Using Python Built-in Functions

Here are two more examples demonstrating the use of lambda functions with the filter() and sorted() functions in Python.

Alternatively, you can pass a more complex custom function instead of a lambda to these high-order functions.

filter()

The filter() function is used to create an iterator from elements of an iterable for which a function returns true. Here's an example using filter() with a lambda function to filter out odd numbers from a list:

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

# Using a lambda function to filter out odd numbers

even_numbers = filter(lambda x: x % 2 == 0, numbers)

# Convert the filter object to a list

even_numbers_list = list(even_numbers)

print(even_numbers_list)


              12345678
            
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Using a lambda function to filter out odd numbers
even_numbers = filter(lambda x: x % 2 == 0, numbers)

# Convert the filter object to a list
even_numbers_list = list(even_numbers)
print(even_numbers_list)

In this example, the lambda function lambda x: x % 2 == 0 checks if a number is even. The filter() function applies this lambda to each element in the list of numbers and returns an iterator of even numbers.

sorted()

The sorted() function returns a new sorted list from the elements of any iterable.

python
sorted(iterable, key=key, reverse=reverse)

iterable is the sequence to sort (list, dict, tuple, etc);
key is a function to execute to decide the order;
reverse is a boolean. False is an ascending order, and True is descending. The default is False.

Here's an example using sorted() with a lambda function to sort a list of tuples based on the second element in each tuple:

tuples = [(1, 'banana'), (2, 'apple'), (3, 'orange')]

# Using a lambda function to sort by the second element of each tuple

sorted_tuples = sorted(tuples, key=lambda x: x[1])

print(sorted_tuples)


              123456
            
tuples = [(1, 'banana'), (2, 'apple'), (3, 'orange')]

# Using a lambda function to sort by the second element of each tuple
sorted_tuples = sorted(tuples, key=lambda x: x[1])

print(sorted_tuples)

In this example, the lambda function lambda x: x[1] returns the second element of each tuple. The sorted() function then sorts the list tuples based on these second elements, resulting in a list sorted alphabetically by the fruit names.

Oppgave

Swipe to start coding

Let's consider a list of dictionaries representing books, and you want to filter out books that have a certain number of pages.

Define the list of books. books is a list of dictionaries. Each dictionary represents a book with two keys: "title" and "pages".
Creating the custom function has_many_pages that accepts book and min_pages arguments.
Use filter() with the custom function.
Converte the filter object to a list, store it to the filtered_books_list variable, and print it.

Løsning

# Step 1: Define the list of books

books = [

{"title": "Harry Potter and the Deathly Hallows", "pages": 640},

{"title": "Rich Dad Poor Dad", "pages": 336},

{"title": "The Great Gatsby", "pages": 160},

{"title": "The Hobbit", "pages": 400}

]

# Step 2: Create a custom function

def has_many_pages(book, min_pages=350):

"""Check if the book has more than min_pages."""

return book["pages"] > min_pages

# Step 3: Use filter() with the custom function

filtered_books = filter(lambda book: has_many_pages(book), books)

# Convert the filter object to a list and print

filtered_books_list = list(filtered_books)

print(filtered_books_list)

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Seksjon 3. Kapittel 3