and excluded. Remaining 6 numbers create 6 items.

Practice

1. What does the following dictionary comprehension do?
{x: x*x for x in range(3)}

easy

A. Creates a list of squares from 0 to 2

B. Creates a set of squares from 0 to 2

C. Creates a dictionary with numbers 0 to 2 as keys and their squares as values

D. Creates a dictionary with keys as squares and values as numbers

Solution

1. Step 1: Understand the loop and key-value pairs

   The comprehension loops over numbers 0, 1, 2 and uses each number as the key.

2. Step 2: Calculate the value for each key

   Each value is the square of the key (x*x), so values are 0, 1, 4 respectively.

3. Final Answer:

   Creates a dictionary with numbers 0 to 2 as keys and their squares as values -> Option C

4. Quick Check:

   Dictionary comprehension = key: x, value: x*x [OK]

Hint: Look for key:value pairs inside braces with a for loop [OK]

Common Mistakes:

• Confusing dictionary with list or set comprehension
• Mixing keys and values in comprehension
• Thinking it creates a list instead of a dictionary

2. Which of the following is the correct syntax for a dictionary comprehension that creates keys from 1 to 3 and values as their double?

easy

A. [x: 2*x for x in range(1, 4)]

B. {x: 2*x for x in range(1, 4)}

C. {x => 2*x for x in range(1, 4)}

D. (x: 2*x for x in range(1, 4))

Solution

1. Step 1: Identify correct dictionary comprehension syntax

   Dictionary comprehensions use curly braces with key:value pairs and a for loop inside.

2. Step 2: Check each option

   {x: 2*x for x in range(1, 4)} uses curly braces and colon correctly. Options B, C, D use wrong brackets or arrow syntax.

3. Final Answer:

   {x: 2*x for x in range(1, 4)} -> Option B

4. Quick Check:

   Curly braces + key:value + for loop = correct syntax [OK]

Hint: Remember dictionary comprehension uses curly braces and colon [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using arrow (=>) instead of colon for key-value
• Using parentheses which create generator expressions

3. What is the output of this code?

nums = [1, 2, 3]
squares = {n: n**2 for n in nums if n % 2 != 0}
print(squares)

medium

A. {1: 1, 3: 9}

B. {2: 4}

C. {1: 1, 2: 4, 3: 9}

D. {}

Solution

1. Step 1: Understand the filtering condition

   The comprehension includes only numbers where n % 2 != 0, meaning odd numbers only (1 and 3).

2. Step 2: Calculate squares for filtered numbers

   Squares are 1*1=1 and 3*3=9, so dictionary is {1:1, 3:9}.

3. Final Answer:

   {1: 1, 3: 9} -> Option A

4. Quick Check:

   Filter odd numbers and square them = {1:1, 3:9} [OK]

Hint: Check the if condition filters keys before squaring [OK]

Common Mistakes:

• Including even numbers by mistake
• Confusing list comprehension output with dictionary
• Ignoring the if condition in comprehension

4. Find the error in this dictionary comprehension:

result = {x, x*2 for x in range(3)}

medium

A. Missing colon between key and value

B. Using parentheses instead of curly braces

C. Range function is incorrect

D. No error, code is correct

Solution

1. Step 1: Check syntax for key-value pairs

   Dictionary comprehension requires a colon ':' between key and value, but here a comma ',' is used.

2. Step 2: Confirm other parts are correct

   Curly braces and range(3) are correct, so the error is the missing colon.

3. Final Answer:

   Missing colon between key and value -> Option A

4. Quick Check:

   Key:value pairs need colon, not comma [OK]

Hint: Use colon ':' to separate key and value in dict comprehension [OK]

Common Mistakes:

• Using comma instead of colon between key and value
• Confusing set comprehension with dictionary comprehension
• Using wrong brackets for comprehension

5. Given a list of words, create a dictionary comprehension that maps each word to its length but only include words longer than 3 letters.
Which code correctly does this?

words = ['cat', 'house', 'dog', 'elephant']

hard

A. {word: len(word) for word in words where len(word) > 3}

B. {word: len(word) for word in words if word > 3}

C. {word: len(word) if len(word) > 3 for word in words}

D. {word: len(word) for word in words if len(word) > 3}

Solution

1. Step 1: Understand filtering condition syntax

   The if condition must come after the for loop to filter words by length correctly.

2. Step 2: Check each option's syntax

   {word: len(word) for word in words if len(word) > 3} correctly places 'if len(word) > 3' after the for loop. Others have syntax errors or wrong condition placement.

3. Final Answer:

   {word: len(word) for word in words if len(word) > 3} -> Option D

4. Quick Check:

   Filter after for loop with if condition [OK]

Hint: Put if condition after for loop in dictionary comprehension [OK]

Common Mistakes:

• Placing if condition before for loop
• Using invalid syntax like 'where' instead of 'if'
• Comparing word string directly to number