List Comprehensions: Magic Loops

Hey there! Today, we’re going to learn about something super cool in Python called “List Comprehensions.” Imagine you’re a magician and you can create lists in a magical and easy way. That’s what list comprehensions are like!

What Are Lists?

First, let’s talk about lists. A list is like a special box where you can store many things. Imagine you have a box where you can put your toys, books, and candies. In Python, a list is a special box where you can store numbers, words, or even other lists.

Here’s how you make a list in Python:

my_list = [1, 2, 3, 4, 5]

This list has the numbers 1, 2, 3, 4, and 5.

What Are Loops?

Now, let’s talk about loops. A loop is like a magic spell that repeats an action over and over. For example, if you want to say “Hello!” five times, you can use a loop:

for i in range(5):
    print("Hello!")

This will print “Hello!” five times.

List Comprehensions: The Magic Loops

List comprehensions are like magic loops that can create lists in a super easy way. They let you write a loop inside a list, which makes your code shorter and easier to understand.

Here’s the magic formula for list comprehensions:

new_list = [expression for item in list]

new_list is the new list you’re creating.
expression is what you want to put in the new list.
item is each element from the original list.
list is the original list.

Let’s see some examples!

Example 1: Creating a List of Squares

Imagine you have a list of numbers, and you want to create a new list with the squares of those numbers. Normally, you would do it like this:

numbers = [1, 2, 3, 4, 5]
squares = []

for num in numbers:
    squares.append(num * num)

print(squares)

But with the magic of list comprehensions, you can do it in one line!

numbers = [1, 2, 3, 4, 5]
squares = [num * num for num in numbers]

print(squares)

Wow! That’s much easier, right?

Example 2: Creating a List of Even Numbers

Let’s say you have a list of numbers, and you want to create a new list with only the even numbers (numbers that can be divided by 2 with no remainder). Normally, you would do it like this:

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = []

for num in numbers:
    if num % 2 == 0:
        even_numbers.append(num)

print(even_numbers)

With list comprehensions, it’s much simpler!

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
even_numbers = [num for num in numbers if num % 2 == 0]

print(even_numbers)

Example 3: Creating a List of Letters

Let’s say you want to create a list with each letter in a word. Normally, you would do it like this:

word = "magic"
letters = []

for letter in word:
    letters.append(letter)

print(letters)

With list comprehensions, you can do it in one line!

word = "magic"
letters = [letter for letter in word]

print(letters)

Practice Questions

Now it’s your turn to practice some magic!

Question 1: Create a List of Cubes

Create a list of the cubes (number * number * number) of numbers from 1 to 5.

Question 2: Create a List of Odd Numbers

Create a list of odd numbers (numbers that can’t be divided by 2 with no remainder) from 1 to 10.

Question 3: Create a List of Uppercase Letters

Create a list with each letter in the word “hello” in uppercase.

Solutions

Solution 1: Create a List of Cubes

numbers = [1, 2, 3, 4, 5]
cubes = [num * num * num for num in numbers]

print(cubes)

Output:

[1, 8, 27, 64, 125]

Solution 2: Create a List of Odd Numbers

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
odd_numbers = [num for num in numbers if num % 2 != 0]

print(odd_numbers)

Output:

[1, 3, 5, 7, 9]

Solution 3: Create a List of Uppercase Letters

word = "hello"
uppercase_letters = [letter.upper() for letter in word]

print(uppercase_letters)

Output:

['H', 'E', 'L', 'L', 'O']

Great job! You’ve learned how to use list comprehensions, the magic loops in Python. Keep practicing, and soon you’ll be a Python magician!