Exercise 1: List Comprehension Mastery

Exercise 1: List Comprehension Mastery
Practice Problem: Write a single-line list comprehension that takes a list of strings, filters out strings shorter than 4 characters, and converts the remaining strings to uppercase.

Exercise Purpose: List comprehensions are a hallmark of Pythonic code. They allow you to replace verbose for loops and .append() calls with a readable, optimized single line. This exercise teaches you how to combine transformation (uppercase) and filtering (length check) in one expression.

Given Input: words = [“apple”, “bat”, “cherry”, “dog”, “elderberry”]

Expected Output: [‘APPLE’, ‘CHERRY’, ‘ELDERBERRY’]

The one-line list comprehension is:

words = ["apple", "bat", "cherry", "dog", "elderberry"]
result = [word.upper() for word in words if len(word) >= 4]
print(result)

Output:

['APPLE', 'CHERRY', 'ELDERBERRY']

How it works

[word.upper() for word in words if len(word) >= 4]

Think of it as:

1. for word in words → iterate through each word
2. if len(word) >= 4 → keep only words with 4+ characters
3. word.upper() → convert the selected words to uppercase

Equivalent traditional loop:

result = []
for word in words:
    if len(word) >= 4:
        result.append(word.upper())

So the key pattern to remember is:

[transformation for item in iterable if condition]