Nested List Comprehensions in Python
List Comprehensions are one of the most amazing features of Python. It is a smart and concise way of creating lists by iterating over an iterable object. Nested List Comprehensions are nothing but a list comprehension within another list comprehension which is quite similar to nested for loops. Show
Let’s take a look at some examples to understand what nested list comprehensions can do: Example 1: I want to create a matrix which looks like below: matrix = [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]The below code uses nested for loops for the given task:
Output: [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]] The same output can be achieved using nested list comprehension in just one line:
Output: [[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]] Explanation:
Example 2: Suppose I want to flatten a given 2-D list: matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] Expected Output: flatten_matrix = [1, 2, 3, 4, 5, 6, 7, 8, 9]This can be done using nested for loops as follows:
Output: [1, 2, 3, 4, 5, 6, 7, 8, 9] Again this can be done using nested list comprehension which has been shown below:
Output: [1, 2, 3, 4, 5, 6, 7, 8, 9] Explanation:
Example 3:
This can be done using an if condition inside a nested for loop which is shown below:
Output: ['Venus', 'Earth', 'Mars', 'Pluto'] This can also be done using nested list comprehensions which has been shown below:
Output: ['Venus', 'Earth', 'Mars', 'Pluto'] Explanation:
Article Tags : Python Technical Scripter python-list Technical Scripter 2018 Practice Tags : python-list How do I create a dynamic nested list in python?You can get python to generate separate-but-identical lists for each row by using a list comprehension. When you use [rowexpr for i in xrange(height)] then rowexpr will be evaluated once per row. The trick then is to use an expression that will result in a unique list each time it is evaluated. How to get dynamic keys from JSON data?0. Add a comment. |. 2. Here is a simple example to get dynamic keys from json response – Get dynamic keys from JSON Data. public void getData (String data) { // Load json data and display JSONObject jsonData = new JSONObject (data); // Use loop to get keys from your response Iterator itr = jsonData.keys (); while (itr.hasNext ()) … Wrong way to create & initialize a list of lists in pythonLet’s start from basic, quickest way to create and initialize a normal list with same values in python is, Output: [5, 5, 5, 5, 5, 5, 5, 5, 5, 5] It created a list of size 10 with the same element i.e. 5. Basically it copied the object inside [], 10 times. Let’s use same logic to create and initialize a list of lists, # Create a list with 4 references of same sub list list_of_num = [[]] * 4 print('List of lists:') print(list_of_num) Output: [[], [], [], []] Here we provided an empty list [] inside the [] and multiplied it by 4. It created 4 different references of [] and inserted them into a new list. Yes, these are not 4 empty lists inside the main list, but these are not different list objects instead these are mere references of first list. Let’s confirm this by iterating over the list and printing id of each sub list object, for elem in list_of_num: print(id(elem)) Output: 200792200 200792200 200792200 200792200 All entries in the list have the same ID i.e. they point to the same list object. Why does it matter ? Let’s inserting an element in the 3rd sub list of main list i.e. Now check the contents of the main list, print('Modified list of lists:') print(list_of_num) Output: Modified list of lists: [[11], [11], [11], [11]] Element was inserted in all the sub lists, because these are not different lists. We didn’t expect this at first place, we just wanted to insert the element in the 3rd sub-list. So, it proves that this is a wrong way to create and initialize a list of list. Let’s look at the correct way, Advertisements How do I merge nested lists in python?First, flatten the nested lists. Take Intersection using filter() and save it to ‘lst3’. Now find elements either not in lst1 or in lst2, and save them to ‘temp’. Finally, append ‘temp’ to ‘lst3’. How do I join a nested list?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. How do I make a nested list one list in Python?The task is to convert a nested list into a single list in python i.e no matter how many levels of nesting is there in python list, all the nested has to be removed in order to convert it to a single containing all the values of all the lists inside the outermost brackets but without any brackets inside. List in PythonLearn about List in Python Z Zeeshan Abbasi 27 Aug 2021-13 mins read by John Sturtz basics python Mark as Completed Tweet Share Email Table of Contents Watch Now This tutorial has a related video course created by the Real Python team. Watch it together with the written tutorial to deepen your understanding: Lists and Tuples in Python Lists and tuples are arguably Python’s most versatile, useful data types. You will find them in virtually every nontrivial Python program. Here’s what you’ll learn in this tutorial: You’ll cover the important characteristics of lists and tuples. You’ll learn how to define them and how to manipulate them. When you’re finished, you should have a good feel for when and how to use these object types in a Python program. Take the Quiz: Test your knowledge with our interactive “Python Lists and Tuples” quiz. Upon completion you will receive a score so you can track your learning progress over time: Take the Quiz » |