Basic list comprehension syntax in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a list of squares from 1 to 5.

Python

squares = [x[1]2 for x in range(1, 6)]
print(squares)

Drag options to blanks, or click blank then click option'

A**

B+

C*

D//

Attempts:

3 left

💡 Hint

Common Mistakes

Using * instead of ** which multiplies but does not square.

Using + which adds instead of powers.

The ** operator is used to raise a number to a power. Here, x**2 means x squared.

2fill in blank

medium

Complete the code to create a list of even numbers from 0 to 10.

Python

evens = [x for x in range(11) if x [1] 2 == 0]
print(evens)

Drag options to blanks, or click blank then click option'

A%

B//

C**

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using // which is floor division, not remainder.

Using ** which is power, not related to even check.

The % operator gives the remainder. If x % 2 == 0, x is even.

3fill in blank

hard

Fix the error in the list comprehension to get lengths of words.

Python

lengths = [[1] for word in words]

Drag options to blanks, or click blank then click option'

Alength(word)

Bword.len()

Cword.length

Dlen(word)

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use word.len() which is not valid in Python.

Using length(word) which is not a Python function.

Use the built-in len() function to get the length of a string in Python.

4fill in blank

hard

Fill both blanks to create a dictionary of word lengths for words longer than 3 letters.

Python

lengths = {word: [1] for word in words if [2] > 3}

Drag options to blanks, or click blank then click option'

Alen(word)

Bword

Dword.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using word.length which is not valid in Python.

Using word instead of len(word) for length.

The key is the word itself, and the value is its length using len(word). The condition checks if the length is greater than 3.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values for words longer than 4 letters.

Python

result = [1]: [2] for w in words if [3] > 4}

Drag options to blanks, or click blank then click option'

Aw.upper()

Bw

Clen(w)

Dw.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using w.lower() instead of uppercase for keys.

Using w.length which is invalid in Python.

The keys are uppercase words using w.upper(), values are the original words w, and the condition checks if the length len(w) is greater than 4.

Practice

1. What does the following list comprehension do?
[x * 2 for x in range(3)]

easy

A. Creates a list of numbers from 1 to 6

B. Creates a list of numbers from 0 to 3

C. Creates a list of numbers doubled from 0 to 2

D. Creates a list of numbers squared from 0 to 2

Solution

1. Step 1: Understand the range and loop variable

   The range(3) generates numbers 0, 1, 2. The variable x takes these values one by one.

2. Step 2: Apply the expression to each value

   Each x is multiplied by 2, so the list becomes [0*2, 1*2, 2*2] = [0, 2, 4].

3. Final Answer:

   Creates a list of numbers doubled from 0 to 2 -> Option C

4. Quick Check:

   List comprehension doubles each number in range(3) [OK]

Hint: Remember: for x in range(3) gives 0,1,2 only [OK]

Common Mistakes:

• Thinking range(3) includes 3
• Confusing doubling with squaring
• Ignoring the for loop variable

2. Which of the following is the correct syntax for a list comprehension that creates a list of squares of numbers 0 to 4?

easy

A. [x**2 x in range(5)]

B. [for x in range(5) x**2]

C. [x^2 in range(5)]

D. [x**2 for x in range(5)]

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is [expression for variable in iterable]. Here, expression is x**2 and iterable is range(5).

2. Step 2: Check each option

   [x**2 for x in range(5)] matches the correct syntax. Options A, B, and C have syntax errors or wrong order.

3. Final Answer:

   [x**2 for x in range(5)] -> Option D

4. Quick Check:

   Correct syntax is [expr for var in iterable] [OK]

Hint: Remember: list comprehension = [expression for variable in iterable] [OK]

Common Mistakes:

• Placing 'for' after expression
• Using ^ instead of ** for power
• Missing 'in' keyword

3. What is the output of this code?

nums = [1, 2, 3, 4]
squares = [n*n for n in nums if n % 2 == 0]
print(squares)

medium

A. [4, 16]

B. [1, 4, 9, 16]

C. [2, 4]

D. [1, 9]

Solution

1. Step 1: Understand the list and condition

   The list nums is [1, 2, 3, 4]. The condition 'if n % 2 == 0' selects even numbers: 2 and 4.

2. Step 2: Calculate squares of selected numbers

   Squares of 2 and 4 are 4 and 16, so the list squares is [4, 16].

3. Final Answer:

   [4, 16] -> Option A

4. Quick Check:

   Filter even numbers, then square them [OK]

Hint: Filter first, then apply expression in list comprehension [OK]

Common Mistakes:

• Including odd numbers by mistake
• Confusing values with their squares
• Ignoring the if condition

4. Find the error in this list comprehension:

result = [x + 1 for in range(5)]

medium

A. Missing variable name after 'for'

B. Using + instead of *

C. range(5) should be range(1,5)

D. List comprehension cannot use range

Solution

1. Step 1: Check the for loop syntax inside comprehension

   The syntax requires a variable name after 'for', like 'for x in range(5)'. Here, 'x' is missing.

2. Step 2: Identify the error

   Because the variable is missing, Python will raise a syntax error.

3. Final Answer:

   Missing variable name after 'for' -> Option A

4. Quick Check:

   for loop needs variable name [OK]

Hint: Always include variable after 'for' in comprehension [OK]

Common Mistakes:

• Omitting the loop variable
• Misplacing 'in' keyword
• Thinking range needs to start at 1

5. You have a list of words: words = ['apple', 'bee', 'cat', 'dog', 'elephant']. Using list comprehension, how do you create a list of the lengths of words that have more than 3 letters?

hard

A. [w for w in words if len(w) > 3]

B. [len(w) for w in words if len(w) > 3]

C. [len(words) for w in words if len(w) > 3]

D. [len(w) > 3 for w in words]

Solution

1. Step 1: Understand the filtering condition

   We want only words with length greater than 3, so use 'if len(w) > 3' to filter.

2. Step 2: Create list of lengths

   For each filtered word, get its length with len(w). So the comprehension is [len(w) for w in words if len(w) > 3].

3. Final Answer:

   [len(w) for w in words if len(w) > 3] -> Option B

4. Quick Check:

   Filter words by length, then get lengths [OK]

Hint: Filter first, then apply len() in comprehension [OK]

Common Mistakes:

• Returning words instead of lengths
• Using len(words) instead of len(w)
• Returning boolean instead of lengths