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. Show
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Python Nested ListsThe 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. # create a nested list nlist1 = [[]] nlist2 = [[1,2],[3,4,5]]List Comprehension with 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 nest list comprehension. # create a list with list comprehension nlist_comp1 = [[i for i in range(5)], [i for i in range(7)], [i for i in range(3)]] nlist_comp2 = [[i for i in range(n)] for n in range(3)] print(nlist_comp1) print(nlist_comp2)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 append() function. In our example, we create a list and append it to one of our existing lists from above. # append a list list1 = [8, 7, 6] nlist_comp1.append(list1) print(nlist_comp1)This should result in this list: [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4, 5, 6], [0, 1, 2], [8, 7, 6]] 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. # concat nested lists concat_nlist = nlist_comp1 + nlist_comp2 print(concat_nlist)The list we should see from this is: [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4, 5, 6], [0, 1, 2], [8, 7, 6], [], [0], [0, 1]] How to Reverse a Nested ListNow 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 reverse() function. # reverse a nested list concat_nlist.reverse() print(concat_nlist)This should print out the nested list: [[0, 1], [0], [], [8, 7, 6], [0, 1, 2], [0, 1, 2, 3, 4, 5, 6], [0, 1, 2, 3, 4]] Okay, so we can easily reverse a list using the reverse function. What if we want to reverse the sub elements of a nested list? We can reverse each of the lists in a nested list by looping through each list in the nested list and calling the reverse function on it. # reverse sub elements of nested list for _list in concat_nlist: _list.reverse() print(concat_nlist)If we call the above code on the original concatenated list, we will see this list: [[4, 3, 2, 1, 0], [6, 5, 4, 3, 2, 1, 0], [2, 1, 0], [6, 7, 8], [], [0], [1, 0]] Now 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. # reverse sub elements + elements of a nested list for _list in concat_nlist: _list.reverse() concat_nlist.reverse() print(concat_nlist)Running this on the original concat_nlist should give: [[1, 0], [0], [], [6, 7, 8], [2, 1, 0], [6, 5, 4, 3, 2, 1, 0], [4, 3, 2, 1, 0]] 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. # flatten list flat_list = [ele for sublist in concat_nlist for ele in sublist] print(flat_list)When running the above code to flatten a list on the original concatenated lists, we should get this list: [0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 8, 7, 6, 0, 0, 1] Reverse a Flattened Nested ListLet’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 reverse function on the flattened list. # reverse elements of a flattened list flat_list.reverse() print(flat_list)After running the reverse function on the list we flattened above, we should get: [1, 0, 0, 6, 7, 8, 2, 1, 0, 6, 5, 4, 3, 2, 1, 0, 4, 3, 2, 1, 0] In 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. 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. Choose an amount Or enter a custom amount Your contribution is appreciated. Your contribution is appreciated. Your contribution is appreciated. DonateDonate monthlyDonate yearlyHow to Create a List of Lists in Python
How do I start a nested list?How to build a nested list
How do you join a nested list in Python?Straightforward way is to run two nested loops – outer loop gives one sublist of lists, and inner loop gives one element of sublist at a time. Each element is appended to flat list object. What are nested lists in python?A nested list is a list within a list. Python provides features to handle nested list gracefully and apply common functions to manipulate the nested lists. In this article we will see how to use list comprehension to create and use nested lists in python. How to create a number of empty nested lists in Python?Note however that the above is for Python 2.x. On Python 3.x., since xrange was removed, you will want this: This will give you the same result like above but the list are not distinct instances,they are just n references to the same instance. Not the answer you’re looking for? How do you remove items from a nested list in Python?Remove items from a Nested List. If you know the index of the item you want, you can use pop () method. It modifies the list and returns the removed item. If you don’t need the removed value, use the del statement. If you’re not sure where the item is in the list, use remove () method to delete it by value. Which is faster to create an empty list in Python?Whereas, [] is just a literal in python that always returns the same result i.e. an empty list. So, no additional name lookup or function call is required, which makes it much faster than list (). Till now we have seen two different ways to create an empty python list, now let’s discuss the different ways to append elements to the empty list. How to iterate through a nested list in Python?Iterate through a Nested List To iterate over the items of a nested list, use simple for loop. L = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]] for list in L: for number in list: print(number, end=’ ‘) # Prints 1 2 3 4 5 6 7 8 9 How do you make an empty nested list in Python?x = [], x[4] = 1 works in perl, throws an error in python). however, just make x a dictionary, and it will work the same way: x = {}, x[4] = 1 now works just fine. you will have to sort the keys, and they won't be monotonic though (i.e. you could have indexes like [0,3,4,7]).
How do I make a nested list in Python?Python 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.
Can you create an empty list in Python?You can create an empty list using an empty pair of square brackets [] or the type constructor list() , a built-in function that creates an empty list when no arguments are passed. Square brackets [] are commonly used in Python to create empty lists because it is faster and more concise.
Is Empty list a nested list?A list within another list is said to be nested. Finally, a list with no elements is called an empty list, and is denoted [].
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