curriculum/challenges/english/blocks/lecture-working-with-loops-and-sequences/6839e38cdf93ee5a00c79676.md
In the previous lesson, you were introduced to the list data type and learned how to access elements from a list as well as list slicing. In this lesson, you will continue to learn about lists and some common methods associated with them like append(), pop(), and sort().
The first method we will look at is the append() method. This is used to add an item to the end of the list. Here is an example of using the append() method to add the number 6 to list of numbers:
numbers = [1, 2, 3, 4, 5]
numbers.append(6)
print(numbers) # [1, 2, 3, 4, 5, 6]
If you want to add one list at the end of another, you can also use the append() method like this:
numbers = [1, 2, 3, 4, 5]
even_numbers = [6, 8, 10]
numbers.append(even_numbers)
print(numbers) # [1, 2, 3, 4, 5, [6, 8, 10]]
Notice how the entire even_numbers list is nested inside of the numbers list.
But if you want to add all of the individual numbers from the even_numbers list at the end of the numbers list, then you can use the extend() method.
The extend() method is similar to the append() method, but with extend() you can add multiple elements from one list to another. Here's an example of adding the numbers 6, 8, and 10 from one list to the end of the numbers list:
numbers = [1, 2, 3, 4, 5]
even_numbers = [6, 8, 10]
numbers.extend(even_numbers)
print(numbers) # [1, 2, 3, 4, 5, 6, 8, 10]
As you can see, the nested list is gone and it's just a list of numbers.
To insert an element at a specific index in a list, you can use the insert() method. This method accepts two arguments: the index where you wish to insert the new item and the item you want to insert.
Here is an example of using the insert() method:
numbers = [1, 2, 3, 4, 5]
numbers.insert(2, 2.5)
print(numbers) # [1, 2, 2.5, 3, 4, 5]
The following code will insert the number 2.5 at index 2 in the numbers list.
If you want to remove an element from a list, you can use the remove() method. The remove() method takes the value of the element to remove as an argument:
numbers = [10, 20, 30, 40, 50, 50]
numbers.remove(50)
print(numbers) # [10, 20, 30, 40, 50]
It is important to note that this method will only remove the first occurrence of an item. Not all of them:
numbers = [10, 20, 30, 40, 50, 50, 50]
numbers.remove(50)
print(numbers) # [10, 20, 30, 40, 50, 50]
To remove an element at a specific index in the list, you can use the pop() method like this:
numbers = [1, 2, 3, 4, 5]
numbers.pop(1) # The number 2 is returned
If you don't specify an element for the pop method, then the last element is removed.
numbers = [1, 2, 3, 4, 5]
numbers.pop() # The number 5 is returned
If you need to empty the list, then you can use the clear() method like this:
numbers = [1, 2, 3, 4, 5]
numbers.clear()
print(numbers) # []
The next method we will take a look at is the sort() method. This method is used to sort the elements in place. Here is an example of sorting a random list of numbers in place:
numbers = [19, 2, 35, 1, 67, 41]
numbers.sort()
print(numbers) # [1, 2, 19, 35, 41, 67]
In contrast to the sort() method, there is the sorted() function which works for any iterable and returns a new sorted list instead of modifying the original list. For example:
numbers = [19, 2, 35, 1, 67, 41]
sorted_numbers = sorted(numbers)
print(numbers) # [19, 2, 35, 1, 67, 41]
print(sorted_numbers) # [1, 2, 19, 35, 41, 67]
As a reminder, an iterable is a special type of object that you can loop over, allowing you to access each item one at a time. You'll learn more about how loops work in Python in a future lesson.
Both the sort() method and sorted() function accept optional key and reverse parameters. You will learn more about these optional parameters in a future lesson when you learn about tuples. You'll also learn more about other built-in functions like sorted() in future lessons.
The next method we will take a look at is the reverse() method. This method, will reverse a list of elements in place like this:
numbers = [6, 5, 4, 3, 2, 1]
numbers.reverse()
print(numbers) # [1, 2, 3, 4, 5, 6]
The last method we will take a look at is the index method. This is used to find the first index where an element can be found in a list. Here is an example of using the index method to find the language 'Java' in a programming_languages list:
programming_languages = ['Rust', 'Java', 'Python', 'C++']
programming_languages.index('Java') # 1
If the element cannot be found, then Python throws a ValueError:
programming_languages = ['Rust', 'Java', 'Python', 'C++']
programming_languages.index('JavaScript')
"""
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: 'JavaScript' is not in list
"""
There are a few more methods for Python lists, but this initial list of methods is a good place to start.
Which of the following examples will correctly insert the number 2.5 at index 2 in the numbers list?
numbers = [1, 2, 3, 4, 5]
numbers.insertInto(2.5, 2)
Remember that the first index represent the index where you want to insert into and the second index represents the item you want to insert.
numbers = [1, 2, 3, 4, 5]
numbers.insert(2, 2.5)
numbers = [1, 2, 3, 4, 5]
numbers.insertInto(2, 2.5)
Remember that the first index represent the index where you want to insert into and the second index represents the item you want to insert.
numbers = [1, 2, 3, 4, 5]
numbers.insert(2.5, 2)
Remember that the first index represent the index where you want to insert into and the second index represents the item you want to insert.
2
Which of the following methods will reverse a list of elements in place?
reverse()
reversing()
Review the last part of the lesson for the answer.
reversedList()
Review the last part of the lesson for the answer.
reversingList()
Review the last part of the lesson for the answer.
1
Which of the following is NOT a list method?
pop()
One of these methods is used in JavaScript. Not Python.
clear()
One of these methods is used in JavaScript. Not Python.
splice()
append()
One of these methods is used in JavaScript. Not Python.
3