Try using a list comprehension within a anycodings_python list comprehension: Show
Note this is preferred to list anycodings_python multiplication because each of these anycodings_python lists is unique. Compare:
vs.
vs.
Where, in the second and third cases, anycodings_python 'a' appears in multiple cells! And using anycodings_python None does not avoid this problem:
tl;dr - use the double list anycodings_python comprehension. In my opinion, it's the anycodings_python most readable option anyway. Nested lists are Python representations of two dimensional arrays. They are used to represent lists of lists. For example, a list of grocery lists for the month or matrices we can multiply. In this post we’re going to go over how to use Python to create and manipulate nested lists. We’ll go over:
The easiest way to create a nested list in Python is simply to create a list and put one or more lists in that list. In the example below we’ll create two nested lists. First, we’ll create a nested list by putting an empty list inside of another list. Then, we’ll create another nested list by putting two non-empty lists inside a list, separated by a comma as we would with regular list elements.
List Comprehension with Python Nested ListsWe can also create nested lists with list comprehension. List comprehension is a way to create lists out of other lists. In our example below, we’ll create two lists with list comprehension in two ways. First we’ll create a nested list using three separate list comprehensions. Second, we’ll create a nested list with nested list comprehension.
The results of the two lists should be:
Adding Lists to a Two Dimensional ArrayNow that we’ve learned how to create nested lists in Python, let’s take a look at how to add lists to them. We work with nested lists the same way we work with regular lists. We can add an element to a nested list with the
This should result in this list:
Concatenating Two Dimensional Lists in PythonOther than adding a list to our 2D lists, we can also add or concatenate two nested lists together. List concatenation in Python is quite simple, all we need to do is add them with the addition sign. Adding nested lists works the same way as adding plain, unnested lists. In our example, we’ll add the two lists we created using list comprehension together.
The list we should see from this is:
How to Reverse a Nested List in PythonNow that we’ve created, added to, and concatenated nested lists, let’s reverse them. There are multiple ways to reverse nested lists, including creating a reversed copy or using list comprehension. In this example though, we’ll reverse a list in place using the built-in
This should print out the nested list:
Reversing the Sub Elements of a Nested ListOkay, so we can easily reverse a list using the
If we call the above code on the original concatenated list, we will see this list:
Reverse the Sub Elements and the Elements of a 2D Python ArrayNow we can reverse the elements in a 2D list as well as reverse the elements of each nested list, we can put them together. To reverse the sub elements and the elements of a 2D list in Python, all we do is loop through each of the inside lists and reverse them, and then reverse the outside list after the loop. We can also do it in the reverse order.
Running this on the original
Turning a 2D List into a Normal ListSo far, we’ve learned how to create, add to and together, and reverse a two-dimensional list in Python. Now, let’s take a look at turning that 2D list into a normal or flattened list. In this example, we’ll use list comprehension to extract each element from each sublist in the list.
When running the above code to flatten a list on the original concatenated lists, we should get this list:
Reverse a Flattened Nested List PythonLet’s put it all together. We just flattened our 2D Python array into a one-dimensional list. Earlier, we reversed our 2D list. Now, let’s reverse our flattened list. Just like with the 2D list, all we have to do to reverse our flattened list is run the
After running the reverse function on the list we flattened above, we should get:
Summary of Python Nested ListsIn this post about nested lists in Python we learned how to create, manipulate, and flatten nested lists. First we learned how to simply create nested lists by just putting lists into a list, then we learned how to create nested lists through list comprehension. Next, we learned how to do some manipulation of 2D arrays in Python. First, how to append a list, then how to concatenate two lists, and finally, how to reverse them. Lastly, we learned how to flatten a 2D list and reverse it. Further Reading
Learn MoreTo learn more, feel free to reach out to me @yujian_tang on Twitter, connect with me on LinkedIn, and join our Discord. Remember to follow the blog to stay updated with cool Python projects and ways to level up your Software and Python skills! If you liked this article, please Tweet it, share it on LinkedIn, or tell your friends! I run this site to help you and others like you find cool projects and practice software skills. If this is helpful for you and you enjoy your ad free site, please help fund this site by donating below! If you can’t donate right now, please think of us next time. I started my professional software career interning for IBM in high school after winning ACSL two years in a row. I got into AI/ML in college where I published a first author paper to IEEE Big Data. After college I worked on the AutoML infrastructure at Amazon before leaving to work in startups. I believe I create the highest quality software content so that’s what I’m doing now. Drop a comment to let me know! Make a one-time donationMake a monthly donationMake a yearly donationChoose an amount Or enter a custom amount Your contribution is appreciated. Your contribution is appreciated. Your contribution is appreciated. DonateDonate monthlyDonate yearly How do you initialize a list in Python?You can initialize a list in Python using square brackets, the list() method, list multiplication, and a list comprehension. Square brackets let you initialize an empty list, or a list which contains some default values. The list() method works in the same way as square brackets.
How do I add elements to a nested list in Python?To add new values to the end of the nested list, use append() method. When you want to insert an item at a specific position in a nested list, use insert() method. You can merge one list into another by using extend() method. If you know the index of the item you want, you can use pop() method.
How do I create a nested list?We can add an element to a nested list with the append() function. In our example, we create a list and append it to one of our existing lists from above. This should result in this list: [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4, 5, 6], [0, 1, 2], [8, 7, 6]]
How do you flatten a nested list in Python?Provide two arguments to the sum() method: my_list and an empty list (i.e. [ ] ). sum() combines my_list and [ ] to produce a flattened list.
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