List comprehension with condition in Python

Introduction

List comprehension with condition helps you create a new list by picking only the items you want from another list, making your code shorter and easier to read.

When you want to filter out unwanted items from a list.

When you want to create a list of squares of only even numbers from another list.

When you want to quickly make a list of words that start with a certain letter.

When you want to transform items but only if they meet a certain rule.

Syntax

Python

new_list = [expression for item in old_list if condition]

The 'expression' is what you want to put in the new list for each item.

The 'condition' is a test that decides if the item should be included.

Examples

Creates a list of even numbers from the original list.

Python

numbers = [1, 2, 3, 4, 5]
even_numbers = [num for num in numbers if num % 2 == 0]

Filters words that start with the letter 'a'.

Python

words = ['apple', 'banana', 'cherry', 'date']
a_words = [word for word in words if word.startswith('a')]

Works fine even if the original list is empty; result is an empty list.

Python

empty_list = []
filtered = [x for x in empty_list if x > 0]

Works with a list of one item, includes it if condition is true.

Python

single_item = [10]
filtered_single = [x for x in single_item if x > 5]

Sample Program

This program takes a list of numbers, then creates a new list with squares of only the even numbers, and prints both lists.

Python

numbers = [10, 15, 20, 25, 30]
print('Original list:', numbers)
even_squares = [num ** 2 for num in numbers if num % 2 == 0]
print('Squares of even numbers:', even_squares)

Important Notes

Time complexity is O(n), where n is the number of items in the original list.

Space complexity is O(m), where m is the number of items that meet the condition.

A common mistake is forgetting the 'if' condition, which includes all items instead of filtering.

Use list comprehension with condition when you want a quick, readable way to filter and transform lists.

Summary

List comprehension with condition lets you make a new list by choosing items that pass a test.

It makes your code shorter and easier to understand than using loops and if statements.

It works well even with empty lists or lists with one item.

Practice

1. What does the following list comprehension do?
[x for x in range(5) if x % 2 == 0]

easy

A. Creates a list of odd numbers from 0 to 4

B. Creates a list of even numbers from 0 to 4

C. Creates a list of numbers from 1 to 5

D. Creates a list of numbers divisible by 3

Solution

1. Step 1: Understand the range and condition

   The range(5) generates numbers 0, 1, 2, 3, 4. The condition x % 2 == 0 selects numbers divisible by 2 (even numbers).

2. Step 2: Apply the condition to each number

   From 0 to 4, the even numbers are 0, 2, and 4. These are included in the new list.

3. Final Answer:

   [0, 2, 4] -> Option B

4. Quick Check:

   Even numbers from 0 to 4 = [0, 2, 4] [OK]

Hint: Look for the condition after 'if' to filter items [OK]

Common Mistakes:

• Confusing even and odd numbers
• Ignoring the condition and taking all numbers
• Misunderstanding the range function

2. Which of the following is the correct syntax for a list comprehension that selects numbers greater than 10 from list nums?

easy

A. [x for x > 10 in nums]

B. [x if x > 10 for x in nums]

C. [if x > 10 for x in nums]

D. [x for x in nums if x > 10]

Solution

1. Step 1: Recall list comprehension syntax

   The correct syntax is [expression for item in iterable if condition]. Here, expression and item are both x, iterable is nums, and condition is x > 10.

2. Step 2: Check each option

   [x for x in nums if x > 10] matches the correct syntax. The other options have misplaced or missing parts.

3. Final Answer:

   [x for x in nums if x > 10] -> Option D

4. Quick Check:

   Correct list comprehension syntax = [x for x in nums if x > 10] [OK]

Hint: Remember: for comes before if in list comprehension [OK]

Common Mistakes:

• Placing 'if' before 'for'
• Omitting 'for' keyword
• Using incorrect order of clauses

3. What is the output of this code?

nums = [3, 7, 12, 5, 20]
result = [n*2 for n in nums if n > 6]
print(result)

medium

A. [14, 24, 40]

B. [3, 7, 12, 5, 20]

C. [6, 14, 24, 10, 40]

D. [12, 24, 40]

Solution

1. Step 1: Identify numbers greater than 6

   From the list, numbers greater than 6 are 7, 12, and 20.

2. Step 2: Multiply each selected number by 2

   7*2=14, 12*2=24, 20*2=40. These form the new list.

3. Final Answer:

   [14, 24, 40] -> Option A

4. Quick Check:

   Filter >6 and double = [14, 24, 40] [OK]

Hint: Apply condition first, then transform each item [OK]

Common Mistakes:

• Multiplying all numbers, ignoring condition
• Including numbers less than or equal to 6
• Confusing multiplication with addition

4. Find the error in this list comprehension:

values = [1, 2, 3, 4]
new_values = [x for x in values if x > 2 else 0]

medium

A. SyntaxError due to incorrect use of else in list comprehension

B. No error, code runs fine

C. TypeError because of wrong condition

D. NameError because x is undefined

Solution

1. Step 1: Understand list comprehension with condition

   In list comprehension, 'if' can be used to filter items, but 'else' must be part of a conditional expression, not after 'if' alone.

2. Step 2: Identify the syntax error

   The code uses 'if x > 2 else 0' incorrectly. The correct way is to use a conditional expression inside the expression part, like [x if x > 2 else 0 for x in values].

3. Final Answer:

   SyntaxError due to incorrect use of else in list comprehension -> Option A

4. Quick Check:

   Else must be inside expression, not after if clause [OK]

Hint: Use if-else inside expression, not after if clause [OK]

Common Mistakes:

• Placing else after if in list comprehension filter
• Confusing filter condition with conditional expression
• Ignoring Python syntax rules for comprehensions

5. You have a list of strings: words = ['apple', '', 'banana', 'cherry', '', 'date']. Which list comprehension creates a new list with only non-empty strings in uppercase?

hard

A. [w for w in words if w.upper()]

B. [w.upper() if w else w for w in words]

C. [w.upper() for w in words if w]

D. [w.upper() for w in words if w == '']

Solution

1. Step 1: Understand filtering non-empty strings

   In Python, empty strings are false in boolean context. Using if w filters out empty strings.

2. Step 2: Convert filtered strings to uppercase

   For each non-empty string w, w.upper() converts it to uppercase.

3. Final Answer:

   [w.upper() for w in words if w] -> Option C

4. Quick Check:

   Filter non-empty and uppercase = [w.upper() for w in words if w] [OK]

Hint: Use 'if w' to filter non-empty strings in list comprehension [OK]

Common Mistakes:

• Filtering empty strings incorrectly
• Using if-else incorrectly inside comprehension
• Not converting strings to uppercase